Immunity to attenuated influenza virus WRL 105 infection induced by heterologous, inactivated influenza A virus vaccines

Temporary Cross-Immunity as a Plausible Driver of Asynchronous Cycles of Dengue Serotypes

Many infectious diseases exist as multiple variants, with interactions between variants potentially driving epidemiological dynamics. These diseases include dengue, which infects hundreds of millions of people every year and exhibits complex multi-serotype dynamics. Antibodies produced in response to primary infection by one of the four dengue serotypes can produce a period of temporary cross-immunity (TCI) to infection by other serotypes. After this period, the remaining antibodies can facilitate the entry of heterologous serotypes into target cells, thus enhancing severity of secondary infection by a heterologous serotype. This represents antibody-dependent enhancement (ADE). In this study, we analyze an epidemiological model to provide novel insights into the importance of TCI and ADE in producing cyclic outbreaks of dengue serotypes. Our analyses reveal that without TCI, such cyclic outbreaks are synchronous across serotypes and only occur when ADE produces high transmission rates. In contrast, the presence of TCI allows asynchronous cycles of serotypes by inducing a time lag between recovery from primary infection by one serotype and secondary infection by another, with such cycles able to occur without ADE. Our results suggest that TCI is a fundamental driver of asynchronous cycles of dengue serotypes and possibly other multi-variant diseases.

Host Factors Impact Vaccine Efficacy: Implications for Seasonal and Universal Influenza Vaccine Programs

Influenza is a global public health problem. Current seasonal influenza vaccines have highly variable efficacy, and thus attempts to develop broadly protective universal influenza vaccines with durable protection are under way. While much attention is given to the virus-related factors contributing to inconsistent vaccine responses, host-associated factors are often neglected. Growing evidences suggest that host factors including age, biological sex, pregnancy, and immune history play important roles as modifiers of influenza virus vaccine efficacy. We hypothesize that host genetics, the hormonal milieu, and gut microbiota contribute to host-related differences in influenza virus vaccine efficacy. This review highlights the current insights and future perspectives into host-specific factors that impact influenza vaccine-induced immunity and protection. Consideration of the host factors that affect influenza vaccine-induced immunity might improve influenza vaccines by providing empirical evidence for optimizing or even personalizing vaccine type, dose, and use of adjuvants for current seasonal and future universal influenza vaccines.

Host Factors Impact Vaccine Efficacy: Implications for Seasonal and Universal Influenza Vaccine Programs

Influenza is a global public health problem. Current seasonal influenza vaccines have highly variable efficacy, and thus attempts to develop broadly protective universal influenza vaccines with durable protection are under way. While much attention is given to the virus-related factors contributing to inconsistent vaccine responses, host-associated factors are often neglected. Growing evidences suggest that host factors including age, biological sex, pregnancy, and immune history play important roles as modifiers of influenza virus vaccine efficacy. We hypothesize that host genetics, the hormonal milieu, and gut microbiota contribute to host-related differences in influenza virus vaccine efficacy. This review highlights the current insights and future perspectives into host-specific factors that impact influenza vaccine-induced immunity and protection. Consideration of the host factors that affect influenza vaccine-induced immunity might improve influenza vaccines by providing empirical evidence for optimizing or even personalizing vaccine type, dose, and use of adjuvants for current seasonal and future universal influenza vaccines.

Is a Universal Influenza Virus Vaccine Possible?

Influenza viruses remain a severe burden to human health because of their contribution to overall morbidity and mortality. Current seasonal influenza virus vaccines do not provide sufficient protection to alleviate the annual impact of influenza and cannot confer protection against potentially pandemic influenza viruses. The lack of protection is due to rapid changes of the viral epitopes targeted by the vaccine and the often suboptimal immunogenicity of current immunization strategies. Major efforts to improve vaccination approaches are under way. The development of a universal influenza virus vaccine may be possible by combining the lessons learned from redirecting the immune response toward conserved viral epitopes, as well as the use of adjuvants and novel vaccination platforms.

Evaluation of Antihemagglutinin and Antineuraminidase Antibodies as Correlates of Protection in an Influenza A/H1N1 Virus Healthy Human Challenge Model

Despite long-term investment, influenza continues to be a significant worldwide problem. The cornerstone of protection remains vaccination, and approved vaccines seek to elicit a hemagglutination inhibition (HAI) titer of ≥1:40 as the primary correlate of protection. However, recent poor vaccine performance raises questions regarding the protection afforded and whether other correlates of protection should be targeted. A healthy volunteer challenge study was performed with a wild-type 2009 A(H1N1)pdm influenza A challenge virus at the NIH Clinical Center to evaluate two groups of participants with HAI titers of ≥1:40 and <1:40. The primary objective was to determine whether participants with HAI titers of ≥1:40 were less likely to develop mild to moderate influenza disease (MMID) after intranasal inoculation. HAI titers of ≥1:40 were protective against MMID but did not reduce the incidence of symptoms alone. Although the baseline HAI titer correlated with some reduction in disease severity measures, overall, the baseline NAI titer correlated more significantly with all disease severity metrics and had a stronger independent effect on outcome. This study demonstrates the importance of examining other immunological correlates of protection rather than solely HAI titers. This challenge study confirms the importance of NAI titer as a correlate and for the first time establishes that it can be an independent predictor of reduction of all aspects of influenza disease. This suggests that NAI titer may play a more significant role than previously thought and that neuraminidase immunity should be considered when studying susceptibility after vaccination and as a critical target in future influenza vaccine platforms.

This study represents the first time the current gold standard for evaluating influenza vaccines as set by the U.S. Food and Drug Administration and the European Medicines Agency Committee for Medicinal Products for Human Use, a “protective” hemagglutination inhibition (HAI) titer of ≥1:40, has been evaluated in a well-controlled healthy volunteer challenge study since the cutoff was established. We used our established wild-type influenza A healthy volunteer human challenge model to evaluate how well this antibody titer predicts a reduction in influenza virus-induced disease. We demonstrate that although higher HAI titer is predictive of some protection, there is stronger evidence to suggest that neuraminidase inhibition (NAI) titer is more predictive of protection and reduced disease. This is the first time NAI titer has been clearly identified in a controlled trial of this type to be an independent predictor of a reduction in all aspects of influenza.

Trivalent influenza vaccine-induced antibody response to circulating influenza a (H3N2) viruses in 2010/11 and 2011/12 seasons

To evaluate antibody response induced by trivalent inactivated influenza vaccine (TIV) against circulating influenza A (H3N2) strains in healthy adults during the 2010/11 and 2011/12 seasons, a hemagglutination-inhibition (HI) assay was utilized to calculate geometric mean antibody titer (GMT), seroprotection rate (post vaccination HI titers of ≥1 :40), and seroresponse rate (4-fold increase in antibody level). In the 2010/11 season, GMT increased 1.8- to 2.0-fold following the first dose of TIV against 3 circulating strains and 2.2-fold following the second compared to before vaccination. The seroresponse rate ranged from 22% to 26% following the first dose of TIV and from 31% to 33% following the second (n = 54 ). The seroprotection rate increased from a range of 6% to 13% to a range of 26% to 33% following the first dose of TIV and to a range of 37% to 42% following the second (n = 54 ). In the 2011/12 season, GMT increased 1.4-fold against A/Osaka/110/2011 and 1.8-fold against A/Osaka/5/2012. For A/Osaka/110/2011, the seroresponse rate was 29%, and the seroprotection rate increased from 26% to 55% following vaccination (n = 31 ). For A/Osaka/5/2012, the seroresponse rate was 26%, and the seroprotection rate increased from 68% to 84% following vaccination (n = 31 ). HI assays with reference antisera demonstrated that the strains in the 2011/12 season were antigenically distinct from vaccine strain (A/Victoria/210/2009). In conclusion, the vaccination increased the seroprotection rate against circulating H3N2 strains in the 2010/11 and 2011/12 seasons. Vaccination of TIV might have potential to induce reactive antibodies against antigenically distinct circulating H3N2 viruses.

Antibody response to influenza A(H1N1)pdm09 among healthcare personnel receiving trivalent inactivated vaccine: effect of prior monovalent inactivated vaccine

Background. Few data are available on the immunogenicity of repeated annual doses of influenza A(H1N1)pdm09-containing vaccines.

Methods. We enrolled healthcare personnel (HCP) in direct patient care during the autumn of 2010 at 2 centers with voluntary immunization. We verified the receipt of A(H1N1)pdm09-containing monovalent inactivated influenza vaccine (MIIV) and 2010–2011 trivalent inactivated vaccine (TIV). We performed hemagglutination inhibition antibody (HI) assays on preseason, post-TIV, and end-of-season serum samples. We compared the proportion of HCPs with HI titer ≥40 against A(H1N1)pdm09 per receipt of prior-season MIIV, current-season TIV, both, or neither.

Results. At preseason (n = 1417), HI ≥ 40 was significantly higher among those who received MIIV (34%) vs those who did not (14%) (adjusted relative risk [ARR], 3.26; 95% confidence interval [CI], 2.72–3.81). At post-TIV (n = 865), HI ≥ 40 was lower among HCP who received MIIV and TIV (66%) than among those receiving only TIV (85%) (ARR, 0.93 [95% CI, .84–.997]). At end-of-season (n = 1254), HI ≥ 40 was 40% among those who received both MIIV and TIV and 67% among those receiving only TIV (ARR, 0.76 [95% CI, .65–.88]), 52% among those who received MIIV only, and 12% among those receiving neither.

Conclusions. HCP immunization programs should consider effects of host immune response and vaccine antigenic distance on immunogenicity of repeated annual doses of influenza vaccines.

Evaluation of influenza virus A/H3N2 and B vaccines on the basis of cross-reactivity of postvaccination human serum antibodies against influenza viruses A/H3N2 and B isolated in MDCK cells and embryonated hen eggs

The vaccine strains against influenza virus A/H3N2 for the 2010-2011 season and influenza virus B for the 2009-2010 and 2010-2011 seasons in Japan are a high-growth reassortant A/Victoria/210/2009 (X-187) strain and an egg-adapted B/Brisbane/60/2008 (Victoria lineage) strain, respectively. Hemagglutination inhibition (HI) tests with postinfection ferret antisera indicated that the antisera raised against the X-187 and egg-adapted B/Brisbane/60/2008 vaccine production strains poorly inhibited recent epidemic isolates of MDCK-grown A/H3N2 and B/Victoria lineage viruses, respectively. The low reactivity of the ferret antisera may be attributable to changes in the hemagglutinin (HA) protein of production strains during egg adaptation. To evaluate the efficacy of A/H3N2 and B vaccines, the cross-reactivities of postvaccination human serum antibodies against A/H3N2 and B/Victoria lineage epidemic isolates were assessed by a comparison of the geometric mean titers (GMTs) of HI and neutralization (NT) tests. Serum antibodies elicited by the X-187 vaccine had low cross-reactivity to both MDCK- and egg-grown A/H3N2 isolates by HI test and narrow cross-reactivity by NT test in all age groups. On the other hand, the GMTs to B viruses detected by HI test were below the marginal level, so the cross-reactivity was assessed by NT test. The serum neutralizing antibodies elicited by the B/Brisbane/60/2008 vaccine reacted well with egg-grown B viruses but exhibited remarkably low reactivity to MDCK-grown B viruses. The results of these human serological studies suggest that the influenza A/H3N2 vaccine for the 2010-2011 season and B vaccine for the 2009-2010 and 2010-2011 seasons may possess insufficient efficacy and low efficacy, respectively.

Correlates of vaccine protection from influenza and its complications

Despite use of influenza vaccines for more than 65 y, influenza and its complications are a major cause of morbidity and mortality worldwide. Most deaths during influenza virus infections are due to underlying co-morbidities or secondary bacterial pneumonia. The measures of immune response currently used for licensure of influenza vaccines are relevant mainly for protection from viral infection in healthy adults. Development of new or improved influenza vaccines will require a definition of novel, and specific correlates of protection. These correlates should associate immune responses with outcomes that are relevant to specific risk groups, such as asthma exacerbation, hospitalization or disruptions to care or daily activities. Assessment of vaccine effectiveness for both viral and bacterial vaccines should include measures of impact on secondary bacterial pneumonia.

Evaluation of the seroprevalence of influenza A(H1N1) 2009 on a university campus: a cross-sectional study

BACKGROUND

Human infection with influenza A(H1N1) 2009 was first identified in the United States on 15 April 2009 and on 11 June 2009, WHO declared that the rapidly spreading swine-origin influenza virus constituted a global pandemic. We evaluated the seroprevalence of influenza A(H1N1) 2009 virus on a large public University campus, as well as disparities in demographic, symptomatic and vaccination characteristics of participants.

METHODS

Using a cross-sectional study design, sera was collected from volunteers and then tested for the presence of antibodies to the virus using a ≥ 1:40 dilution cut-off by hemagglutination inhibition assay. In conjunction, participants were asked to complete a questionnaire allowing us to estimate risk factors for infection in this population, as well as distinguish artificially derived antibodies from naturally derived antibodies.

RESULTS

300 total participants were recruited and tested. 158 (52.6%) tested positive for influenza A(H1N1) 2009 via hemagglutination inhibition assay using a ≥ 1:40 dilution cut-off. 86 people (54.4%) tested positive for H1N1 but did not report experiencing symptoms during the pandemic meeting the May 2010 CDC definition of influenza-like illness. Furthermore, of those individuals who reported that they had received the H1N1 vaccine, 16% did not test positive.

CONCLUSIONS

Overall, 52.7% of the total study population tested positive for influenza A(H1N1) 2009. 54.4% of those who tested positive for influenza A(H1N1) 2009 using the ≥ 1:40 dilution cut-off on the hemagglutination inhibition assay in this study population did not report experiencing symptoms during the pandemic meeting the May 2010 CDC definition of influenza-like illness. 16% of those who reported receiving the H1N1 vaccine did not test positive by HAI. We also found that vaccination coverage for H1N1 vaccine was poor among Blacks and Latinos, despite the fact that vaccine was readily available at no cost.

Influenza vaccine immunology

Studying the spread of influenza in human populations and protection by influenza vaccines provides important insights into immunity against influenza. The 2009 H1N1 pandemic has taught the most recent lessons. Neutralizing and receptor-blocking antibodies against hemagglutinin are the primary means of protection from the spread of pandemic and seasonal strains. Anti-neuraminidase antibodies seem to play a secondary role. More broadly cross-reactive forms of immunity may lessen disease severity but are insufficient to prevent epidemic spread. Priming by prior exposure to related influenza strains through infection or immunization permits rapid, potent antibody responses to immunization. Priming is of greater importance to the design of immunization strategies than the immunologically fascinating phenomenon of dominant recall responses to previously encountered strains (original antigenic sin). Comparisons between non-adjuvanted inactivated vaccines and live attenuated vaccines demonstrate that both can protect, with some advantage of live attenuated vaccines in children and some advantage of inactivated vaccines in those with multiple prior exposures to influenza antigens. The addition of oil-in-water emulsion adjuvants to inactivated vaccines provides enhanced functional antibody titers, greater breadth of antibody cross-reactivity, and antigen dose sparing. The MF59 adjuvant broadens the distribution of B-cell epitopes recognized on HA and NA following immunization.

The mucosal and systemic immune responses elicited by a chitosan-adjuvanted intranasal influenza H5N1 vaccine

BACKGROUND

Development of influenza vaccines that induce mucosal immunity has been highlighted by the World Health Organisation as a priority (Vaccine 2005;23:1529). Dose-sparing strategies and an efficient mass-vaccination regime will be paramount to reduce the morbidity and mortality of a future H5N1 pandemic.

OBJECTIVES

This study has investigated the immune response and the dose-sparing potential of a chitosan-adjuvanted intranasal H5N1 (RG-14) subunit (SU) vaccine in a mouse model.

METHODS

Groups of mice were intranasally immunised once or twice with a chitosan (5 mg/ml)-adjuvanted SU vaccine [7·5, 15 or 30 μg haemagglutinin (HA)] or with a non-adjuvanted SU vaccine (30 μg HA). For comparison, another group of mice were intranasally immunised with a whole H5N1 (RG-14) virus (WV) vaccine (15 μg HA), and the control group consisted of unimmunised mice.

RESULTS

The chitosan-adjuvanted SU vaccine induced an immune response superior to that of the non-adjuvanted SU vaccine. Compared with the non-adjuvanted SU group, the chitosan-adjuvanted SU vaccine elicited higher numbers of influenza-specific antibody-secreting cells (ASCs), higher concentrations of local and systemic antibodies and correspondingly an improved haemagglutination inhibition (HI) and single radial haemolysis (SRH) response against both the homologous vaccine strain and drifted H5 strains. We measured a mixed T-helper 1/T-helper 2 cytokine response in the chitosan-adjuvanted SU groups, and these groups had an increased percentage of virus-specific CD4(+) T cells producing two Thelper 1 (Th1) cytokines simultaneously compared with the non-adjuvanted SU group. Overall, the WV vaccine induced higher antibody concentrations in sera and an HI and SRH response similar to that of the chitosan-adjuvanted SU vaccine. Furthermore, the WV vaccine formulation showed a stronger bias towards a T-helper 1 profile than the SU vaccine and elicited the highest frequencies of CD4(+) Th1 cells simultaneously secreting three different cytokines (INFγ(+) , IL2(+) and INFα(+) ). As expected, two immunisations gave a better immune response than one in all groups. The control group had very low or not detectable results in the performed immunoassays.

CONCLUSION

The cross-clade serum reactivity, improved B- and T-cell responses and dose-sparing potential of chitosan show that a chitosan-adjuvanted intranasal influenza vaccine is a promising candidate vaccine for further preclinical development.

Immunogenicity of trivalent influenza vaccine in extremely low-birth-weight, premature versus term infants

BACKGROUND

Influenza vaccine immunogenicity in premature infants is incompletely characterized.

OBJECTIVE

To assess the immunogenicity of trivalent, inactivated influenza vaccine (TIV) in extremely low-birth-weight (≤ 1000 g birth weight) premature (<30 weeks gestation) infants. We hypothesized that geometric mean titers of influenza antibody would be lower in premature than in full-term (FT) (≥ 37 week) infants.

DESIGN/METHODS

In this prospective multicenter study, former premature and FT infants who were 6 to 17 months of age received 2 doses of TIV during the 2006-2007 or 2007-2008 influenza seasons. Sera were drawn before dose 1, and 4 to 6 weeks after dose 2. Antibody was measured by hemagglutination inhibition.

RESULTS

Over 2 years, 41 premature and 42 FT infants were enrolled; 36 and 33 of these infants, respectively, had postvaccination titers available. Premature infants weighed less (mean, 1.3-1.8 kg difference) at the time of immunization than FT infants. Prevaccination titers did not differ between groups. Premature infants had higher postvaccination antibody geometric mean titers than FT infants to H1 (2006-2007, 1:513 vs. 1:91, P = 0.03; 2007-2008, 1:363 vs. 1:189, P = 0.02) and B/Victoria (2006-2007, 1:51 vs. 1:10, P = 0.02). More premature than FT infants had antibody titers ≥ 1:32 to B/Victoria (85% vs. 60%, P = 0.04) in 2007-2008. Two (5%) premature and 8 (19%) FT infants had adverse events, primarily fever, within 72 hours after vaccination. No child had medically diagnosed influenza.

CONCLUSIONS

Former premature infants had antibody responses to 2 TIV doses higher than or equal to those of FT children. Two TIV doses are immunogenic and well tolerated in extremely low-birth-weight, premature infants 6 to 17 months old.

Projection of seasonal influenza severity from sequence and serological data

Severity of seasonal influenza A epidemics is related to the antigenic novelty of the predominant viral strains circulating each year. Support for a strong correlation between epidemic severity and antigenic drift comes from infectious challenge experiments on vaccinated animals and human volunteers, field studies of vaccine efficacy, prospective studies of subjects with laboratory-confirmed prior infections, and analysis of the connection between drift and severity from surveillance data. We show that, given data on the antigenic and sequence novelty of the hemagglutinin protein of clinical isolates of H3N2 virus from a season along with the corresponding data from prior seasons, we can accurately predict the influenza severity for that season. This model therefore provides a framework for making projections of the severity of the upcoming season using assumptions based on viral isolates collected in the current season. Our results based on two independent data sets from the US and Hong Kong suggest that seasonal severity is largely determined by the novelty of the hemagglutinin protein although other factors, including mutations in other influenza genes, co-circulating pathogens and weather conditions, might also play a role. These results should be helpful for the control of seasonal influenza and have implications for improvement of influenza surveillance.

Risk factors and immunity in a nationally representative population following the 2009 influenza A(H1N1) pandemic

BACKGROUND

Understanding immunity, incidence and risk factors of the 2009 influenza A(H1N1) pandemic (2009 H1N1) through a national seroprevalence study is necessary for informing public health interventions and disease modelling.

METHODS AND FINDINGS

We collected 1687 serum samples and individual risk factor data between November-2009 to March-2010, three months after the end of the 2009 H1N1 wave in New Zealand. Participants were randomly sampled from selected general practices countrywide and hospitals in the Auckland region. Baseline immunity was measured from 521 sera collected during 2004 to April-2009. Haemagglutination inhibition (HI) antibody titres of ≥1:40 against 2009 H1N1 were considered seroprotective as well as seropositive. The overall community seroprevalence was 26.7% (CI:22.6-29.4). The seroprevalence varied across age and ethnicity. Children aged 5-19 years had the highest seroprevalence (46.7%;CI:38.3-55.0), a significant increase from the baseline (14%;CI:7.2-20.8). Older adults aged ≥60 had no significant difference in seroprevalence between the serosurvey (24.8%;CI:18.7-30.9) and baseline (22.6%;CI:15.3-30.0). Pacific peoples had the highest seroprevalence (49.5%;CI:35.1-64.0). There was no significant difference in seroprevalence between both primary (29.6%;CI:22.6-36.5) and secondary healthcare workers (25.3%;CI:20.8-29.8) and community participants. No significant regional variation was observed. Multivariate analysis indicated age as the most important risk factor followed by ethnicity. Previous seasonal influenza vaccination was associated with higher HI titres. Approximately 45.2% of seropositive individuals reported no symptoms.

CONCLUSIONS

Based on age and ethnicity standardisation to the New Zealand Population, about 29.5% of New Zealanders had antibody titers at a level consistent with immunity to 2009 H1N1. Around 18.3% of New Zealanders were infected with the virus during the first wave including about one child in every three. Older people were protected due to pre-existing immunity. Age was the most important factor associated with infection followed by ethnicity. Healthcare workers did not appear to have an increased risk of infection compared with the general population.

A new approach to estimate vaccine efficacy based on immunogenicity data applied to influenza vaccines administered by the intradermal or intramuscular routes

BACKGROUND

Despite their pivotal role in the assessment of influenza vaccines, limited attempts have been made to use haemagglutination inhibition (HI) titers for predicting vaccine efficacy against laboratory-confirmed influenza. We present here the second step of a two-step approach allowing performing such predictions and use it to compare a new trivalent inactivated influenza vaccine administered by the intradermal (ID) route (INTANZA® /IDFlu®) with the vaccine administered by the classical intramuscular (IM) route.

METHODS

The first step corresponding to the estimation of the level of protection against laboratory-confirmed influenza that can be linked to each HI titer, referred to as the HI protection curve, was achieved by using a meta-analytical approach based on published information. Vaccine efficacy and differences in vaccine efficacy are predicted in a second step using this HI protection curve alongside the results of two randomized clinical trials providing comparative information on the immunogenicity of trivalent inactivated influenza vaccines administered ID or IM in 3503 & 1645 elderly participants, respectively.

RESULTS

Pooling all available immunogenicity data, the predicted vaccine efficacy was 63.3% [CI: 58.1; 68.7] for ID route and 54.4% [CI: 49.4; 59.2] for IM route. The corresponding relative increase in efficacy that is of 16.5% [CI: 12.7; 20.1]. Predicted vaccine efficacies decreased with age for both vaccines, but the decrease was less marked by ID route: the relative increase in efficacy for subjects aged 70 years and above is of 18.0% [CI:12;24].

CONCLUSION

The analysis performed confirmed that the superior immune response provided by the vaccine using the ID route should translate into a higher vaccine efficacy against laboratory-confirmed influenza.

Serologic survey of swine workers for exposure to H2N3 swine influenza A

BACKGROUND

Of the 16 influenza A hemagglutinin (H) subtypes, only H1, H2 and H3 viruses have been shown to cause sustained human infection. Whereas H1 and H3 viruses currently circulate seasonally in humans, H2 viruses have not been identified in humans since 1968. In 2006, an H2N3 influenza virus was isolated from ill swine in the United States.

OBJECTIVE

To assess the potential for zoonotic influenza transmission, the current study looked for serologic evidence of H2 influenza infection among workers at two swine facilities, some exposed and some unexposed to H2N3-positive pigs.

METHODS

The sera were assessed for antibodies to swine H2 influenza and currently circulating seasonal human influenza A subtypes H1N1 and H3N2. Workers were interviewed to obtain details such as age, influenza vaccination history, experiences of influenza-like-illness, and use of personal protective equipment and hygiene when working with pigs. Exposure and risk factors for positive antibody titers were compared for exposed and unexposed individuals as well as for H2 antibody-positive and H2 antibody-negative individuals.

RESULTS

Blood was taken from 27 swine workers, of whom four had positive H2 antibody titers (> or = 1:40). Three of the positive employees were born before 1968 and one had an unknown birth date. Only one of these workers had been exposed to H2N3-positive pigs, and he was born in 1949.

CONCLUSIONS

These data do not support the hypothesis that swine workers were infected with the emergent swine H2N3 influenza A virus.

Efficacy of live attenuated influenza vaccine in children 6 months to 17 years of age

BACKGROUND

It has been suggested that live attenuated influenza vaccine (LAIV) may be less effective in older individuals because of prior wild-type influenza infections. LAIV is currently approved in the United States, South Korea and Hong Kong for individuals 2-49 years of age.

OBJECTIVE

To examine data from previously published pediatric studies to determine the efficacy of LAIV in various age groups.

METHODS

Four studies in which the subject age range exceeded 36 months were identified: one 2-year study comparing LAIV with placebo and three 1-year studies comparing LAIV with trivalent inactivated influenza vaccine (TIV). Efficacy against any strain regardless of antigenic similarity to vaccine was analyzed by age; age groups were based on the study design and sample size. A logistic regression model was used to assess whether age, as a continuous variable, was an effect modifier on LAIV efficacy.

RESULTS

The efficacy of LAIV did not vary with age in children aged 15-84 months compared with placebo or in children aged 6 months to 17 years compared with TIV.

CONCLUSIONS

The available data from prospective, randomized studies in children does not support the concept that prior repeated exposure to influenza, either through wild-type infection or vaccination with live, attenuated or inactivated vaccines, reduces the efficacy of LAIV compared with placebo or TIV. The decreased immunologic responses to LAIV reported in older individuals or those with pre-existing immunity do not appear to translate into reduced protection from influenza in children.

Relationship between haemagglutination-inhibiting antibody titres and clinical protection against influenza: development and application of a bayesian random-effects model

Background

Antibodies directed against haemagglutinin, measured by the haemagglutination inhibition (HI) assay are essential to protective immunity against influenza infection. An HI titre of 1:40 is generally accepted to correspond to a 50% reduction in the risk of contracting influenza in a susceptible population, but limited attempts have been made to further quantify the association between HI titre and protective efficacy.

Methods

We present a model, using a meta-analytical approach, that estimates the level of clinical protection against influenza at any HI titre level. Source data were derived from a systematic literature review that identified 15 studies, representing a total of 5899 adult subjects and 1304 influenza cases with interval-censored information on HI titre. The parameters of the relationship between HI titre and clinical protection were estimated using Bayesian inference with a consideration of random effects and censorship in the available information.

Results

A significant and positive relationship between HI titre and clinical protection against influenza was observed in all tested models. This relationship was found to be similar irrespective of the type of viral strain (A or B) and the vaccination status of the individuals.

Conclusion

Although limitations in the data used should not be overlooked, the relationship derived in this analysis provides a means to predict the efficacy of inactivated influenza vaccines when only immunogenicity data are available. This relationship can also be useful for comparing the efficacy of different influenza vaccines based on their immunological profile.

Quantifying the impact of immune escape on transmission dynamics of influenza

Influenza virus evades prevailing natural and vaccine-induced immunity by accumulating antigenic change in the haemagglutinin surface protein. Linking amino acid substitutions in haemagglutinin epitopes to epidemiology has been problematic because of the scarcity of data connecting these scales. We use experiments on equine influenza virus to address this issue, quantifying how key parameters of viral establishment and shedding increase the probability of transmission with genetic distance between previously immunizing virus and challenge virus. Qualitatively similar patterns emerge from analyses based on antigenic distance and from a published human influenza study. Combination of the equine data and epidemiological models allows us to calculate the effective reproductive number of transmission as a function of relevant genetic change in the virus, illuminating the probability of influenza epidemics as a function of immunity.

Correlation of cellular immune responses with protection against culture-confirmed influenza virus in young children

The highly sensitive gamma interferon (IFN-gamma) enzyme-linked immunosorbent spot (ELISPOT) assay permits the investigation of the role of cell-mediated immunity (CMI) in the protection of young children against influenza. Preliminary studies of young children confirmed that the IFN-gamma ELISPOT assay was a more sensitive measure of influenza memory immune responses than serum antibody and that among seronegative children aged 6 to <36 months, an intranasal dose of 10(7) fluorescent focus units (FFU) of a live attenuated influenza virus vaccine (CAIV-T) elicited substantial CMI responses. A commercial inactivated influenza virus vaccine elicited CMI responses only in children with some previous exposure to related influenza viruses as determined by detectable antibody levels prevaccination. The role of CMI in actual protection against community-acquired, culture-confirmed clinical influenza by CAIV-T was investigated in a large randomized, double-blind, placebo-controlled dose-ranging efficacy trial with 2,172 children aged 6 to <36 months in the Philippines and Thailand. The estimated protection curve indicated that the majority of infants and young children with >or=100 spot-forming cells/10(6) peripheral blood mononuclear cells were protected against clinical influenza, establishing a possible target level of CMI for future influenza vaccine development. The ELISPOT assay for IFN-gamma is a sensitive and reproducible measure of CMI and memory immune responses and contributes to establishing requirements for the future development of vaccines against influenza, especially those used for children.

Influenza vaccines have a short but illustrious history

Isolation of the causative virus of influenza in 1933, followed by the discovery of embryonated hen eggs as a substrate, quickly led to the formulation of vaccines. Virus-containing allantoic fluid was inactivated with formalin. The phenomenon of antigenic drift of the virus HA was soon recognized and, as WHO began to coordinate the world influenza surveillance, it became easier for manufacturers to select an up-to-date virus. Influenza vaccines remain unique in that the virus strain composition is reviewed yearly but modern attempts are being made to free manufacturers from this yolk by investigating internal virus proteins including M2e and NP as “universal” vaccines covering all virus sub types. Recent technical innovations have been the use of Vero and MDCK cells as the virus cell substrate, the testing of two new adjuvants and the exploration of new presentations to the nose or epidermal layers as DNA or antigen mixtures. The international investment into public health measures for a global human outbreak of avian H5N1 influenza is leading to enhanced production of conventional vaccine and to a new research searchlight on T cell epitope vaccines, viral live attenuated carriers of influenza proteins and even more innovative substrates to cultivate virus, including plant cells.

Model answers or trivial pursuits? The role of mathematical models in influenza pandemic preparedness planning

The panzootic of H5N1 influenza in birds has raised concerns that the virus will mutate to spread more readily in people, leading to a human pandemic. Mathematical models have been used to interpret past pandemics and outbreaks, and to thus model possible future pandemic scenarios and interventions. We review historical influenza outbreak and transmission data, and discuss the way in which modellers have used such sources to inform model structure and assumptions. We suggest that urban attack rates in the 1918-1919 pandemic were constrained by prior immunity, that R(0) for influenza is higher than often assumed, and that control of any future pandemic could be difficult in the absence of significant prior immunity. In future, modelling assumptions, parameter estimates and conclusions should be tested against as many relevant data sets as possible. To this end, we encourage researchers to access FluWeb, an on-line influenza database of historical pandemics and outbreaks.

Shaping the phylogenetic tree of influenza by cross-immunity

Cross-immunity among related strains can account for the selection producing the slender phylogenetic tree of influenza A and B in humans. Using a model of seasonal influenza epidemics with drift (Andreasen, 2003. Dynamics of annual influenza A epidemics with immuno-selection. J. Math. Biol. 46, 504-536), and assuming that two mutants arrive in the host population sequentially, we determine the threshold condition for the establishment of the second mutant in the presence of partial cross-protection caused by the first mutant and their common ancestors. For fixed levels of cross-protection, the chance that the second mutant establishes increases with rho the basic reproduction ratio and some temporary immunity may be necessary to explain the slenderness of flu's phylogenetic tree. In the presence of moderate levels of temporary immunity, an asymmetric situation can arise in the season after the two mutants were introduced and established: if the offspring of the new mutant arrives before the offspring of the resident type, then the mutant-line may produce a massive epidemic suppressing the original lineage. However, if the original lineage arrives first then both strains may establish and the phylogenetic tree may bifurcate.

The SIRC model and influenza A

We develop a simple ordinary differential equation model to study the epidemiological consequences of the drift mechanism for influenza A viruses. Improving over the classical SIR approach, we introduce a fourth class (C) for the cross-immune individuals in the population, i.e., those that recovered after being infected by different strains of the same viral subtype in the past years. The SIRC model predicts that the prevalence of a virus is maximum for an intermediate value of R(0), the basic reproduction number. Via a bifurcation analysis of the model, we discuss the effect of seasonality on the epidemiological regimes. For realistic parameter values, the model exhibits a rich variety of behaviors, including chaos and multi-stable periodic outbreaks. Comparison with empirical evidence shows that the simulated regimes are qualitatively and quantitatively consistent with reality, both for tropical and temperate countries. We find that the basins of attraction of coexisting cycles can be fractal sets, thus predictability can in some cases become problematic even theoretically. In accordance with previous studies, we find that increasing cross-immunity tends to complicate the dynamics of the system.

Modelling antigenic drift in weekly flu incidence

Since influenza in humans is a major public health threat, the understanding of its dynamics and evolution, and improved prediction of its epidemics are important aims. Underlying its multi-strain structure is the evolutionary process of antigenic drift whereby epitope mutations give mutant virions a selective advantage. While there is substantial understanding of the molecular mechanisms of antigenic drift, until now there has been no quantitative analysis of this process at the population level. The aim of this study is to develop a predictive model that is of a modest-enough structure to be fitted to time series data on weekly flu incidence. We observe that the rate of antigenic drift is highly non-uniform and identify several years where there have been antigenic surges where a new strain substantially increases infective pressure. The SIR-S approach adopted here can also be shown to improve forecasting in comparison to conventional methods.

Population dynamics of rapid fixation in cytotoxic T lymphocyte escape mutants of influenza A

The dynamics of cellular immunity against pathogens, and its interaction with the human MHC system, is a key area for empirical research, both within individual hosts and in population genetic surveys. However, in contrast with humoral immunity, the dynamics of cellular immunity have not been modeled at the population level. Here, we address this lacuna with a model of recently observed dramatic invasions of cytotoxic T lymphocyte escape mutants in human influenza A. In particular, we offer an explanation for the rapid fixation of a HLA-B27 restricted cytotoxic T lymphocyte escape mutant on the nucleoprotein that emerged in the 1993-1994 season. We find that the dynamics within a single season of influenza do not provide a realistic description, but a model of the full annual dynamics can offer a possible explanation. Our model is deterministic for the winter epidemic, and stochastic for the summer period. An escape mutant that leads to a slightly longer infection in a small proportion of hosts has a substantial advantage through summer persistence. Furthermore, if a small number of founding cases are responsible for initiating each epidemic, then this effect of rapid mutant fixation is amplified.

Traveling waves in a model of influenza A drift

Between major pandemics, the influenza A virus changes its antigenic properties by accumulating point mutations (drift) mainly in the RNA genes that code for the surface proteins hemagglutinin (HA) and neuraminidase (NA). The successful strain (variant) that will cause the next epidemic is selected from a reduced number of progenies that possess relatively high transmissibility and the ability to escape from the immune surveillance of the host. In this paper, we analyse a one-dimensional model of influenza A drift (Z. Angew. Math. Mech. 76 (2) (1996) 421) that generalizes the classical SIR model by including mutation as a diffusion process in a phenotype space of variants. The model exhibits traveling wave solutions with an asymptotic wave speed that matches well those obtained from numerical simulations. As exact solutions for these waves are not available, asymptotic estimates for the amplitudes of infected and recovered classes are provided through an exponential approximation based on the smallness of the diffusion constant. Through this approximation, we find simple scaling properties to several parameters of relevance to the epidemiology of the disease.

An endemic model with variable re-infection rate and applications to influenza

An epidemic model is considered, where immunity is not absolute, but individuals that have recovered from the disease can be re-infected at a rate which depends on the time that has passed since their recovery (recovery age). Such a model, e.g., can account for the genetic drift in the influenza virus. In the special case that the model has no vital dynamics, there is no obvious disease-free equilibrium and so the model lacks the usual interplay between the basic replacement ratio being >1 and the disease-free equilibrium being unstable. In fact, this relatively simple model which combines ordinary differential equations with a transport equation shares with general structured population models the feature that the appropriate state space of the solution semiflow is a space of measures, here on the compacted right real half line, with the weak* topology. The disease-free equilibrium, in terms of recovered individuals, is then represented as a Dirac measure concentrated at infinity. Still it is difficult to linearize about it. This makes the concept of persistence very important, for one can show the following: if the basic replacement ratio is >1, the disease is uniformly strongly persistent, i.e., the number of infectives is ultimately bounded away from 0 with the bound not depending on the initial data. We also derive various conditions for the local and global stability of the endemic equilibrium in terms of the re-infection rate. For instance, the endemic equilibrium is likely to be locally asymptotically stable if the re-infection rate is a highly sub-homogeneous function of recovery age. Conversely, if the re-infection rate is a step function which is zero at small recovery age, the endemic equilibrium can be unstable.

Influenza vaccines: present and future

Immunization is the most feasible method for preventing influenza. Vaccination against influenza is recommended for everyone 65 years of age and older and for persons less than 65 years of age who are at risk for developing complications of influenza. Immune correlates of protection have been established, and a global network is in place to monitor the appearance and circulation of antigenic variants of influenza viruses, as well as the appearance of novel subtypes of influenza A. Antigenic and genetic analyses of circulating viruses and testing of serum from vaccine recipients guide vaccine composition updates. The efficacy of influenza vaccines depends in part on the closeness of the antigenic match between the vaccine strain and the epidemic strain. Currently licensed influenza vaccines are trivalent, formalin-inactivated, egg-derived vaccines; their efficacy ranges from 70 to 90% in young, healthy populations when there is a close antigenic match between vaccine strains and epidemic strains. Development of intranasally administered alternative vaccines and improvement of the existing vaccine are areas of active research. A trivalent, ca live vaccine is the most promising LAIV candidate. In a field trial, efficacy rates of LAIV in young children were 96% against influenza A (H3N2) and 91% against influenza B. However, few data are available to compare this formulation of the trivalent ca live vaccine with the trivalent, inactivated vaccine. Influenza vaccine recommendations will most likely be revised on licensure of LAIV; each vaccine may offer distinct advantages in specific populations.

Altered antibody response to influenza H1N1 vaccine in healthy elderly people as determined by HI, ELISA, and neutralization assay

To determine the influence of ageing per se as well as of priming histories on the antibody response to influenza vaccination, haemagglutination inhibition (HI), ELISA IgG, IgA, IgM and neutralizing antibody titres were studied in 43 healthy young subjects (mean age 23 years) and 55 healthy elderly people (mean age 79 years). The HI and ELISA lgG responses to the A/Guizhou/54/89 strain (H3N2) for which both the young and the elderly had similar priming histories were equal. By contrast, the HI and IgG responses to A/Taiwan/1/86 (H1N1), where the priming histories were different, were lower in the elderly (P < 0.05). Influenza-specific IgA responses in the elderly tended to be higher for all vaccine strains. Influenza-specific postvaccination IgM titres were similar or tended to be higher in the elderly. A subgroup of elderly subjects (18%) who did not express HI activity to the A/Taiwan/1/86 (H1N1) vaccine strain, reacted in the HI assay with the closely related A/Singapore/6/86 (H1N1) strain. These elderly people, however, produced lgG antibodies which neutralized A/Taiwan/1/86 virus in vitro. It is concluded that the elderly are capable of mounting antibody responses similar to those observed in the young. Moreover, the observed age-related differences in antibody responses to H1N1 strains are probably not due to ageing of the immune system itself, but are determined by differences in priming histories.

Influenza vaccines: the effect of vaccine dose on antibody response in primed populations during the ongoing interpandemic period. A review of the literature

Health authorities tend to favour an increase of the antigen dose in inactivated influenza vaccines from < or = 10 micrograms haemagglutinin (HA) per vaccine strain to 15 micrograms HA/strain. The increased dose is expected to yield a meaningful increase in the number of subjects to be protected after vaccination. To verify this expectation, we have reviewed 20 published reports (1978-1991) of serological studies in which anti-HA-IgG antibody after different doses was measured. In the review, stratification groups of previously primed subjects were formed and the antibody response was estimated for doses of 10 and 15 micrograms HA by linear k*2-chi 2 model. Despite a considerable heterogenicity of study populations, study designs, vaccine types and strains, and antibody assays, the results were consistent in revealing high protection rates (> or = 75%) for a 10 micrograms HA dose of influenza A vaccine components. For both response and protection rates, an increase of the antigenic load from 10 to 15 micrograms HA was not associated with a meaningful increase of seroresponse: in 38 out of 39 stratification groups, the increase of response and/or protection rate varied between -9% and +8%, with a median of 1.5%. These results do not justify the expectation that a vaccine dose of 15 micrograms HA per strain would be clinically superior to a dose of 10 micrograms HA. Only in a group of immune-compromised patients on chronic intermittent haemodialysis were results in favour of a higher dose found, which may justify further evaluation in this special population.

Serological responses in volunteers to inactivated trivalent subunit influenza vaccine: antibody reactivity with epidemic influenza A and B strains and evidence of a rapid immune response

A study of the immunogenicity of the inactivated trivalent subunit influenza vaccine for the 1989/90 season was performed in what proved to be an influenza epidemic year. One hundred student volunteers at The London Hospital Medical College participated in the study and the findings indicated that there was an excellent serological match between the epidemic strain of influenza A (H3N2) and the vaccine strain. Before vaccination, the geometric mean titre (GMT) to A/England/308/89, a representative H3N2 epidemic strain in the United Kingdom from the 1989/90 season, was 46. Post-vaccination the antibody levels rose and 99% of vaccinees had HI titres of greater than or equal to 40, the GMT being 131. The serological responses were also investigated against other circulating influenza A (H3N2 and H1N1) and B strains. Preliminary results of an evaluation of the rapidity of the immune response showed that in three of six subjects rises in HI antibody appeared within two days of vaccination.

Antibody induction by influenza vaccines in the elderly: a review of the literature

Conflicting results have been reported concerning the association between high age and response to influenza vaccines. Some authors have found a reduced response in aged subjects, others have found no difference or even better results as compared with younger control subjects. Seventeen papers were selected from international literature published in the period 1968-1988 for a review of the anti-haemagglutinin-IgG sero-response following vaccination: among 30 cases in which vaccine components could be studied independently, ten revealed a better immune response in young subjects than in the elderly, four found more favourable results in the elderly, and 16 could not detect any significant between-group-differences, the latter most probably because of a high type-2-error. Nine of these 16 cases tended to favour young subjects. These results were relativated by the finding that each paper had at least one of three methodological limitations: (1) the failure to exclude subjects with illnesses or using drugs influencing the immune system, (2) the failure to exclude subjects with previous vaccinations against influenza, (3) the failure to exclude subjects with high prevaccination antibody titres. The direction of these biases is such that failure to address any one issue will lead to an underestimate of the response of aged subjects. In view of the failure to control these biases, it was not surprising that the papers reviewed presented a heterogeneous picture. Thus, the association between high age per se and response to influenza vaccines, if any, has not yet been established. Suggestions are made for future studies in which admission criteria should control health state and previous exposure to influenza antigens.

Characterization of subtype-specific and cross-reactive helper-T-cell clones recognizing influenza virus hemagglutinin

The specificity and function of two T-cell clones derived from A/Memphis/1/71 (H3) influenza virus (Mem 71)-immune BALB/c spleen cells have been compared. One clone, X-31 clone 1, was subtype specific, proliferating in response to influenza strains of the H3 subtype only. The other, Jap clone 3, cross-reacted in proliferation assays with heterologous subtypes of influenza A, but not type B. Both clones recognized the HA1 chain of the hemagglutinin (HA) molecule and their proliferation in response to detergent-disrupted virus could be specifically inhibited by monoclonal antibodies to the HA. The T-cell clones were of the L3T4+ phenotype. Both recognized antigen in association with I-Ed, as indicated by studies with H-2 recombinant strains of mice and by blocking with monoclonal anti-I-E antibody. In vivo, both clones elicited a delayed-type hypersensitivity (DTH) reaction when inoculated into mouse footpads together with virus, X-31 clone 1 again displaying subtype specificity and Jap clone 3 being cross-reactive. The clones were also able to provide factor-mediated help in vitro to virus-primed B cells in an anti-HA antibody response. The cross-reactive T-cell clone provided help not only for B cells primed with influenza A subtype H3 and responding to H3 virus in culture, but also for H2 virus-primed B cells making anti-H2 antibody.

An evolutionary epidemiological mechanism, with applications to type A influenza

In this paper I develop a model that describes an evolutionary epidemiological mechanism and apply this model to the epidemiology of type A influenza. This evolutionary epidemiological model differs from the classical nonevolutionary epidemiological model which has been applied to diseases like measles, rubella, and whooping cough in having a novel mechanism which causes susceptible individuals to be introduced into the host population. In the nonevolutionary model, susceptibles are continually introduced into the host population by demographic processes: most hosts that die are immune, while newborn hosts are susceptible. In this evolutionary model, the susceptible class is continually replenished because the pathogen changes genetically, and hence immunologically, from one epidemic to the next, causing previously immune hosts to become susceptible. I derive formulae which describe how the equilibrium number of infected hosts, the interepidemic period, and the probability that a host will become reinfected depend on the rate of amino acid substitution in the pathogen, m, a parameter describing the effect of these substitutions on host immunity, gamma, as well as the host population size, N, and the recovery rate, r. To apply the model to influenza, I show how the nondimensional parameter epsilon = m gamma N/r2 may be estimated from four types of data. The methods are applied to several data sets, and I conclude that epsilon much less than 1; sampling variation and inconsistencies between the various data sets do not permit epsilon to be estimated more precisely. The evolutionary epidemiological model has no threshold host population size, in contrast to the nonevolutionary model.

Interpretation of responses and protective levels of antibody against attenuated influenza A viruses using single radial haemolysis

Antibody determinations against H3N2 and H1N1 type A influenza viruses were carried out on paired sera obtained from volunteers taking part in influenza virus vaccine studies, using both the haemagglutination-inhibition (HI) and single radial haemolysis (SRH) test. Good correlation between the HI and SRH test was found for both H3N2 and H1N1 antibody and the zone area increases corresponding to significant SRH antibody rises determined for both virus strains. In both H3N2 and H1N1 vaccine studies, intranasal infection of the volunteers with live attenuated viruses was involved and by the measurement of HI and SRH antibodies prior to and following infection, levels of antibody equating with protection against the infecting viruses could be estimated. For the HI test the antibody titres associated with 50% protection were 42 for H1N1, and 44 for H3N2 viruses; for the SRH test, 50% protection was associated with zone areas of 20.0-25.0 mm2 for both H1N1 and H3N2 viruses.

A one-year study of trivalent influenza vaccines in primed and unprimed volunteers: immunogenicity, clinical reactions and protection

Three hundred volunteers were divided into two age groups, 14-30 years and 31-60 years. Each participant was immunized intramuscularly with a subunit, whole virus or absorbed whole virus vaccine, containing A/Bangkok/1/79 (H3N2), A/Brazil/11/78 (H1N1) and B/Singapore/222/79 influenza virus. Serum haemagglutination-inhibition (HI) antibody response, protection, and reactogenicity were studied after one and two doses of the vaccines. Primary immunization induced much higher percentages of HI antibody titres greater than or equal to 100 against all three vaccine viruses and much higher geometric mean titres (GMT) in volunteers with pre-immunization titres greater than or equal to 18 as compared to those with pre-immunization titres less than 18. Secondary immunization did not result in an increase of GMTs or antibody titres greater than or equal to 100 in volunteers with pre-immunization titres less than 18. On the whole, the response to the subunit vaccine was similar to that to the other two vaccines. To influenza B/Singapore/222/79 virus the response was lowest after administration of the whole virus vaccine in the age group 31-60 years. Over 50% of the HI titres greater than or equal to 100 found after immunization in the different vaccine and age groups were still present after one year. Serologically established infections during the winter months following immunization amounted to 15% in the subunit vaccine group, 6% in the whole virus vaccine group, and 10% in the adsorbed whole virus vaccine group. Local and systemic reactions to all three vaccines were mild in nature. Local reactions after primary immunization were much less frequent following administration of the subunit vaccine as compared to the other two vaccines, especially in the younger age group. In comparison to primary immunization, after booster immunization the incidence of local reactions was higher for the subunit vaccine and lower for the adsorbed whole virus vaccine. In the age group 14-30 years the incidence of local reactions after primary as well as booster immunization was much greater in females than in males, especially when the adsorbed whole virus vaccine was used.

A graded-dose study of inactivated, surface antigen influenza B vaccine in volunteers: reactogenicity, antibody response and protection to challenge virus infection

One hundred and nineteen volunteers were divided into five groups, and each volunteer inoculated subcutaneously with an aqueous subunit B/Hong Kong/73 vaccine containing 40, 20, 10, or 5 micrograms of HA or saline alone in a 0.5 ml volume. The incidence of reactions was recorded 24 h after inoculation. One month following immunization the serum HI antibody to B/Hong Kong/73 virus was measured; each volunteer was inoculated intranasally with live, attenuated influenza B (RB77) virus; and the incidence of infection by the challenge virus was determined by HI antibody response. The results showed that the incidence of reactions to all doses of vaccine were relatively low, the severity mild, and the duration short. However, the incidence of reactions was highest for those given 40 micrograms HA and least for those given 5 micrograms HA. The serum HI antibody responses to vaccine showed a dose-response relationship. For volunteers given 40 micrograms HA, 22 (96%) showed a fourfold rise in antibody titre and all volunteers had antibody titres of greater than 40 following immunization: for volunteers given 5 micrograms HA the g.m.t. increased from 16.6 to 86.1; and for those given 10 and 20 micrograms HA the response was intermediate. Following challenge, the lowest incidence of infection was seen in volunteers given the highest dose of vaccine. However, all doses of vaccine induced some protection against challenge virus infection, and the incidence of infection was directly related to the serum antibody titre at the time of challenge. The 50% protection titre of serum HI antibody was estimated as 15 to 20.

Interference following dual inoculation with influenza A (H3N2) and (H1N1) viruses in ferrets and volunteers

The effects of simultaneous inoculation with two attenuated influenza A viruses was studied in ferrets and volunteers. Groups of ferrets were inoculated with an influenza A (H3N2) or (H1N1), virus or a combination of both viruses: the temperature response, serum and local antibody response, and the change in nasal wash protein concentration was determined. The results showed that both viruses were attenuated for ferrets, and that inoculation with both viruses together did not cause clinical reactions. Serological studies on paired serum samples obtained from ferrets showed that both viruses when given separately infected all the inoculated animals; however, dual infection resulted in all ferrets being infected with the influenza A (H3N2) virus strain, but this infection interfered with infection by the influenza A (H1N1) strain. Similar investigations were carried out in volunteers. Again, the clinical reactions and temperature response of volunteers to infection by one or other of the viruses showed both strains to be attenuated for man even when given together. In addition, no adverse clinical reactions were seen in volunteers inoculated with both viruses simultaneously. Serum antibody studies showed that infection by influenza A (H1N1) virus interfered with infection by the influenza A (H2N2) virus strain. These results show evidence of interference by influenza A viruses; however, the direction of interference was one-way, and differed for ferrets and for volunteers.

Strain specificity of serum antibody to the haemagglutinin of influenza A (H3N2) viruses in children following immunization or natural infection

The specificity of serum anti-HA antibody from children immunized or infected with A/Victoria/75 (H3N2 or A/Texas/77 (H3N2) virus was examined using the single radial haemolysis test together with adsorption of antibody with three antigenic variants A/Hong Kong/68 (H3N2), A/Port Chalmers/73 (H3N2) and A/Victoria/75 (H3N2). The majority of young children reacted to vaccination or infection by producing strain-specific (SS) antibody to the homologous virus. A small proportion of children's sera contained cross-reacting (CR) antibodies capable of reacting with the haemagglutinins of all antigenic variants of the sub-type including A/HK/1/68. In contrast, most adults reacted immunologically to either vaccination or infection by producing CR antibody, reacting with all variants of the antigenic subtype including the prototype virus A/HK/1/68 (H3N2).

Relationship between surface antigens of two variants of influenza A (H3N2) virus, as revealed by hemagglutination inhibition, kinetic neutralization, and neuraminidase inhibition

Rabbit antisera were raised against plaque-purified influenza virus strains of A/Victoria/75 and A/Texas/77 isolated from Seattle influenza patients. The antigenic specificity of hemagglutinins was compared by hemagglutination inhibition (HI) and kinetic neutralization tests. Anti-A/Victoria/75 had equally high HI titers and neutralization rate constants (kappa values) for A/Victoria/75 and A/Texas/77. In contrast, anti-A/Texas/77 had a high HI titer and kappa value to A/Texas/77 and a low HI titer and kappa value to A/Victoria/75. Similar results were obtained with antisera to recombinants with hemagglutinin specific for A/Victoria/3/75 or A/Texas/1/77 and with irrelevant neuraminidase. Seven wild-type isolates, three each of A/Texas and A/Victoria, and one strain characterized as a bridging strain were tested by HI and kinetic neutralization. Characterization as A/Texas or A/Victoria was confirmed by the results. No significant difference in neuraminidase specific for A/Victoria/75 or A/Texas/77 was hown when recombinants with an irrelevant hemagglutinin were compared by the neuraminidase inhibition test. These results suggest that A/Victoria/75 strains are "senior" to A/Texas/77 strains. The epidemiological implications of this observation are discussed.

The specificity of the anti-haemagglutinin antibody response induced in man by inactivated influenza vaccines and by natural infection

The anti-haemagglutinin antibody response in adult human volunteers to inactivated whole virus or tween ether split influenza A/Victoria/75 (H3N2) and A/Scotland/74 (H3N2) virus vaccines was investigated using antibody absorption and single-radial-haemolysis (SRH) techniques. The concentrations of haemagglutinin (HA), nucleoprotein (NP) and matrix (M) antigens measured by single radial diffusion (SRD) and rocket immunoelectrophoresis were similar for both the whole virus and split vaccines. Whole virus and split vaccines induced crossreactive (CR) antibody in 87% of vaccinees. Strain specific (SS) antibody to A/Hong Kong/1/68 of the homologous virus was induced less frequently than CR antibody. Higher anti-haemagglutinin antibody titres were detected in persons receiving the split virus vaccines than in those receiving the whole virus vaccines. No antibody to the type-specific matrix protein was detectable, but 33% of volunteers developed an antibody rise to type-specific nucleoprotein antigen. The specificity of the anti-haemagglutinin antibody response in human adults to natural infection with A/Port Chalmers/73 (H3N2) virus was similar to that induced by inactivated vaccines in that a high proportion of subjects developed CR anti-haemagglutinin antibody, which reacted with A/Hong Kong/68 virus and the homologous A/Port Chalmers/73 virus, and SS antibody for A/Hong Kong/68 virus but SS antibody for A/Port Chalmers/73 virus was infrequently stimulated by natural infection.