Reduced infectivity of cold-adapted influenza A H1N1 viruses in the elderly: correlation with serum and local antibodies

Methodical Design of Viral Vaccines Based on Avant-Garde Nanocarriers: A Multi-Domain Narrative Review

The current health crisis caused by coronavirus 2019 (COVID-19) and associated pathogens emphasize the urgent need for vaccine systems that can generate protective and long-lasting immune responses. Vaccination, employing peptides, nucleic acids, and other molecules, or using pathogen-based strategies, in fact, is one of the most potent approaches in the management of viral diseases. However, the vaccine candidate requires protection from degradation and precise delivery to the target cells. This can be achieved by employing different types of drug and vaccine delivery strategies, among which, nanotechnology-based systems seem to be more promising. This entry aims to provide insight into major aspects of vaccine design and formulation to address different diseases, including the recent outbreak of SARS-CoV-2. Special emphasis of this review is on the technical and practical aspects of vaccine construction and theranostic approaches to precisely target and localize the active compounds.

Intranasal powder live attenuated influenza vaccine is thermostable, immunogenic, and protective against homologous challenge in ferrets

Influenza viruses cause annual seasonal epidemics and sporadic pandemics; vaccination is the most effective countermeasure. Intranasal live attenuated influenza vaccines (LAIVs) are needle-free, mimic the natural route of infection, and elicit robust immunity. However, some LAIVs require reconstitution and cold-chain requirements restrict storage and distribution of all influenza vaccines. We generated a dry-powder, thermostable LAIV (T-LAIV) using Preservation by Vaporization technology and assessed the stability, immunogenicity, and efficacy of T-LAIV alone or combined with delta inulin adjuvant (Advax™) in ferrets. Stability assays demonstrated minimal loss of T-LAIV titer when stored at 25 °C for 1 year. Vaccination of ferrets with T-LAIV alone or with delta inulin adjuvant elicited mucosal antibody and robust serum HI responses in ferrets, and was protective against homologous challenge. These results suggest that the Preservation by Vaporization-generated dry-powder vaccines could be distributed without refrigeration and administered without reconstitution or injection. Given these significant advantages for vaccine distribution and delivery, further research is warranted.

Intranasal administration of influenza vaccines: current status

AbstractThis review article focuses on intranasal immunisation against influenza,although it also encompasses antigen uptake and processing in the nasopharyngealpassages, host defence from influenza and current influenza vaccination practices.Improvement of current vaccination strategies is clearly required; current proceduresinvolve repeated annual injections that sometimes fail to protect the recipient. It isenvisaged that nonpercutaneous immunisation would be more attractive to potentialvaccinees, thus improving uptake and coverage. As well as satisfying noninvasivecriteria, intranasal influenza immunisation has a number of perceived immunologicaladvantages over current procedures. Perhaps one of the greatest attributes of thisapproach is its potential to evoke the secretion of haemagglutinin-specific IgAantibodies in the upper respiratory tract, the main site of viral infection. Inactivated influenza vaccines have the advantage that they have a long historyof good tolerability as injected immunogens, and in this respect are possibly morelikely to be licensed than attenuated viruses. Inert influenza vaccines are poormucosal immunogens, requiring several administrations, or prior immunologicalpriming, in order to engender significant antibody responses. The use of vaccinedelivery systems or mucosal adjuvants serves to appreciably improve theimmunogenicity of mucosally applied inactivated influenza vaccines. As is the casewhen they are introduced parenterally, inactivated influenza vaccines are relativelypoor stimulators of virus-specific cytotoxic T lymphocyte activity following nasalinoculation. Live attenuated intranasal influenza vaccines are at a far moreadvanced stage of clinical readiness (phase III versus phase I). With the use of liveattenuated vaccines, it is possible to stimulate mucosal and cell-mediatedimmunological responses of a similar kind to those elicited by natural influenzainfection. In children, recombinant live attenuated cold-adapted influenza viruses arewell tolerated. Moreover, cold-adapted influenza viruses usually stimulate protectiveimmunity following only a single nasal inoculation. Safety of recombinant liveattenuated cold-adapted influenza viruses has also been demonstrated in high riskindividuals with cystic fibrosis, asthma, cardiovascular disease and diabetes mellitus.They are not suitable for immunising immunocompromised patients, however, andare poorly efficacious in individuals with pre-existing immunity to strains closelyantigenically matched with the recombinant virus. According to the reviewedliterature, it is apparent that intranasal administration of vaccine as an aerosol issuperior to administration as nose drops. The information reviewed in this papersuggests that nasally administered influenza vaccines could make a substantialimpact on the human and economic cost of influenza.

Intramuscular immunization of mice with live influenza virus is more immunogenic and offers greater protection than immunization with inactivated virus

BACKGROUND

Influenza virus continues to cause significant hospitalization rates in infants and young children. A 2-dose regime of trivalent inactivated vaccine is required to generate protective levels of hemagglutination inhibiting (HAI) antibodies. A vaccine preparation with enhanced immunogenicity is therefore desirable.

METHODS

Mice were inoculated intramuscularly (IM) with live and inactivated preparations of A/Wisconsin/67/2005 (H3N2). Serum cytokine levels, hemagglutinin (HA)-specific antibody responses and nucleoprotein (NP)-specific CD8+ T cell responses were compared between vaccinated groups, as well as to responses measured after intranasal infection. The protective efficacy of each vaccine type was compared by measuring virus titers in the lungs and weight loss of mice challenged intranasally with a heterosubtypic virus, A/PR/8/34 (H1N1).

RESULTS

Intramuscular administration of live virus resulted in greater amounts of IFN-α, IL-12 and IFN-γ, HA-specific antibodies, and virus-specific CD8+ T cells, than IM immunization with inactivated virus. These increases corresponded with the live virus vaccinated group having significantly less weight loss and less virus in the lungs on day 7 following challenge with a sublethal dose of a heterosubtypic virus.

CONCLUSIONS

Inflammatory cytokines, antibody titers to HA and CD8+ T cell responses were greater to live than inactivated virus delivered IM. These increased responses correlated with greater protection against heterosubtypic virus challenge, suggesting that intramuscular immunization with live influenza virus may be a practical means to increase vaccine immunogenicity and to broaden protection in pediatric populations.

Live attenuated vaccines for pandemic influenza

In this chapter, we will review the development of and clinical experience with the currently licensed seasonal live attenuated influenza vaccines (LAIV) and preclinical studies of H5, H7, and H9 live attenuated pandemic influenza vaccine candidates. Vectored vaccine approaches will not be reviewed in this chapter. Experience with seasonal influenza vaccination has demonstrated the safety and efficacy of LAIV in both children and adults; moreover, cross-protection among antigenically distinct viruses within the same subtype may be induced by LAIV. While clinical studies and further characterization of the immunologic response to avian influenza viruses are still needed, the experience with seasonal LAIV underscores the potential of live attenuated vaccines to play an important role in the event of a pandemic.

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.

[Immunity and antiviral vaccinations. Example: the respiratory mucosa]

OBJECTIVE

As the mucosal surfaces of the respiratory tract represent a major portal of entry for most human viruses and many bacteria, they seem to be a critical component of the mammalian immunologic repertoire. Thus, vaccines stimulating this local immunity could represent an interesting approach to prevent these infections. After detailing the different mechanisms implied in this mucosal immunity, the aim of this study is to analyze the basis of such a vaccination and the different vaccines available to mucosal respiratory tract use.

MUCOSAL IMMUNITY

The major antibody isotype in external secretions is secretory immunoglobin A (S-IgA); the role of IgM (S-IgM) and IgG (S-IgG) are actually questionned. It is, however, interesting that the major effector cells in the mucosal surfaces are not IgA B cells, but T lymphocytes that may represent up to 80% of the entire mucosal lymphoid cell population.

IMMUNOPROPHYLAXIS BY THE MUCOSAL ROUTE

Passive antibodies were shown to protect against mucosal viral infections, such as those caused by RSV, but very high quantities of passive antibodies are needed to restrict virus replication on mucosal surface.In general, factors which favor development of mucosal antibody and cell mediated immune responses include the oral or respiratory immunization and the replicating nature of the vaccine agents. However, to date only a few vaccines have become available to mucosal respiratory tract use, and cold-adapted influenza virus vaccines is the only one available using nasal route. Other parenteral licensed vaccines have not been recommended for mucosal administration. Some of them have been experimentally used with nasal administration of replicating agents (varicella and measles vaccines) or non replicating agents (influenza inactivated vaccine), but have been found to induce a very low mucosal response.

CONCLUSION

Based on the experience with existing vaccines, the development of mucosal immunity or administration of vaccines via the mucosal route is clearly not a prerequisite today for control or prevention of most viral infectious respiratory diseases or diseases with respiratory tract as a route of contamination. But the example of live attenuated intranasal influenza vaccine inducing both systemic and local immune response without immunopathology, is promising for the future of the mucosal immunization against respiratory viral infections.

Phase I evaluation of intranasal trivalent inactivated influenza vaccine with nontoxigenic Escherichia coli enterotoxin and novel biovector as mucosal adjuvants, using adult volunteers

Trivalent influenza virus A/Duck/Singapore (H5N3), A/Panama (H3N2), and B/Guandong vaccine preparations were used in a randomized, controlled, dose-ranging phase I study. The vaccines were prepared from highly purified hemagglutinin and neuraminidase from influenza viruses propagated in embryonated chicken eggs and inactivated with formaldehyde. We assigned 100 participants to six vaccine groups, as follows. Three intranasally vaccinated groups received 7.5-microg doses of hemagglutinin from each virus strain with either 3, 10, or 30 microg of heat-labile Escherichia coli enterotoxin (LTK63) and 990 microg of a supramolecular biovector; one intranasally vaccinated group was given 7.5-microg doses of hemagglutinin with 30 microg of LTK63 without the biovector; and another intranasally vaccinated group received saline solution as a placebo. The final group received an intramuscular vaccine containing 15 microg hemagglutinin from each strain with MF59 adjuvant. The immunogenicity of two intranasal doses, delivered by syringe as drops into both nostrils with an interval of 1 week between, was compared with that of two inoculations by intramuscular delivery 3 weeks apart. The intramuscular and intranasal vaccine formulations were both immunogenic but stimulated different limbs of the immune system. The largest increase in circulating antibodies occurred in response to intramuscular vaccination; the largest mucosal immunoglobulin A (IgA) response occurred in response to mucosal vaccination. Current licensing criteria for influenza vaccines in the European Union were satisfied by serum hemagglutination inhibition responses to A/Panama and B/Guandong hemagglutinins given with MF59 adjuvant by injection and to B/Guandong hemagglutinin given intranasally with the highest dose of LTK63 and the biovector. Geometric mean serum antibody titers by hemagglutination inhibition and microneutralization were significantly higher for each virus strain at 3 and 6 weeks in recipients of the intramuscular vaccine than in recipients of the intranasal vaccine. The immunogenicity of the intranasally delivered experimental vaccine varied by influenza virus strain. Mucosal IgA responses to A/Duck/Singapore (H5N3), A/Panama (H3N2), and B/Guandong were highest in participants given 30 microg LTK63 with the biovector, occurring in 7/15 (47%; P=0.0103), 8/15 (53%; P=0.0362), and 14/15 (93%; P=0.0033) participants, respectively, compared to the placebo group. The addition of the biovector to the vaccine given with 30 microg LTK63 enhanced mucosal IgA responses to A/Duck/Singapore (H5N3) (P=0.0491) and B/Guandong (P=0.0028) but not to A/Panama (H3N2). All vaccines were well tolerated.

[Antiviral vaccination and respiratory mucosal immunity: still disappointing results from a seductive idea]

Mucosal surfaces of the respiratory tract represent a major portal of entry for most human viruses and a critical component of the mammalian immunologic repertoire. The major antibody isotype in external secretions is secretory immunoglobin A (S-IgA). The major effector cells in mucosal surfaces, however, are not IgA B cells, but T lymphocytes, which may account for up to 80% of the mucosal lymphoid cell population. Mucosal immunoprophylaxis is theoretically an important approach to control infections acquired through these portals. Passive antibodies can protect against mucosal viral infections, as shown for respiratory syncytial virus, but very high quantities of passive antibodies are needed to restrict virus replication on mucosal surface. Factors likely to induce mucosal antibody and cell-mediated immune responses include oral or respiratory routes of immunization and active (effectively replicating) vaccine agents. Very few antiviral vaccines have been developed to protect the mucosal surface of the respiratory tract, and only an attenuated influenza virus vaccine uses the nasal route. Other vaccines, approved for parenteral use, have been administered experimentally by the nasal route; these include active (replicating) and inactive (nonreplicating) vaccines. By this route they induce only a moderate local mucosal response. Neither the development of mucosal immunity nor the administration of vaccines via the mucosal route is essential for control or prevention of most respiratory viral infections and diseases acquired through the respiratory tract. Nonetheless, the example of the live attenuated intranasal influenza vaccine, which induces both systemic and local immune response, is promising for the future of mucosal immunization against respiratory viral infections.

Live cold-adapted influenza A vaccine produced in Vero cell line

The African green monkey kidney (Vero) cell line was used as a substrate for the development of a live cold-adapted (ca) reassortant influenza vaccine. For that purpose, a new master strain was generated by an adaptation of the wild type (wt) A/Singapore/1/57 virus to growth at 25 degrees C in a Vero cell line. The resulting cold-adapted (ca) muster strain A/Singapore/1/57ca showed temperature sensitive (ts) phenotype and was attenuated in animal models and protective in the challenge experiments in ferrets. Two vaccine candidates of influenza A(H1N1) and A(H3N2) subtypes (6/2 reassortants) inheriting six genes coding internal proteins from the new master strain and the surface antigens hemagglutinin (HA) and neuraminidase (NA) from the epidemic viruses were obtained by a standard method of genetic reassortment. All steps of the vaccine preparation were done exclusively in Vero cells, including the isolation of the epidemic viruses. Both vaccine strains were used for immunization of young adult volunteers in a limited clinical trial and appeared to be safe, well tolerated and immunogenic after intranasal administration.

Influenza vaccines generated by reverse genetics

Influenza viruses cause annual epidemics and occasional pandemics of acute respiratory disease. Vaccination is the primary means to prevent and control the disease. However, influenza viruses undergo continual antigenic variation, which requires the annual reformulation of trivalent influenza vaccines, making influenza unique among pathogens for which vaccines have been developed. The segmented nature of the influenza virus genome allows for the traditional reassortment between two viruses in a coinfected cell. This technique has long been used to generate strains for the preparation of either inactivated or live attenuated influenza vaccines. Recent advancements in reverse genetics techniques now make it possible to generate influenza viruses entirely from cloned plasmid DNA by cotransfection of appropriate cells with 8 or 12 plasmids encoding the influenza virion sense RNA and/or mRNA. Once regulatory issues have been addressed, this technology will enable the routine and rapid generation of strains for either inactivated or live attenuated influenza vaccine. In addition, the technology offers the potential for new vaccine strategies based on the generation of genetically engineered donors attenuated through directed mutation of one or more internal genes. Reverse genetics techniques are also proving to be important for the development of pandemic influenza vaccines, because the technology provides a means to modify genes to remove virulence determinants found in highly pathogenic avian strains. The future of influenza prevention and control lies in the application of this powerful technology for the generation of safe and more effective influenza vaccines.

Intranasal Cold-Adapted Influenza Virus Vaccine Combined with Inactivated Influenza Virus Vaccines

Although influenza vaccine delivery strategies have improved coverage rates to unprecedented levels nationally among persons aged 65 years and older, influenza remains one of the greatest vaccine-preventable threats to public health among elderly in the US. A new, intranasal live attenuated influenza vaccine (LAIV) was recently approved by the US FDA for use in persons aged 5-49 years, which excludes the elderly population. Limitations of immune response to inactivated influenza vaccine (IAIV) and effectiveness of current influenza vaccination strategies among the elderly suggest that a combined approach using LAIV and/or the IAIV in various permutations might benefit this group. We explore characteristics of the LAIV, data regarding its utility in protecting against influenza in the elderly, and challenges and opportunities regarding potential combined inactivated/live attenuated vaccination strategies for the elderly. Although LAIV appears to hold promise either alone or in combination with IAIV, large well conducted randomised trials are necessary to define further the role of LAIV in preventing influenza morbidity and mortality among the elderly. We also suggest that innovative vaccine coverage strategies designed to optimise prevention and control of influenza and minimise viral transmission in the community must accompany, in parallel, the acquisition of clinical trials data to best combat morbidity and mortality from influenza.

Mutant Escherichia coli heat-labile enterotoxin [LT(R192G)] enhances protective humoral and cellular immune responses to orally administered inactivated influenza vaccine

Influenza vaccines capable of inducing both systemic and mucosal antibody responses are highly desirable. Optimal induction of mucosal IgA is accomplished by mucosal delivery of vaccine. Mucosal adjuvants may improve the immunogenicity and efficacy of vaccines delivered by this route. Here, we compare the adjuvant activities of a mutant of heat-labile enterotoxin from Escherichia coli [LT(R192G)] with those of the wildtype LT (wtLT) for oral vaccination with inactivated influenza vaccine in BALB/c mice. Compared with administration of oral influenza vaccine alone, co-administration of vaccine with LT(R192G) provided enhanced protection from infection in the upper and lower respiratory tract equivalent to and at similar doses as that obtained with wtLT. Likewise, LT(R192G) augmented virus-specific IgG and IgA responses in serum, lung and nasal washes and the numbers of virus-specific antibody-forming cells in spleen, lung and Peyer's patches in a manner comparable to wtLT. Virus-specific splenic CD4(+) cells from mice administered oral vaccine with either adjuvant produced a mixed Th1- and Th2-type cytokine response pattern. Taken together, these results indicate that LT(R192G), like wtLT, is a potent adjuvant for oral vaccination of mice with influenza vaccine.

Nasal immunization with subunit proteosome influenza vaccines induces serum HAI, mucosal IgA and protection against influenza challenge

The immunogenicity of a mucosally delivered subunit influenza vaccine was assessed in mice. Split influenza virus vaccine (sFlu) was formulated with proteosomes (Pr-sFlu), administered intranasally, and the induced immunity was compared with the responses elicited by sFlu alone given either intramuscularly or intranasally. Intranasal (i.n.) immunization with Pr-sFlu induced specific serum IgG and hemagglutination inhibition (HAI) titers comparable to or better than those induced by intramuscular (i.m.) sFlu, and in contrast to sFlu alone, i.n. Pr-sFlu also induced high levels of influenza-specific IgA in lung and nasal washes. Mice receiving i.n. Pr-sFlu were completely protected against live virus challenge, as were mice immunized by injection with sFlu alone. The i.n. Pr-sFlu elicited cytokine responses polarized towards a type 1 phenotype whereas those elicited by sFlu alone were of a mixed type 1/type 2 phenotype. The data strongly suggest that i.n. proteosome-formulated influenza antigens are highly effective and are excellent candidates for a non-invasive human vaccine.

Randomized controlled trial of seroresponses to double dose and booster influenza vaccination in frail elderly subjects

Responses to influenza vaccination are poor in frail elderly subjects who suffer the greatest morbidity and mortality due to infection. Therefore, a randomized clinical trial was performed to determine the effect of a double dose and booster vaccination on antibody responses after influenza vaccination. A total of 815 patients (median age 83 years, median disability score 8, median disease categories 2 and median number of medications 4) residing in 14 nursing homes in the Netherlands were vaccinated during the influenza season 1997-98. The first vaccine dose (15 or 30 microg) was given on Day 0 followed by a booster dose (placebo or 15 microg) on Day 84. Blood samples were taken before and 25 days after vaccination. There were four treatment groups: (i) 15 microg and placebo, (ii) 15 microg and 15 microg booster, (iii) 30 microg and placebo and (iv) 30 microg and 15 microg booster. Geometric mean antibody titers of those receiving the double vaccine dose was 15% (95% CI, 6% to 24%, P = 0.001) higher as compared to the standard 15 microg dose. A booster dose, given 84 days after the first vaccination, yielded postvaccination titters that were 14% (95% CI, 9% to 19%, P = 0.001) higher as compared to placebo. Subgroup analysis did not reveal patient groups that had a proportionally greater benefit from adapted vaccination strategies. It is concluded that higher antibody responses can be achieved in frail elderly people by a double vaccine dose or a booster vaccination.

Immunogenicity and efficacy of Russian live attenuated and US inactivated influenza vaccines used alone and in combination in nursing home residents

The immunogenicity and efficacy of Russian live attenuated and US inactivated trivalent influenza vaccines administered alone or in three different combinations were evaluated in a randomized, placebo-controlled, double-blinded study of 614 elderly or chronically ill nursing home residents in St. Petersburg, Russia during the 1996-97 influenza season. Postvaccination serum antibody responses were more frequent among individuals administered the combination vaccines than among those vaccinated with live or inactivated vaccine alone. Only individuals who received live vaccine, alone or in combination with inactivated vaccine, achieved significant postvaccination increases in virus-specific nasal IgA. Efficacy in preventing laboratory-confirmed influenza in vaccinated versus nonvaccinated individuals was 67% (95%CI, 36-81%) for recipients of a combination of the vaccines compared with 51% (95%CI, -17-79%) for recipients of live vaccine alone and 50% (95%CI, -26-80%) for recipients of inactivated vaccine alone. These results suggest that administration of a combination of influenza vaccines may provide a strategy for improved influenza vaccination of elderly people.

Respiratory viral infections in the elderly

Viral respiratory infections represent a significant challenge for those interested in improving the health of the elderly. Influenza continues to result in a large burden of excess morbidity and mortality. Two effective measures, inactivated influenza vaccine, and the antiviral drugs rimantadine and amantadine, are currently available for control of this disease. Inactivated vaccine should be given yearly to all of those over the age of 65, as well as younger individuals with high-risk medical conditions and individuals delivering care to such persons. Live, intranasally administered attenuated influenza vaccines are also in development, and may be useful in combination with inactivated vaccine in the elderly. The antiviral drugs amantadine and rimantadine are effective in the treatment and prevention of influenza A, although rimantadine is associated with fewer side-effects. Recently, the inhaled neuraminidase inhibitor zanamivir, which is active against both influenza A and B viruses, was licensed for use in uncomplicated influenza. The role of this drug in treatment and prevention of influenza in the elderly remains to be determined. Additional neuraminidase inhibitors are also being developed. In addition, to influenza, respiratory infections with respiratory syncytial virus, parainfluenza virus, rhinovirus, and coronavirus have been identified as potential problems in the elderly. With increasing attention, it is probable that the impact of these infections in this age group will be more extensively documented. Understanding of the immunology and pathogenesis of these infections in elderly adults is in its infancy, and considerable additional work will need to be performed towards development of effective control measures.

Dose-dependent antibody response to influenza H1N1 vaccine component in elderly nursing home patients

The effects of an increased antigen dose on HI, IgG, IgA, and IgM antibody responses to influenza A/Taiwan/1/86 (H1N1) were investigated in 92 elderly nursing-home residents (mean age 81 years) and 104 young subjects (mean age 20 years). At a standard 10-microg dose, HI and IgG titer rises were lower in the elderly. HI titers did not improve at higher vaccine dosages. By contrast, influenza-specific IgG and IgA antibody responses were dose dependent in elderly subjects, but not in young. In the young subjects, IgM antibody responses were dose dependent. The improved antibody responses in the elderly as observed in IgG and IgA were not reflected in the HI response. Therefore, the evaluation of antibody production by HI only may lead to an underestimate of the immune response in elderly people.

Evaluation of live, cold-adapted influenza A and B virus vaccines in elderly and high-risk subjects

We have evaluated the use of live cold-adapted influenza A and B virus vaccines in the elderly. Cold-adapted influenza A and B virus vaccines are safe and modestly immunogenic in individuals over 65 years of age. However, our studies and those of other groups have shown that immune response to cold-adapted vaccines in this age group are modest. Administration of combined cold-adapted influenza A and inactivated influenza vaccine has resulted in slightly higher frequencies of local and systemic humoral immune responses than inactivated vaccine alone in some, but not all, studies. In a double-blind field trial conducted in nursing homes over a 3 year period, combined cold-adapted influenza A (H3N2) and trivalent inactivated influenza vaccine resulted in a 60% decrease (95% CI, 18-82%) in the rate of laboratory documented influenza A compared with inactivated vaccine alone. Further studies of multivalent cold-adapted influenza vaccines used in combination with inactivated vaccine should be performed.

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.

Antibody efficacy as a keen index to evaluate influenza vaccine effectiveness

The efficacy of the influenza vaccine is often understimated, due to the dilution of the outcome by noninfluenzal illnesses. We thus explored the methodology to evaluate the effect of the inactivated influenza vaccine under the following strict conditions: an assessment of the effectiveness on clinical illness among healthy adults in a small-scale mixed epidemic during the 1991-1992 season. The vaccine antigens included were A/Yamagata/32/89 (H1N1), A/Beijing/352/89 (H3N2), and B/Bangkok/163/90. Two indices were analyzed: "vaccine efficacy", a comparison between the vaccinees and the nonvaccinees; and "antibody efficacy", a comparison between those with and those without a protective level of pre-epidemic hemagglutination-inhibition (HAI) antibody. The odds ratio (OR) and its 95% confidence interval (95% CI) was calculated by the logistic regression model. A decrease in the age-adjusted OR of vaccination was not statistically significant: 0.54 (95% CI: 0.19-1.53) corresponding to vaccine efficacy (1-OR) of 46% (-53% to 81%). Among the vaccinees, a significantly decreased OR in those with a higher titer to A/Beijing was observed: 0.14 (0.02-0.92) adjusted for the mutual effects of pre-epidemic antibodies to different vaccine antigens. The adjusted ORs thus calculated for A/Yamagata and B/Bangkok were not found to be statistically significant. The antibody efficacy (1-OR) was estimated to be 86% (8% to 98%) against illnesses related to A/Beijing-like viruses. The product of antibody efficacy (86%) and the proportion of those who achieved a protective level of antibody after vaccination (73% for A/Beijing strain) was 63%, which is theoretically equivalent to the vaccine efficacy. Thus, the antibody efficacy is considered to be an important index, while the vaccine efficacy against clinical illnesses is easily disturbed by extraneous factors in the field trials.

Genetically engineered live attenuated influenza A virus vaccine candidates

We have generated new influenza A virus live attenuated vaccine candidates by site-directed mutagenesis and reverse genetics. By mutating specific amino acids in the PB2 polymerase subunit, two temperature-sensitive (ts) attenuated viruses were obtained. Both candidates have 38 degrees C shutoff temperatures in MDCK cells, are attenuated in the respiratory tracts of mice and ferrets, and have very low reactogenicity in ferrets. Infection of mice or ferrets with either mutant conferred significant protection from challenge with the homologous wild-type virus. Three tests for genetic stability were used to assess the propensity for reversion to virulence: 14 days of replication in nude mice, growth at 37 degrees C in tissue culture, and serial passage in ferrets. One candidate, which contains mutations intended to reduce the ability of PB2 to bind to cap structures, was stable in all three assays, whereas the second candidate, which contains mutations found only in other ts strains of influenza virus, lost its ts phenotype in the last two assays. This approach has therefore enabled the creation of live attenuated influenza A virus vaccine candidates suitable for human testing.

Temperature sensitive mutants of influenza A virus generated by reverse genetics and clustered charged to alanine mutagenesis

Temperature sensitive (ts) mutants of influenza A virus have the potential to serve as live attenuated (att) virus vaccines. Previously, ts mutants were isolated by chemical mutagenesis or arose spontaneously, and most likely contained point mutations in one or more genes. While sufficiently attenuated, even the most genetically stable of these viruses was found to revert to a more virulent form in a seronegative vaccinee. Recently developed technology, however, allows the introduction of engineered mutations into the genome of influenza A and B viruses, permitting the rational design of attenuated mutants with the potential for increased genetic stability. To accomplish this goal, we have introduced ts mutations into the PB2 gene of A/Los Angeles/2/87 (H3N2) and rescued the mutated genes into infectious viruses. We have used clustered charged to alanine mutagenesis (substitution of alanine for charged amino acid residues which are present in clusters) of the PB2 gene to generate novel ts mutants. Viruses containing such ts PB2 genes were attenuated in mice and ferrets. This approach has thus yielded several vaccine candidates with ts and attenuated characteristics in animal models. Combination of these mutations with each other or with other ts mutations may lead to a high level of genetic stability.

Neuraminidase-specific antibody responses to inactivated influenza virus vaccine in young and elderly adults

Little information is available on the potential role of antibody to influenza virus neuraminidase (NA) in vaccine-induced immunity. In the present study, serologic responses to the N1Texas/91 and N2Beijing/92 NA components of trivalent inactivated influenza virus vaccine were measured by NA inhibition (NI) and enzyme-linked immunosorbent assay (ELISA), and the results for adults aged 18 to 45 (young) or > or = 65 (elderly) years were compared. The two age groups had comparable rates (32 to 50%) of NI response. In contrast, ELISA immunoglobulin G (IgG) antibody responses to N1 and N2 NAs occurred in 70 to 71 and 67 to 83%, respectively, of young subjects but in only 3 to 18 and 18 to 35%, respectively, of elderly subjects. prevaccination mean ELISA IgG and IgA NA antibody titers were generally lower for the young adults than they were for the elderly, whereas the corresponding NI titers were comparable. In young adults, plaque size-reducing NA antibody increases were positively associated with ELISA but not with NI antibody increases. There were no apparent age-related differences in the immunoglobulin isotype distribution of the anti-NA response, with IgG being the dominant class and IgG1 the dominant subclass of serum antibody. Anti-hemagglutinin antibody responses to H1Texas/91 and H3Beijing/92 were greater in magnitude and frequency than the corresponding NA-specific responses to N1Texas/91 and N2Beijing/92 when measured by hemagglutination inhibition and NI, respectively, but not when measured by ELISA. The discordance between NI and ELISA for measurement of NA-specific vaccine responses may reflect the relative insensitivity of NI in discriminating differences when initial antibody titers are low.