A comparison of live and inactivated influenza A (H1N1) virus vaccines. 1. Short-term immunity

Influenza Vaccination Strategies: Comparing Inactivated and Live Attenuated Influenza Vaccines

Influenza is a major respiratory pathogen causing annual outbreaks and occasional pandemics. Influenza vaccination is the major method of prophylaxis. Currently annual influenza vaccination is recommended for groups at high risk of complications from influenza infection such as pregnant women, young children, people with underlying disease and the elderly, along with occupational groups such a healthcare workers and farm workers. There are two main types of vaccines available: the parenteral inactivated influenza vaccine and the intranasal live attenuated influenza vaccine. The inactivated vaccines are licensed from 6 months of age and have been used for more than 50 years with a good safety profile. Inactivated vaccines are standardized according to the presence of the viral major surface glycoprotein hemagglutinin and protection is mediated by the induction of vaccine strain specific antibody responses. In contrast, the live attenuated vaccines are licensed in Europe for children from 2-17 years of age and provide a multifaceted immune response with local and systemic antibody and T cell responses but with no clear correlate of protection. Here we discuss the immunological immune responses elicited by the two vaccines and discuss future work to better define correlates of protection.

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.

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.

Cold-adapted live influenza vaccine versus inactivated vaccine: systemic vaccine reactions, local and systemic antibody response, and vaccine efficacy. A meta-analysis

Since the 1940s, influenza vaccines are inactivated and purified virus or virus subunit preparations (IIV) administered by the intramuscular route. Since decades, attempts have been made to construct, as an alternative, attenuated live influenza vaccines (LIV) for intranasal administration. Presently, the most successful LIV is derived from the cold-adapted master strains A/Ann Arbor/6/60 (H2N2) and B/Ann Arbor/1/66 (AA-LIV, for Ann-Arbor-derived live influenza vaccine). It has been claimed that AA-LIV is more efficacious than IIV. In order to assess differences between the two vaccines with respect to systemic reactogenicity, antibody response, and efficacy, we performed a meta-analysis on eighteen randomised comparative clinical trials involving a total of 5000 vaccinees of all ages. Pooled odds ratios (AA-LIV versus IIV) were calculated according to the random effects model. The two vaccines were associated with similarly low frequencies of systemic vaccine reactions (pooled odds ratio: 0.96, 95% confidence interval: 0.74-1.24). AA-LIV induced significantly lower levels of serum haemagglutination inhibiting antibody and significantly greater levels of local IgA antibody (influenza virus-specific respiratory IgA assayed by ELISA in nasal wash specimens) than IIV. Yet, although they predominantly stimulate different antibody compartments, the two vaccines were similarly efficacious in preventing culture-positive influenza illness. In all trials assessing clinical efficacy, the odds ratios were not significantly different from one (point of equivalence). The pooled odds ratio for influenza A-H3N2 was 1.50 (95% CI: 0.80-2.82), and for A-H1N1, 1.03 (95% CI: 0.58-1.82). The choice between the two vaccine types should be based on weighing the advantage of the attractive non-invasive mode of administration of AA-LIV, against serious concerns about the biological risks inherent to large-scale use of infectious influenza virus, in particular the hazard of gene reassortment with non-human influenza virus strains.

Vaccination of macaques against pathogenic simian immunodeficiency virus with Venezuelan equine encephalitis virus replicon particles

Vaccine vectors derived from Venezuelan equine encephalitis virus (VEE) that expressed simian immunodeficiency virus (SIV) immunogens were tested in rhesus macaques as part of the effort to design a safe and effective vaccine for human immunodeficiency virus. Immunization with VEE replicon particles induced both humoral and cellular immune responses. Four of four vaccinated animals were protected against disease for at least 16 months following intravenous challenge with a pathogenic SIV swarm, while two of four controls required euthanasia at 10 and 11 weeks. Vaccination reduced the mean peak viral load 100-fold. The plasma viral load was reduced to below the limit of detection (1,500 genome copies/ml) in one vaccinated animal between 6 and 16 weeks postchallenge and in another from week 6 through the last sampling time (40 weeks postchallenge). The extent of reduction in challenge virus replication was directly correlated with the strength of the immune response induced by the vectors, which suggests that vaccination was effective.

Immune response of human volunteers and animals to vaccination with egg-grown influenza A (H1N1) virus is influenced by three amino acid substitutions in the haemagglutinin molecule

Inactivated subunit vaccines were prepared from high-growth reassortants derived from two separate egg isolates from a single clinical specimen of influenza A (H1N1) virus. One of these reassortants, NIB-14, was antigenically indistinguishable from isolates made in tissue culture, while the other, NIB-17, was antigenically different and typical of egg isolates. The viruses differed by three amino acid residues in the haemagglutinin (HA) molecule and the anti-HA serological response induced was studied in animal models and human volunteers. In the volunteer groups both vaccines induced very high levels of circulating haemagglutination inhibition antibodies but with different serological specificities. Both NIB-14 and NIB-17 vaccines induced high levels of cross-reactive antibodies capable of reacting with both strains, but only NIB-14 vaccine induced significant levels of strain-specific antibodies capable of reacting exclusively with the homologous strain. Antisera containing only cross-reactive antibodies proved as capable of virus neutralization as antisera containing high levels of strain-specific antibodies. We extended the argument that epidemic strains are antigenically more closely related to tissue culture isolates and established that viruses which differ by only single amino acids at critical points in the HA structure can induce a significantly different immune response when used as inactivated vaccines.

Development and persistence of local and systemic antibody responses in adults given live attenuated or inactivated influenza A virus vaccine

An enzyme-linked immunosorbent assay was used to measure nasal-wash and serum isotype-specific hemagglutinin antibody responses in 109 seronegative (hemagglutination-inhibiting titer less than or equal to 1:8) adults vaccinated intranasally with live attenuated A/Washington/897/80 (H3N2) or A/California/10/78 (H1N1) cold-adapted (ca) virus or with licensed subvirion vaccine subcutaneously. Live and inactivated virus elicited serum immunoglobulin A (IgA) responses in 83 and 96% of vaccinees, respectively, and elicited serum IgG responses in 72 and 100% of vaccinees. Inactivated virus induced higher titers of serum antibodies than did live virus and stimulated a nasal-wash IgG response more often than did live virus (94 versus 59%, P less than 0.01). In contrast, only 38% of inactivated virus vaccinees had local IgA responses compared with 83% of live virus vaccinees. Serum IgA and IgG and nasal IgG antibody titers remained elevated above prevaccination levels for at least 6 months in most of the live and inactivated vaccine responders, but the mean level of local IgA antibody induced by infection with live virus vaccine, in particular, decreased substantially. Considered in the context of previous work, the finding that live virus vaccine induced relatively long-lasting antibody in both local and serum compartments suggested that this vaccine may be a suitable alternative to inactivated vaccine for use in healthy persons.

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 comparison of live and inactivated influenza A (H1N1) virus vaccines. 2. Long-term immunity

Groups of volunteers were immunized with one of three influenza virus vaccines, and the resistance to challenge infection with attenuated influenza A (H1N1) virus was measured 8 months later. The vaccines were aqueous subunit influenza A/USSR/77 (H1N1) vaccine, aqueous subunit influenza B/Hong Kong/73 vaccine, or attenuated influenza virus A (H1N1) vaccine. The B virus vaccine was included as a control to assess the incidence of natural A virus infection during the study period. A proportion of the B virus vaccinees had pre-existing A (H1N1) virus antibody and were used to study the immunity conferred by natural infection to the live virus challenge. The serum antibody responses were measured at 1 and 8 months after immunization. The results showed that all the vaccines induced serum HI antibody in a proportion of the volunteers; however, after 1 month, higher titres of serum antibody were found in volunteers given inactivated A vaccine than in those given live attenuated A virus vaccine. Eight months post-immunization the titres of serum antibody in volunteers given inactivated vaccine had declined significantly, but there were no changes in the antibody titres of those given live virus vaccine. The incidence of infection by the challenge virus at 8 months post-immunization was directly related to the serum antibody titres 1 month post-immunization; no evidence was obtained to suggest that those given live virus vaccine had a more solid immunity than those given inactivated vaccine.