Seasonal influenza vaccination status among children with laboratory evidence of pandemic H1N1 infection

Partial protection against 2009 pandemic influenza A (H1N1) of seasonal influenza vaccination and related regional factors: Updated systematic review and meta-analyses

This updated systematic review and meta-analyses aims to systematically evaluate the cross-protection of seasonal influenza vaccines against the 2009 pandemic A (H1N1) influenza infection, and investigate the potential effect of the influenza strains circulating previous to the pandemic on the association between vaccine receipt and pandemic infection. In addition, subgroup analysis was performed based on the study locations and previous circulating influenza viruses. Relevant articles in English and Chinese from 2009 to October 2013 were systematically searched, and 21 eligible studies were included. For case-control studies, an insignificant 20% reduced risk for pandemic influenza infection based on combined national data (OR = 0.80; 95%CI: 0.60, 1.05) was calculated for people receiving seasonal influenza vaccination. However, for RCTs, an insignificant increase in the risk of seasonal influenza vaccines was observed (RR = 1.27; 95% CI: 0.46, 3.53). For the subgroup analysis, a significant 35% cross-protection was observed in the subgroup where influenza A outbreaks were detected before the 2009 pandemic. Moreover, the results indicated that seasonal influenza vaccination may reduce the risk of influenza-like illnesses (ILIs) (RR = 0.91; 95% CI: 0.84, 0.99). Our findings partially support the hypothesis that seasonal vaccines may offer moderate cross-protection for adults against laboratory-confirmed pandemic influenza A (H1N1) infection and ILIs. Further immunological studies are needed to understand the mechanism underlying these findings.

Pandemic influenza A(H1N1)pdm09: risk of infection in primary healthcare workers

BACKGROUND

Healthcare workers in primary care are at risk of infection during an influenza pandemic. The 2009 influenza pandemic provided an opportunity to assess this risk.

AIM

To measure the prevalence of seropositivity to influenza A(H1N1)pdm09 among primary healthcare workers in Canterbury, New Zealand, following the 2009 influenza pandemic, and to examine associations between seropositivity and participants' sociodemographic characteristics, professional roles, work patterns, and seasonal influenza vaccination status.

DESIGN AND SETTING

An observational study involving a questionnaire and testing for influenza A(H1N1)pdm09 seropositivity in all primary healthcare workers in Canterbury, New Zealand between December 2009 and February 2010. Method Participants completed a questionnaire that recorded sociodemographic and professional data, symptoms of influenza-like illness, history of seasonal influenza vaccination, and work patterns. Serum samples were collected and haemagglutination inhibition antibody titres to influenza A(H1N1)pdm09 measured.

RESULTS

Questionnaires and serum samples were received from 1027 participants, from a workforce of 1476 (response rate 70%). Seropositivity was detected in 224 participants (22%). Receipt of seasonal influenza vaccine (odds ratio [OR] = 2.0, 95% confidence interval [CI] = 1.2 to 3.3), recall of influenza (OR = 1.9, 95% CI = 1.3 to 2.8), and age ≤45 years (OR = 1.4, 95% CI = 1.0 to 1.9) were associated with seropositivity.

CONCLUSION

A total of 22% of primary care healthcare workers were seropositive. Younger participants, those who recalled having influenza, and those who had been vaccinated against seasonal influenza were more likely to be seropositive. Working in a dedicated influenza centre was not associated with an increased risk of seropositivity.

Annual influenza vaccination affects the development of heterosubtypic immunity

Annual vaccination of healthy children >6 months of age against seasonal influenza has been recommended by public health authorities of some countries. However, currently used seasonal vaccines provide only limited protection against (potentially) pandemic influenza viruses. Furthermore, we recently hypothesized that annual vaccination may hamper the development of cross-reactive immunity against influenza A viruses of novel subtypes, that would otherwise be induced by natural infection. Here we summarize our findings in animal models in which we demonstrated that vaccination against influenza A/H3N2 virus reduced the induction of heterosubtypic immunity against highly pathogenic avian influenza A/H5N1 virus, otherwise induced by a prior infection with influenza A/H3N2 virus. The reduction of heterosubtypic immunity correlated with reduced virus-specific CD8+ T cell responses. An additional study was performed in humans, in which we collected peripheral blood mononuclear cells from annually vaccinated children with cystic fibrosis (CF) and age-matched unvaccinated healthy control children to study the virus-specific T cell response. An age-related increase of the virus-specific CD8+ T cell response was observed in unvaccinated children that was absent in vaccinated children with CF. These findings highlight the importance of the development of vaccines that provide protection against influenza A viruses of all subtypes.

Impacts on influenza A(H1N1)pdm09 infection from cross-protection of seasonal trivalent influenza vaccines and A(H1N1)pdm09 vaccines: systematic review and meta-analyses

Cross-protection by seasonal trivalent influenza vaccines (TIVs) against pandemic influenza A H1N1 2009 (now known as A[H1N1]pdm09) infection is controversial; and the vaccine effectiveness (VE) of A(H1N1)pdm09 vaccines has important health-policy implications. Systematic reviews and meta-analyses are needed to assess the impacts of both seasonal TIVs and A(H1N1)pdm09 vaccines against A(H1N1)pdm09.We did a systematic literature search to identify observational and/or interventional studies reporting cross-protection of TIV and A(H1N1)pdm09 VE from when the pandemic started (2009) until July 2011. The studies fulfilling inclusion criteria were meta-analysed. For cross-protection and VE, respectively, we stratified by vaccine type, study design and endpoint. Seventeen studies (104,781 subjects) and 10 studies (2,906,860 subjects), respectively, reported cross-protection of seasonal TIV and VE of A(H1N1)pdm09 vaccines; six studies (17,229 subjects) reported on both. Thirteen studies (95,903 subjects) of cross-protection, eight studies (859,461 subjects) of VE, and five studies (9,643 subjects) of both were meta-analysed and revealed: (1) cross-protection for confirmed illness was 19% (95% confident interval=13-42%) based on 13 case-control studies with notable heterogeneity. A higher cross-protection of 34% (9-52%) was found in sensitivity analysis (excluding five studies with moderate/high risk of bias). Further exclusion of studies that recruited early in the pandemic (when non-recipients of TIV were more likely to have had non-pandemic influenza infection that may have been cross-protective) dramatically reduced heterogeneity. One RCT reported cross-protection of 38% (19-53%) for confirmed illness. One case-control study reported cross-protection of 50% (40-59%) against hospitalisation. (2) VE of A(H1N1)pdm09 for confirmed illness was 86% (73-93%) based on 11 case-control studies and 79% (22-94%) based on two cohort studies; VE against medically-attended ILI was 32% (8-50%) in one cohort study. TIVs provided moderate cross-protection against both laboratory-confirmed A(H1N1)pdm09 illness (based on eight case-control studies with low risk of bias and one RCT) and also hospitalisation. A finding of increased risk from seasonal vaccine was limited to cases recruited early in the pandemic. A(H1N1)pdm09 vaccines were highly effective against confirmed A(H1N1)pdm09 illness. Although cross-protection was less than the direct effect of strain-specific vaccination against A(H1N1)pdm09, TIV was generally beneficial before A(H1N1)pdm09 vaccine was available.