Variable definitions of the influenza season and their impact on vaccine effectiveness estimates

WHO Collaborating Centre for Reference and Research on Influenza, Melbourne, Australia. Electronic address: .
Vaccine (Impact Factor: 3.62). 07/2013; 31(40). DOI: 10.1016/j.vaccine.2013.06.103
Source: PubMed


Vaccine effectiveness (VE) studies are often made for a "season" which may refer to different analysis periods in different systems. We examined whether the use of four different definitions of season would materially affect estimates of influenza VE using data from the Victorian general practice (GP) sentinel surveillance network for 2007-2012. In general, the choice of analysis period had little effect on VE estimates (≤five percentage points) when there was a statistically significant protective effect of vaccination (2007, 2010 and 2012). In contrast, for years when the analysis period varied widely depending on the method used and when VE estimates were imprecise, the change in VE estimate was as much as 43 percentage points (2008). Studies of influenza VE should clearly define the analysis period used and, where possible, provide sensitivity analyses to align this definition with other VE studies.

3 Reads
  • [Show abstract] [Hide abstract]
    ABSTRACT: Twice each year the World Health Organization makes a recommendation for the composition of the influenza vaccine, based on circulating strains of influenza A(H3N2), A(H1N1) and B. Strain selection has always been based on immunogenicity studies with limited human data. Immunogenicity can be considered as a proxy for vaccine effectiveness (VE). However, only interim VE estimates for the target hemisphere can be considered in time for the strain selection meeting.Using surveillance data from Victoria, Australia, we retrospectively estimated and compared interim and final VE estimates for 2007 to 2012. In general, interim estimates were within five percentage points of final estimates. However, estimates made too early or in years of low influenza activity may be unreliable.
    Eurosurveillance: bulletin europeen sur les maladies transmissibles = European communicable disease bulletin 10/2013; 18(41). DOI:10.2807/1560-7917.ES2013.18.41.20605 · 5.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: SUMMARY Influenza surveillance enables systematic collection of data on spatially and demographically heterogeneous epidemics. Different data collection mechanisms record different aspects of the underlying epidemic with varying bias and noise. We aimed to characterize key differences in weekly incidence data from three influenza surveillance systems in Melbourne, Australia, from 2009 to 2012: laboratory-confirmed influenza notified to the Victorian Department of Health, influenza-like illness (ILI) reported through the Victorian General Practice Sentinel Surveillance scheme, and ILI cases presenting to the Melbourne Medical Deputising Service. Using nonlinear regression, we found that after adjusting for the effects of geographical region and age group, characteristics of the epidemic curve (including season length, timing of peak incidence and constant baseline activity) varied across the systems. We conclude that unmeasured factors endogenous to each surveillance system cause differences in the disease patterns recorded. Future research, particularly data synthesis studies, could benefit from accounting for these differences.
    Epidemiology and Infection 04/2014; 143(02):1-13. DOI:10.1017/S0950268814000764 · 2.54 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Vaccine effectiveness may wane with increasing time since vaccination. This analysis used the Victorian sentinel general practitioner (GP) network to estimate vaccine effectiveness for trivalent inactivated vaccines in the 2012 season. A test-negative design was used where patients presenting to GPs with influenza-like illness who tested positive for influenza were cases and noncases were those who tested negative. Vaccination status was recorded by GPs. Vaccine effectiveness was calculated as (1-odds ratio) × 100%. Estimates were compared early versus late in the season and by time since vaccination. Virus isolates were assessed antigenically by hemagglutination inhibition assay in a selection of positive samples and viruses from healthy adults who experienced a vaccine breakthrough were analyzed genetically. The adjusted vaccine effectiveness estimate for any type of influenza was 45% (95% CI: 8,66) and for influenza A(H3) was 35% (95% CI: -11,62). A non-significant effect of waning effectiveness by time since vaccination was observed for A(H3). For those vaccinated <93 days of presentation vaccine effectiveness was 37% (95% CI: -29,69), while for those vaccinated ≥93 days before presentation it was 18% (95% CI: -83,63). Comparison of early versus late in the season estimates was very sensitive to the cut off week chosen for analysis. Antigenic data suggested that low vaccine effectiveness was not associated with poor vaccine match among the A(H3) viruses. However, genetic analysis suggested nucleotide substitutions in antigenic sites. In 2012, the trivalent influenza vaccine provided moderate protection against influenza and showed limited evidence for waning effectiveness. Antigenic and genetic data can provide additional insight into understanding these estimates. J. Med. Virol. © 2013 Wiley Periodicals, Inc.
    Journal of Medical Virology 06/2014; 86(6). DOI:10.1002/jmv.23847 · 2.35 Impact Factor
Show more