17 August 2012
Early estimates of seasonal influenza vaccine effectiveness in Europe among target groups for vaccination: results from the I-MOVE multicentre case–control study, 2011/12
Kissling and Valenciano present the results of an influenza vaccine effectiveness (VE) multicenter case-control study based on seven sentinel surveillance networks throughout Europe(1). This paper provides early season estimates of the effectiveness of the 2011/12 trivalent vaccine against medically attended influenza like illness (ILI) that was laboratory confirmed as influenza. Particular interest is shown in the effectiveness of the vaccine against influenza A(H3N2) among subpopulations identified as target groups for vaccination (1). Kissling & Valenciano defined their study population as non-institutionalized patients of all ages, who consulted a participating practitioner for ILI and having a naso-pharyngeal swab taken less than eight days after ILI symptom onset. Cases were considered ILI patients who returned positive swabs for influenza A(H3N2). Controls were ILI patients who tested negative for any influenza virus. Individuals were considered vaccinated if they had received a dose of the 2011/12 seasonal vaccine more than 14 days before the date of ILI symptom onset (1). During the 12 weeks of data collection, 867 (92.7%) of 935 laboratory-confirmed influenza cases recruited in the study were identified as influenza A(H3N2). This finding was consistent with data from the Community Network of Reference Laboratories (CNRL) for Human Influenza in Europe: of the 11,159 viruses detected from week 40/2011 to week 7/2012, 95.9% were influenza type A, and of 6,238 influenza A viruses subtyped, 97.5% were influenza A(H3N2) (2). The adjusted VE derived from the 7 sentinel surveillance networks of Europe was 43% (95%CI -0.4 to 67.7). This result suggests low to moderate VE against influenza A(H3N2) in the early 2011/12 season in Europe, which is consistent with other studies in Spain for the 2011/12 influenza season and Australia in the 2011 season (3,4).
Households provide a strategic setting to track influenza virus infection among close contacts of influenza cases [2,3]. When implemented, household studies give insight into novel viruses; the full range of illness associated with the virus; and the transmission characteristics of the virus in a confined setting , which could influence pandemic response and real-time policy interventions. As Lau et al recommend, household studies prepared prior to pandemic outbreak may enable swift implementation during a pandemic, providing valuable information at early stages of a pandemic. However the heterogeneous nature of household study designs and implementation may impede the comparability of studies. A standardized protocol for household studies could assist in reducing this potential impediment.
Influenza vaccination is a vital public health measure to limit morbidity and mortality, particularly in at-risk populations. Accordingly, vaccine efficacy is vital to ensure appropriate protection against influenza for the wider public and at-risk populations. Studies of VE, such as Kissling & Valenciano’s are critical to elucidating seasonal VE and will impact future vaccine strategy decisions. Case control studies, although the best type of study for vaccine efficacy evaluation, do have methodological constraints (5,6). Bias and confounding in case-control studies have the potential to skew VE estimates and hence these studies should be generally viewed with caution. Kissling & Valenciano identify such methodological constraints and attempt to alleviate them by collecting extensive data on confounders and controlling for specific factors (1). Due to the detections of influenza A(H3N2) in the temperate region of the southern hemisphere, this study of vaccine efficacy has particular relevance for countries experiencing influenza A(H3N2) transmission. In the northern hemisphere 2011/12 influenza season, A(H3N2) drift was identified during the season, potentially contributing to lower VE (7). Studies such as Kissling & Valenciano’s provide much needed monitoring of vaccine efficacy, assisting seasonal vaccine strategy decisions. Regular and accurate surveillance data is the best tool available to better provide for informed decisions on strategies for seasonal vaccination. Until a universal vaccine is developed, surveillance continues to be important in monitoring of the seasonal changes of influenza viruses and the vaccine strains selection process. Influenza vaccine effectiveness against influenza viruses varies among populations and is highly dependent on matching vaccines to circulating viruses. Vaccine efficacy studies provide valuable insights into these variances in sub-populations. Due to seasonal changes of influenza viruses, the WHO advises on vaccine strains for the northern and southern hemispheres and recommends annual influenza vaccination for populations at-risk for severe infections. Even vaccinated, individuals with chronic medical conditions should seek early treatment when ill with ILI symptoms if influenza virus circulation is known in the community.
1) Kissling E, Valenciano M, I-MOVE case–control studies team. Early estimates of seasonal influenza vaccine effectiveness in Europe among target groups for vaccination: results from the I-MOVE multicentre case–control study, 2011/12. Euro Surveill. 2012;17(15):pii=20146.
2) European Centre for Disease Prevention and Control (ECDC). Weekly influenza surveillance overview. Main surveillance developments in week 7/2012 (13–19 February 2012). Stockholm: ECDC; 24 Feb 2012.
3) Jiménez-Jorge S, de Mateo S, Pozo F, Casa I, Garcia Cenoz M, Castilla J, et al. Early estimates of the effectiveness of the 2011/12 influenza vaccine in the population targeted for vaccination in Spain, 25 December 2011 to 19 February 2012. Eurosurveillance. 2012;17(12):pii=20129.
4) Fielding JE, Grant KA, Tran T, Kelly HA. Moderate influenza vaccine effectiveness in Victoria, Australia, 2011. Euro Surveill. 2012;17(11):pii=20115. h
5) Valenciano M, Kissling E, Ciancio BC, Moren A. Study designs for timely estimation of influenza vaccine effectiveness using European sentinel practitioner networks. Vaccine. 2010;28(46):7381-8.
6) Smith PG, Rodrigues LC, Fine PEM. Assessment of the protective efficacy of Vaccines against common diseases using case-control and cohort studies. International Journal of Epidemiology. 1984;13(1):87-93.
7) Weekly Epidemiological Record: Review of the 2011–2012 winter influenza season, northern hemisphere. 2012 Jun 15;87(24):233-40.