Following the 2009 pandemic, pre-pandemic influenza A/H1N1 strains were almost entirely displaced by the A(H1N1)pdm09 virus pandemic strain, which continues to cause significant seasonal epidemics. Starting in the 2010/11 season, the WHO recommended including the pandemic (A/California/07/2009) strain in every year’s seasonal vaccine for both hemispheres. We undertook a systematic review of the literature to summarize the epidemiologic evidence on the effectiveness of seasonal influenza vaccines against A(H1N1)pdm09 infection in post-pandemic seasons.
We searched the electronic databases of Medline, Scopus and Google Scholar for articles on influenza A/H1N1 vaccine effectiveness (VE) published in English between January 1, 2010 and March 31st, 2014. The literature search was limited to articles that reported on studies conducted in one or more post-pandemic season (2010/11 to 2013/14 seasons). The results of 27 studies conducted in North America, Western Europe and Australia were summarized.
Most (18 of 27) studies were conducted during the 2010/11 season, which saw significant activity, especially in Europe. During that season, estimates of vaccine effectiveness against laboratory confirmed A(H1N1)pdm09 infection among outpatient populations ranged from 46% to 79% (median=60%; average=61%). Only one study examined VE against hospitalization due to laboratory-confirmed A(H1N1)pdm09 infection with similar estimates (58%, 16-79%) to those against any infection.1
Seasonal vaccines were slightly more effective among children than adults. Reported VE estimates among high-risk populations (e.g., those with chronic diseases) in 6 studies were, unexpectedly, equal to or higher than overall VE estimates ranging from 47% to 85%. No studies specifically examined VE against ICU admission, severe illness, or death due to influenza A(H1N1)pdm09 and no randomized clinical trials were found.
There were fewer studies that reported on VE against A(H1N1)pdm09 infection during the 2011/12, 2012/13 and 2013/14 seasons (2). Their estimates were generally comparable with the 2010/11 season.
This review suggests that seasonal influenza vaccines were moderately effective (about 60%) in preventing A(H1N1)pdm09 infection in post-pandemic seasons. These estimates are comparable to seasonal influenza vaccines used before the pandemic.
Evidence from studies reviewed in this report as well as from national surveillance data indicate that the circulating A(H1N1)pdm09 virus remains genetically and antigenically similar to the A/California/07/2009 virus that caused the 2009 pandemic, as well as the 2013/14 season’s reference vaccine strain. These findings support the decision of the WHO to retain the A/California/07/2009-like strain as the H1N1 component in subsequent seasons.
Evidence concerning VE among persons at high-risk of influenza complications remain very limited. Despite significant progress achieved recently by several national networks in measuring type-specific VE, we are still far away from achieving the objective of having accurate estimates of the effectiveness of influenza vaccines in preventing severe illness especially among high-risk populations. Achieving this objective will require expanding and strengthening national influenza surveillance networks.
This summary is based upon a review commissioned by the National Collaborating Centre for Infectious Diseases and prepared as a manuscript for publication in a peer-reviewed open access journal. Information on the studies reviewed and a complete list of references are included in the full manuscript. Please contact the author, Salah Mahmud, or NCCID for details.
1. Castilla JM-A, V.; Salcedo, E.; Martinez-Baz, I.; Garcia Cenoz, M.; Guevara, M.; Alvarez, N.; Irisarri, F.; Moran, J.; Barricarte, A.; Network for Influenza Surveillance in Hospitals of Navarre. Vaccine effectiveness in preventing influenza hospitalizations in Navarre, Spain, 2010-2011; Cohort and case-control study. Vaccine. 2012;30:195-200.
2. Kissling EV, M.; Bucholz, U.; Larrauri, A.; Cohen, J.M.; Nunes, B.; Rogalska, J.; Pitigol, D.; Paradowska-Stankiewicz, I.; Reuss, A.; Jimenez-Jorge, S.; Daviaud, J.; Guiomar, R.; O’Donnell, J.; Necula, G.; Gluchowska, M.; Moren, A.; . Influenza vaccine effectiveness estimates in Europe in a season with three influenza type/subtypes circulating:the I-MOVE multicentre case–control study, influenza season 2012/13. Eurosurveillance. 2014 February 13, 2014;19(6).
Production of this document has been made possible through a financial contribution from the Public Health Agency of Canada through funding for the National Collaborating Centres for Public Health (NCCPH).
The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada. Information contained in the document may be cited provided that the source is mentioned.