Peer-reviewed literature

04 November 2011

Efficacy and effectiveness of influenza vaccines:
a systematic review and meta-analysis

Vaccination is a key mechanism for prevention and control of seasonal and pandemic influenza around the world. Osterholm et al. (2011) undertook a systematic review and meta-analysis to evaluate the efficacy and effectiveness of licensed influenza vaccines in the United States of America. Of 5 707 articles reviewed, 31 articles published between January 1, 1967 and February 15, 2011 were included in the analysis; all used accurate laboratory testing techniques, specifically RT-PCR (Reverse Transcription Polymerase Chain Reaction) or culture for confirmation of influenza illness. The median effectiveness of the 2009 H1N1 monovalent vaccine was 69% against medically attended illness with H1N1 pdm09 for people under 65 years if age. It was found that seasonal influenza vaccines provided moderate protection against confirmed influenza illness, with 59% efficacy of the trivalent influenza vaccines in healthy adults, and 83% efficacy of the live attenuated influenza vaccine (LAIV) in children under 8 years of age. Importantly, however, it was noted that protection can be lower -sometimes substantially lower- in some influenza seasons.

The review noted insufficient evidence regarding the protection of adults aged 65 years and older. New influenza vaccines with improved clinical efficacy and effectiveness are recommended to further reduce influenza-related morbidity and mortality. Public support for present vaccines should also be maintained.


The effectiveness of the influenza vaccine can vary from season to season, depending primarily on the similarity between the virus strains contained in the vaccine and that circulating in the community. The influenza virus evolves over time changing its surface proteins in order to evade antibodies produced in response to infection, the body’s primary defense. When the vaccine "match" or antigenic relatedness to the circulating virus is not close, the effectiveness of that season's vaccine will be lower. Much work goes into vaccine strain selection each year and the accuracy of this process is greatly enhanced by the free and open sharing of surveillance data by national governments with the WHO Global Influenza Surveillance and Response System. In addition, it has long been known that influenza vaccine is less effective in protecting persons at highest risk for developing severe influenza-related disease than it is for young healthy adults. The recent development of a live attenuated vaccine appears to have increased the effectiveness of influenza vaccination for children. Work with adjuvants, which boost the body's response to vaccination, and higher dose vaccines for the elderly hold promise for increasing the effectiveness in other high-risk individuals. The relatively low response in high-risk individuals has prompted many programs to recommend vaccination of health care workers and others with close exposure to persons susceptible to severe complications to decrease the likelihood of transmission to them. Until such a time as more effective vaccines are available, those who care for or have close contact with individuals who are at high risk of severe influenza complications, such as the elderly, persons with chronic medical conditions, and pregnant women, should strongly consider being vaccinated themselves to decrease the chance of spread to these susceptible individuals. Annual vaccination of high-risk individuals themselves is also currently the most effective means of decreasing the risk of severe disease and death in these persons.

Additional studies can assist researchers to fully understand the benefits of influenza vaccination in the elderly and people at high risk for complications; however, efficacy studies involving randomized controlled trials targeting these groups pose ethical challenges. The alternative, observational studies, may be confounded with sampling biases, such as access to care and differences in vaccination rates between high-risk and health individuals. Therefore, results of observational studies must be interpreted with caution. The World Health Assembly in 2003 recommended increased and timely immunization coverage of high-risk groups, including the elderly, in those countries where influenza vaccination policies exist (WHA 56.19).


Osterholm., M.T. et al. 2011. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. The Lancet; doi:10.1016/S0140-6736(08)61345-8).

The WHO Consultation on the Composition of Influenza Vaccines for the Southern Hemisphere 2012. Recommended composition of influenza virus vaccines for use in the 2012 southern hemisphere influenza season.

Fifty-Sixth World Health Assembly 28 May 2003. WHA 56.19. Prevention and control of influenza pandemics and annual epidemics.

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