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Available influenza vaccines contain inactivated viruses, either whole or in parts, are administered parenterally or intranasally. Their composition varies yearly because of viral antigenic shifts and drifts. Vaccines with a composition matching yearly World Health Organization recommendations are 72% [95% confidence interval (CI) = 54-83%] efficacious (prevention of influenza cases caused by influenza viruses A and B). Their effectiveness [capacity to prevent clinical influenza, or influenza-like illness (ILI)] is lower, at 37% (95% CI = 18-52%). A decision to vaccinate an adult population has to take into account the efficacy of the vaccines and their effectiveness (the likely proportion of ILI caused by influenza A and B viruses, amenable to prevention by vaccination), as well as costs and likely compliance. As the yearly levels of circulating A and B viruses are difficult to predict during the decision time for a vaccination campaign, there is a considerable element of uncertainty regarding the likely effectiveness of 'this year's' vaccine.

Original publication




Journal article


Occupational Medicine

Publication Date





255 - 258