Headlines - The flu vaccine has lost some of its effectiveness this year

Since 2009, the French Institute for Public Health Surveillance (InVS), in collaboration with French anesthesiologists and intensivists and the National Reference Center for Influenza Viruses (CNR), has conducted surveillance each winter of patients hospitalized for influenza in hospital intensive care units (both adult and pediatric). During the first weeks of this year’s flu epidemic, which occurred from February to mid-March, the CNR reported the circulation of a significant proportion of viruses that were antigenically variant compared to the vaccine strain. The vaccine may be less effective against these viruses. Using data from this unique surveillance system, it was possible to estimate the vaccine’s effectiveness. The principle of the analysis involves comparing the proportion of vaccinated individuals among patients admitted to intensive care for influenza with the proportion of vaccinated individuals of the same age and sex in France. Vaccine efficacy is then measured by the percentage of vaccinated individuals who are actually protected by the vaccine.Vaccine efficacy was estimated at 53% (95% CI: 40–67) for the 2010–2011 season and at 30% (95% CI: 22–39) for the 2011–2012 season. There is thus a clear decline in vaccine efficacy from one year to the next. Since vaccination status is known for only 75% of patients in intensive care—with a likely high proportion of unvaccinated individuals among those with unknown status—these estimates of vaccine efficacy are likely to be conservative.These data support the World Health Organization’s recommendation to include this year’s A(H3N2) strain in next year’s vaccines.