As part of the World Health Organization Global Influenza Surveillance Response System, 144 national influenza centers around the globe conduct year-round surveillance of the flu virus. Twice a year, a committee made up of representatives from five of the top participating centers come together to review the data and determine which strains to include in the next round of flu vaccines. They base their recommendations on:
Which flu viruses are actively making people sickHow much these viruses are spreadingHow effective the previous season’s flu vaccine is likely to be against themWhether or not the vaccine can provide cross-protection for other related viruses
This year’s shot is a quadrivalent vaccine, meaning it protects against four viruses — two influenza A viruses and two influenza B viruses. (Previous vaccines have been trivalent and formulated to protect against two influenza A and one influenza B virus.) When the flu vaccine is a good match with the circulating virus, it can reduce the risk of flu by up to 60 percent for those who are vaccinated. That means 60 percent fewer vaccinated people will contract the flu after they encounter the virus compared to the unvaccinated. The flu is highly contagious, so if you’re not vaccinated, you’re more likely to develop the flu. Because the recommendations for which strains to include in this seasons’ vaccines were provided in March, and flu season isn’t in full swing, how effective they will be is an unknown. But that isn’t a reason not to get a flu shot. Even if the flu vaccine isn’t perfectly matched to the dominant strain circulating, you do get some residual protection, particularly against severe disease, emphasizes Schaffner.
