For years, flu vaccines were designed to protect against three different flu viruses (trivalent). This included an influenza A H1N1 virus, an influenza A H3N2 virus and one B virus. Experts had to choose one B virus, even though there are two very different lineages of B viruses that both circulate during most seasons. This meant the vaccine did not protect against the group of B viruses not included in the vaccine. Adding another B virus to the vaccine aims to give broader protection against circulating flu viruses.
Flu vaccines that protect against four flu viruses are made in the same way as the flu vaccines that have been around for years that protect against three flu viruses. The difference is the addition of another vaccine virus. Studies have shown that vaccines made to protect against four flu viruses have a safety profile similar to seasonal flu vaccines made to protect against three viruses, with similar—mostly mild—side effects. Hundreds of millions of people have safely gotten flu vaccines that protect against three flu viruses. Like all seasonal flu vaccines, vaccines that protect against four flu viruses will be monitored for their safety and effectiveness.
Quadrivalent vaccine cannot cause influenza illness, because the vaccine viruses used to make it are ‘inactivated’ (killed) or attenuated (weakened). See “Can the flu vaccine give me the flu?” for information about flu vaccine side effects.