Twice a year, WHO officials and an advisory group of experts hold a consultation and information meeting on the composition of influenza virus vaccines for each hemisphere’s upcoming flu season. In February they meet to issue recommendations for the Northern Hemisphere vaccine, and in September, they meet for the Southern Hemisphere vaccine. At these sessions, health experts from around the world review data on currently circulating strains of influenza to help better anticipate the virus’ activity, and use that information to inform vaccine makers on the best components for the seasonal flu shot. WHO’s recent global surveillance has shown newly emerging H1N1 subgroups and thus, changing the vaccine components may be critical to preventing a pandemic virus. A report on the recent WHO meeting notes that, “the periodic replacement of viruses contained in influenza vaccines is necessary in order for the vaccines to be effective due to the constant evolving nature of influenza viruses, including those circulating and infecting humans.”
Recommendations based on influenza surveillance cannot be confused with prediction. Influenza is a virus prone to change, undergoing gradual antigenic drift and sometimes sudden antigenic shifts that can result in big differences in the viruses’ surface proteins, hemagglutinin, and the neuraminidase. Global surveillance of circulating influenza viruses can detect drifts which occur over years, such as the one from A/California to A/Michigan, and use that data to inform decisions on vaccine components. It is an imperfect system incapable of perfectly forecasting what the upcoming flu season will look like, due to the occurrence of sudden virus shifts. Flu vaccines work by causing the body to develop antibodies that provide protection against infection from viruses in the vaccine. When vaccine components do not properly reflect the viruses that are circulating, they can be rendered ineffective at preventing infections and large outbreaks. By using the most up-to-date influenza data, health experts say the flu shot can stave off dangerous and deadly pandemics and recommend that everyone ages 6 months and older receive the vaccine.
According to the Centers for Disease Control and Prevention (CDC), in a bad flu season the virus has caused as many as 49,000 deaths in the United States. Flu season in the United States can begin as early as October and can run until as late as May, while Southern Hemisphere countries typically see a flu season occurring between April and September. The vaccine for the Northern Hemisphere’s 2016-2017 upcoming flu season has retained the component inoculating for A/California, though the following season’s vaccines may include the WHO-recommended change to A/Michigan.
While improvements in surveillance and data gathering help health officials and vaccine makers develop better vaccines, the CDC says that fewer than half of the people in the United States ages 6 months and older received the flu shot last season. Health experts around the world say that vaccination is the most effective way to prevent influenza infection and potentially severe outcomes caused by the flu.
