The seasonal influenza vaccine is designed to protect against the influenza viruses research indicates are most likely to spread and cause illness among people during the upcoming flu season. Flu viruses are constantly changing, so the vaccine is updated each year based on which influenza viruses are making people sick, how those viruses are spreading, and how well the previous season’s vaccine protects against those viruses.
More than 100 national influenza centers in over 100 countries conduct year-round surveillance for influenza. This involves receiving and testing thousands of influenza virus samples from patients with suspected flu illness. The laboratories send representative viruses to five World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza, which are located in the following places:
- Atlanta, Georgia, USA (Centers for Disease Control and Prevention, CDC);
- London, United Kingdom (National Institute for Medical Research);
- Melbourne, Australia (Victoria Infectious Diseases Reference Laboratory);
- Tokyo, Japan (National Institute for Infectious Diseases); and
- Beijing, China (National Institute for Viral Disease Control and Prevention).
In February of each year, WHO consults with experts from WHO Collaborating Centers, Essential Regulatory Laboratories, and other partners to review data generated by the worldwide network of influenza laboratories. Afterward, WHO makes recommendations for the composition of the seasonal influenza vaccine for the Northern Hemisphere.