Each year, the seasonal influenza viruses change slightly which leads to a less perfect match between the vaccine viruses and the circulating viruses, rendering the vaccine used in previous years less effective. Therefore, each year, a new vaccine must be prepared that will be effective against the influenza virus strains that are expected to circulate in the next season. On the other hand, the vaccine takes about six months to manufacture, so the formula for the vaccine is developed on the basis of viruses that have circulated in the previous season. Hence, there is always a slight chance that the match will be imperfect.
To make the selection, researchers in the WHO’s Global Influenza Surveillance Network study the influenza viruses that are circulating in humans. Based on information collected by the network, the WHO recommends a vaccine that targets these circulating viruses bi-annually — for the northern and the southern hemispheres. The WHO Collaborating Centres then release suitable prototype viruses or primary seed viruses to the vaccine manufacturers. These viruses carry the H and N proteins of viruses that were originally isolated from humans suffering from influenza.