According to the latest reports, U.S. flu activity has continued to increase in intensity and expand geographically in many areas of the country. Flu season typically peaks between December and February and lasts until May.
Surveillance information indicates that influenza A (H3N2) is the strain of flu virus circulating most widely. The severity of flu disease this season also appears to be similar to previous seasons in which influenza A (H3N2) viruses have circulated predominantly.
Question arose last month regarding the effectiveness of the this year’s flu vaccine. Traditional flu vaccines are made to protect against three different flu viruses (called “trivalent” vaccines). This year’s trivalent flu vaccine protects against two influenza A viruses (H1N1 and H3N2) and an influenza B virus.
Influenza viruses are constantly changing so it’s common for new strains of influenza viruses to appear each year. The effectiveness of the vaccine depends in part on the match between the viruses in the vaccine and influenza viruses that are circulating in the community. If these are closely matched, vaccine effectiveness is higher. If they are not closely matched, vaccine effectiveness can be reduced. However, even when the viruses are not closely matched, the vaccine can still protect many people and prevent flu-related complications. Such protection is possible because antibodies made in response to the vaccine can provide some protection (called cross-protection) against different, but related strains of influenza viruses.
Vaccination remains the best method for preventing influenza and its potentially severe complications in children and adults even in years where there is a suboptimal match between vaccine and circulating strains of influenza viruses. The CDC continues to recommend vaccination as long as flu viruses are circulating and encourages prompt treatment with flu antiviral drugs for people at high risk of serious flu complications.