Every year vaccine makers make a new influenza vaccine. They do this every year because influenza viruses change enough so that the vaccine from the previous year might not protect against the current year’s circulating strains.
To determine which influenza strains to put into these yearly vaccines, researchers at the Centers for Disease Control and Prevention (CDC) try to predict the strains that will be circulating in North America by examining those that are circulating in South America. Usually, they’re right. This year, they weren’t. Shortly after influenza viruses entered the U.S. in 2014, the predominant strain, H3N2, started to mutate, or drift. As a consequence, about two thirds of the H3N2 strains in circulation had drifted far enough away from the H3N2 in the vaccine that it no longer protected against disease. The result was that only about 23 percent of Americans who received the influenza vaccine in 2014 were protected against disease (B. Dennis, “CDC: Flu Vaccine Only 23 Percent Effective This Season, But Still Better than Nothing,” Washington Post, January 15, 2015).
To determine the level of protection, the CDC studied 2,321 children and adults who visited doctors in five different states: Michigan, Pennsylvania, Texas, Washington and Wisconsin. Vaccine effectiveness in children 6-17 months of age was 26 percent. Protection was even worse in adults; only 12 percent among those 18-49 years of age, and 14 percent of those older than 50 years of age were protected by vaccine.
Despite these woeful numbers, the CDC continues to recommend the influenza vaccine for two reasons. First, 23 percent efficacy afforded by vaccination is better than the 0 percent efficacy afforded by not getting a vaccine. Second, the CDC estimates that even a vaccine that is only 10 percent effective will prevent about 13,000 hospitalizations in older adults over the course of an influenza season.
It is likely that next year’s influenza vaccine will include this drifted H3N2 strain.