Research asks why last year’s flu shot only 19% effective
Last year’s flu vaccine was only 19 percent effective in preventing medical visits due to flu-related complications, according to the U.S. Centers for Disease Control and Prevention. Until now, researchers did not know why.
But Scott Hensley, of Philadelphia’s Wistar Institute, has identified a hot spot on the flu virus where mutations in the virus’ genome occurred last year. His team’s work suggests that vaccine developers should look closely at this spot when creating the annual flu vaccine.
“Viruses don’t accumulate mutations for the fun of it,” said Hensley, whose work was published in the journal Cell Reports. “They acquire mutations in order to deal with the body’s immune response.”
The body’s immune system is always in an arms race with flu viruses. Researchers develop vaccines that, when injected, produce antibodies that bind to viruses, protecting us from illness.
In response, the virus mutates to prevent the immune system from recognizing it, and the cycle continues. The flu virus can mutate very quickly, which is why people need a seasonal flu shot every year.
Hensley, an assistant professor at the Wistar medical center, said the flu strains from last season had at least five new mutations. One in particular significantly reduced the immune system’s ability to detect the virus and protect people from the flu, he said.
“The thing about the flu is that it’s so unpredictable, so selection of vaccine strains becomes an educated guess,” Hensley said.
After the flu season peaks in December and January each year, the World Health Organization surveys what strains have developed. In February, experts decide where to focus their efforts on the next winter’s vaccine. This decision is typically based on the most prevalent virus strains.
Last year, however, the problem was that strains with the mutations Hensley’s group identified were present, but in such small amounts they got little attention.
Something about those low-frequency strains caused them to predominate in the virus population. That something was their enhanced ability to “avoid human antibody responses generated against past strains” of the flu, Hensley said.
This study “was a neat way using molecular techniques for showing why our antibodies didn’t protect us for this particular strain,” said David Pegues, professor of medicine at the University of Pennsylvania. “While it can’t be used to predict the future, it can reassure us that we’re making the best decisions moving forward.”