PHILADELPHIA — Despite the sweltering heat of summer, some researchers are still preoccupied by cold and flu season, especially Scott Hensley of Philadelphia’s Wistar Institute, who hopes that vaccines being developed for the 2015-16 flu season will provide more protection than last year’s versions.

Last year’s 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.

Hensley’s group 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.”

Seasonal flu is a serious problem every year and contributes to more than 30,000 annual deaths in the United States alone.

One reason flu is so dangerous is that the body’s immune system and the viruses that try to override it engage in an evolutionary arms race. 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 especially can mutate very quickly, which is why people need a seasonal flu shot every year.