Last season's flu shot protected as few as one in five people and this year's could be similarly ineffective, researchers said Monday, calling for a better way to make the vaccine.

The reason just 20 to 30 percent of people were protected by the 2016-2017 flu shot was a mutation in the H3N2 strain of the virus, which did not show up in the mass-produced vaccine that is grown using eggs, said the report in the Proceedings of the National Academy of Sciences.

Flu vaccines work by delivering purified proteins from the outer layer of dead flu viruses, which primes the immune system to fight off a new invasion.

But if a virus mutates, and the vaccine doesn't change to match it, effectiveness is lost, said lead author Scott Hensley, associate professor of microbiology at the University of Pennsylvania.

"The 2017 vaccine that people are getting now has the same H3N2 strain as the 2016 vaccine, so this could be another difficult year if this season is dominated by H3N2 viruses again."

The majority of flu vaccine proteins are purified from a virus grown in chicken eggs.

According to the study, a strain of the H3N2 virus with a different outer layer protein emerged during the 2014-2015 flu season, and this version of H3N2 remains prevalent today.

The flu vaccine delivered last season was updated to include the new version of this protein, but Hensley and colleagues found the egg-grown version of this protein acquired a new mutation.

Researchers found that antibodies elicited by the 2016-2017 vaccine in ferrets and humans "did a poor job of neutralizing H3N2 viruses that circulated last year," said the report.

However, when a version of the 2016-2017 vaccine produced without eggs was delivered to ferrets and people, antibodies "were able to effectively recognize and neutralize the new H3N2 virus."

Hensley said the findings show that a new method needs to be developed for widespread use.

"Current H3N2 viruses do not grow well in chicken eggs, and it is impossible to grow these viruses in eggs without adaptive mutations," he said.

"Our data suggest that we should invest in new technologies that allow us to ramp up production of influenza vaccines that are not reliant on eggs," Hensley said.

Nevertheless, he urged people to go ahead and get the flu vaccine this year.

"Some protection against H3N2 viruses is better than nothing and other components of the vaccine, like H1N1 and influenza B, will likely provide excellent protection this year," he said.

A seasonal flu vaccine is recommended once a year for everyone over six months of age to lower the risk of pneumonia and other life-threatening complications, according to the US Centers for Disease Control and Prevention.

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