Last year’s flu season will go down as one of the worst in history. According to the Centers for Disease Control, the 2017-2018 flu season sickened millions and killed 80,000 Americans, including 180 children.
The impact was so harsh, in part, because the strain that dominated last season mutated in the six months between the development of the flu vaccine and when doctors started giving it to patients. The CDC estimated last season’s flu vaccine was about 40 percent effective.
But researchers are cautiously optimistic about this season. They have a good hunch about which strain will dominate and there is no evidence it has mutated drastically, which means the vaccine could be very effective.
“Get your flu shot,” Scott Hensley, an associate professor of microbiology at the University of Pennsylvania, told The Daily Beast.
Figuring out which flu strain will spread and creating a vaccine to combat it is something of an elaborate guessing game that begins months before the start of the season. In fact, the FDA committee that advises on the vaccine met Wednesday to start work on next year’s shot.
The flu shot is generally composed of two subtypes of influenza A—H1N1 and H3N2—and influenza B. (There are two other strains of influenza that are non-epidemic: C, which can cause mild respiratory illness, and D, which only affects cattle.)
The molecular makeup of each subtype can vary and the closer the vaccine matches it, the better it works. But making that match is tricky, and not entirely in the scientists’ control.
“We might expect one strain to come and another one comes instead," said Kathryn Edwards, a professor of pediatrics at Vanderbilt University and chair of the FDA committee.
"We might not be prepared with a perfect match for the vaccine. Perfect would be a shot that's exactly like the flu that's circulating and that's very effective. But we can't always predict that."
The last two years, H3N2 was the dominant virus. A generally vicious subtype, it still shocked epidemiologists with its ferocity—causing more damage than even 2009-2010’s swine flu pandemic, which infected about 59 million Americans and killed 12,000.
The silver lining is that if you got the flu in the last couple of years, you could have pretty good immunity if H3N2 surges again this season.
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But researchers say they don’t think that will happen anyway.
“I don’t want to predict that it’s a mild season, but it’s a good chance that it’s an H1N1 year,” Hensley said.
As far as the scientists can tell, H1N1 has not mutated as drastically as H3N2 did, but only time will tell how well the vaccine matches the strain.
“Perfect would be a shot that's exactly like the flu that's circulating and that's very effective,” said Edwards, who has studied influenza for more than four decades. “But we can't always predict that."
One reason why it’s hard to make a perfect vaccine is that flu shot development takes place six months in advance. The composition of the vaccine being distributed now was decided on in February.
Researchers often look to South America to see what types of influenza are circulating there, Edwards said; Hensley also pointed to Australia. Both continents are in the southern hemisphere and have their flu seasons at opposite times from North America, offering a sort of flu preview.
Armed with this information, the FDA committee votes on which molecular versions of each strain should be in the final U.S. flu shot. It’s up to the FDA to make the final decision. "They take a recommendation but they can disagree," Edwards said.
Even though last year’s flu shot was a dud, researchers urge Americans—even healthy ones without compromised immune systems—to get the vaccine.
"We did a study on hospitalized pneumonia patients and whether they had gotten flu shots, and we found that it [the flu shot] prevented severe disease more effectively," Edwards said. "Around 200 pediatric influenza patients died [in the 2017-2018 flu year] and around half of them did not have chronic or underlying problems."
The lesson? "Healthy people can get very sick from the flu," she said. "You don't have to be high risk to be really sick or die. We might not have perfect vaccines but the best way to prevent severe influenza is the vaccine."