Shortly before Christmas, labs across the country started to see more positive flu tests. That uptick is often the first sign that flu season is underway.
“It definitely looks like flu season is here,” says Lynnette Brammer, an epidemiologist who is in charge of tracking the flu at the CDC.
The strain that’s causing most flu infections so far this year is an “A” type strain called H3N2. That’s reason for concern, experts say.
“The dominant H3N2 A strain is the variety that causes more severe disease, especially among older persons,” says William Schaffner, MD, an infectious disease specialist at Vanderbilt University in Nashville.
“A lot of the bad years are H3 years. The elderly and the very young may have a harder time this year,” she says.
The good news is that all forms of this year’s flu vaccine (including both the 3-strain and 4-strain shots) contain H3N2, and so should provide some protection.
The bad news is that flu vaccines don’t tend to work as well against this particular strain, so you can still get it even if you’ve had your shot.
An analysis of multiple studies, published in 2016, found that even when the vaccine is a close match to the circulating H3N2 strain, it only cuts a person’s chances of getting sick by an average of 33%. The same study found that flu vaccines are better at protecting against the “B” strains of the virus. When B strains are circulating, a flu vaccine can cut a person’s chances of getting sick by about 61%.
Imperfect as it is, the flu vaccine is still the best weapon to blunt a virus that can land you in the hospital. And even though it takes 2 weeks for the vaccine to become fully effective, you still have time to get it.
“What you hope is that even though it didn’t prevent the illness, it will prevent more severe outcomes,” like hospitalization and death, Brammer says.