science of us

What to Know About Influenza B, the ‘Second Wave’ of Flu Season

Photo: TommL/Getty Images

The good news: We’re officially past the peak of flu season. From here on out, doctors say reported cases will start to decrease, and your chances of getting the flu will drop significantly — until next December, anyway.

The bad news, though, is that we’re not totally out of the woods just yet: There’s a “second wave” flu going around. The CDC’s weekly flu report for week 12 of the 2017–2018 flu season (the week ending on March 24) showed flu activity in general decreasing around the country — but of the thousands of flu-positive cases reported, almost 58 percent were influenza B, a different strain than the virus that broke records for doctor’s visits and hospitalizations earlier this year.

Here’s everything you need to know about flu season’s second round (hint: it’s not too late to get a flu shot).

So this is a different type of flu?

It sure is. The virus hitting people left and right this year was a strain of influenza A, specifically the H3N2 strain. Influenza A is more infectious to the general population, and it also tends to be more severe than some other forms of flu, resulting in more hospitalizations.

When an A strain predominates a flu season, a B strain usually isn’t far behind, according to Stephen Ferrara, a flu expert and associate dean of clinical affairs at Columbia University School of Nursing.

“This is typical in a flu season,” he says. “[Influenza] A strains drop and then we see a second wave of influenza B. We’re not sure why that happens — there are hypotheses that it could be weather related, and as temperatures warm up, influenza B becomes more prevalent — but we just don’t know.”

Is it more or less severe than influenza A?

The short (and unsatisfying) answer: We don’t totally know. For a long time, Ferrara says, researchers believed influenza B was less severe. More recent research, though, is challenging that notion. While the B strain may, in fact, be less contagious than the A strain, some studies found the B strain to be more deadly, particularly for children. A 2016 study of the past eight flu seasons determined the “mortality associated with pediatric influenza B infection was greater than that of influenza A.”

We do know this much: Influenza B definitely disproportionately affects children, whose underdeveloped immune systems make them more susceptible to any aggressive virus. It also makes them more likely to develop complications.

“We don’t want to say, ‘Oh, it’s only influenza B, you don’t need to worry,’” Ferrara says. “We don’t want to put out a false sense that it’s any less severe. Unfortunately, there will still be pediatric deaths this flu season, but there’s no need to panic. Influenza B tends to not be as contagious from person to person, so it’s slightly easier to prevent.”

Parents should be on high alert for flu symptoms, at least through the end of April (though it’s important to note, Ferrara says, that the flu can strike at any time of year). Children who develop a sudden fever of 101 degrees or higher, or who have body aches, chills, shakes, a dry cough, are acting lethargic and refusing to drink should see a doctor right away.

Will my flu shot protect me?

Again, not the most desirable answer, but — maybe. The formula for the flu vaccine changes every year based on scientists’ predictions for which strain will be the most predominant. But there are actually two different forms of that vaccine developed. There’s a trivalent vaccine, which protects against two A strains and a B strain, and a quadrivalent vaccine that protects against two A strains and two B strains.

“Depending on which vaccine you received, you probably got immunized against the A strain H3N2, and against at least one B strain as well,” Ferrara says. “But there are all types of variables.”

One such variable is the fact that researchers haven’t yet definitively determined which strain of influenza B people in the “second wave” are catching. Both the trivalent and quadrivalent vaccines given this year protect against the B strain that seems to be popping up most often, but there could still be other strains out there.

And because it takes 10 to 14 days for immunity to actually build up following a vaccination, even if you run out and get the shot today, you could get the flu in the next two weeks.

That doesn’t mean you shouldn’t still get the shot, though. It bears repeating: If you haven’t already, you absolutely should. Even if you still come down with the flu, it will have been worth it.

“For people who do get influenza following vaccination, their case of the flu tends to not be as severe,” Ferrara says. “There’s not as high a fever, and it doesn’t have as long a duration.”

What do I do to avoid this second wave?

The same thing you’ve been doing since December: wash your hands, take your vitamins, and avoid sharing close spaces with people who are sick. And if the last four months of flu reports haven’t convinced you to go get a flu shot, let this be the one that finally does.

All About Influenza B, the ‘Second Wave’ of Flu Season