By Julia Ries

Two flu strains are overlapping each other this flu season.

This means you can get sick twice from different flu strains.

While the flu vaccine isn't a perfect match, it's the best defense against the flu.

To say this flu season has been abnormal is an understatement.

For one, the flu season got its earliest start in 16 years.

Up to 18 million people have gotten the flu this year, according to the Centers for Disease Control and Prevention's (CDC) latest estimates. Up to 210,000 people have been hospitalized and thousands have died, including 39 children.

We're also seeing B strains of the flu dominate, something that hasn't happened in the United States in nearly 30 years.

And, unfortunately, the vaccine missed the mark with B/Victoria, the most common strain we're seeing this year. The CDC believes the shot only covers about 58 percent of B-linked cases.

Now, halfway through flu season, A strains are picking up, increasing the odds we'll have a "double-barreled flu season," in which two strains strike back to back — a pattern health experts say is extremely rare.

Between the early start, rise in B strains, and recent spike in A-strain illnesses, this flu season officially has infectious disease experts stumped.

"This season has turned a lot of [what we know about flu] on its head," said Dr. William Schaffner, an infectious disease specialist with Vanderbilt University Medical Center and the medical director at the National Foundation for Infectious Diseases. "There's a lot we know, and even more we don't know about flu."

Double-Barreled Flu Season A double-barreled flu season occurs when two flu outbreaks overlap one another, a pattern which is very unusual, according to flu experts. Last year, for example, we saw A/H1N1 infections peak early, followed by another wave of A/H3N2 infections. Though the predominant strains are different this year, we're seeing the same pattern play out: Activity took off with B/Victoria and now that second wave of A/H1N1 is coming for us, according to Schaffner. "Around the country, my colleagues and I are seeing H1N1 come up strong, and it's now about 50-50 [with B/Victoria]," Schaffner told Healthline. The most worrisome part of a double-barreled flu season is that you can get sick twice. Just because you caught a B-strain flu doesn't mean that you're immune from the A strains. "There will be the rare person who gets two flu infections in the same season — one with B and one with H1N1," Schaffner said. Though there will be some protection within each strain — in that contracting an A strain will protect you against other A strains, and B strains will protect against other B's — there's not much cross protection. A double-barreled season also means we're more likely to see a prolonged influenza season.

What to Know About B and A Strains The fact that B strains are predominating this year isn't just confusing, it's concerning as well. B strains haven't hit this hard for nearly 30 years, since during the 1992–1993 season, the CDC told Healthline. Additionally, we didn't see much of the B strain in the past couple of years, according to Dr. Norman Moore, the director of infectious diseases for Abbott. This means that many people — especially kids — have never been exposed to the strain, and consequently, don't have residual immunity against it. "When there's a rarity, it actually sets you up for another bigger push to get it, because at that point, we really don't have anybody with any strong immunity going around, so we're all potential vessels for getting exposed and transmitting it," Moore said. This is one of the reasons kids are being hit harder this year. They've never been exposed to this type of the flu — it's their first go around. "These kids are just brand new to getting flu B," Moore said. And because we haven't seen much of the B/Victoria strain in the past few years, this year's vaccine missed the mark. "We thought initially the match was perfect, but it's not. It's off a little bit, and that means in many populations the vaccine is not going to function optimally," Schaffner explained. Fortunately, the vaccine covers H1N1 well. According to Schaffner, the match to H1N1 is right on. And because A strains circulate every year, most people have built up at least some "immune memory" to it — despite the fact these strains change and mutate each year. "Our past experience with influenza viruses does give us some residual protection that lasts," Schaffner said.

There’s Still Time to Get Vaccinated "It's not too late," Moore said about the vaccine, noting that we still don't know for sure what's going to happen next. If flu A continues to get worse, as predicted, the flu shot will protect you through the rest of the season. And even though the vaccine isn't a perfect match to B strains, it can still help lessen the severity of the flu. "If you've been vaccinated, and even if there is a mismatch, you are likely to have a less severe infection when you get it," Schaffner said. Remember: By getting immunized, you're not only protecting yourself, but others as well who may be more at risk for developing severe complications — like the elderly, pregnant women, children under 2, and immunosuppressed people. "When we protect ourselves, we are really protecting those around us," Moore said.

The Bottom Line Health experts say this has been an extremely unusual flu season. It started very early with a strain that we typically don't see much of. Now, another strain is building momentum and creating a path for what's known as a double-barreled flu season, in which two types of flu strike back to back. With a second wave coming, flu experts say it's not too late to get vaccinated before things pick up again.

Reposted with permission from Healthline. For detailed source information, please view the original article on Healthline.

