The flu vaccine is working well this year. It's not too late to get it.

Current flu trends for 2018-19. Brown shows H1N1 strains,
red shows H3N2, and yellow indicates the strain was not
genotyped.

The flu is widespread and increasing right now, according to the CDC.  At least 42 states were reporting high levels of flu activity as of the end of December 2018, and the rates are still climbing. In other words, we're in the midst of flu season.

Other than that, though, the news is relatively good. Here's why.

First, the dominant strain of flu this year is H1N1, which is the "swine flu" that first appeared as a pandemic in 2009. But pandemics don't have to come with high mortality rates, and as it turned out–luckily for humankind–the 2009 flu was milder than the previous dominant strain, H3N2, which first appeared way back in 1968.

This season, nearly 90% of the flu cases tested by the CDC are turning out to be H1N1, the milder variety. Although 10% of people are still getting the much-nastier H3N2 flu, it's good news compared to last year, when H3N2 dominated.

Back to the bad news (although this is old news): the 2009 swine flu (H1N1) didn't completely displace the older flu strain. Instead, we now have both types of influenza circulating, along with two strains of the even milder influenza B virus. Since 2009, the flu vaccine has to combat all 4 of these flu viruses, which is why you might see the term "quadrivalent" associated with the vaccine. That just means it targets all 4 different strains.

Back to the good news again: the vaccine this year contains just the right strains! This doesn't always happen; actually it happens much less frequently than anyone would like. But now that the flu season is under way, the CDC can test the circulating flu viruses and compare them to the strains that are targeted by this year's vaccine. This year, both the H1N1 and the H3N2 viruses match the vaccine strains really well, which means that if you got the shot, you are likely to be very well protected.

(Keep in mind that even in a good year, the vaccine isn't 100% effective, and you can still get the flu. But you are much less likely to get it than anyone who is unvaccinated.)

While I've got your attention, let me answer one of the top 10 health questions of the year: "how long is the flu contagious?" According to the CDC,

• the flu is most contagious in the first 3-4 days after becoming sick.

It continues to be contagious for up to a week, so if you have the flu, stay home! And make sure those around you avoid physical contact, as much as possible, and wash their hands frequently.

And while I'm at it, let's debunk a common myth:

• No, you can't get the flu from the vaccine.

So if you've put off getting the flu vaccine, it's not too late! The season is in full swing, but if you get the vaccine today, you'll likely have excellent protection for the rest of the season. Go get it.