The 2019-2020 flu season is up and running—and so far, it's off to a weird start.

Flu activity has been elevated since the start of November and is only expected to continue climbing, the Centers for Disease Control and Prevention reports in its latest flu update. That's a few weeks earlier than in past years.

Flu season in the United States can ramp up in the fall and peak anywhere between December and March, then drag itself out as late as May. In the last 36 years, flu most often ramped up in December and January and peaked in February. But for this winter, the CDC says there's a 40 percent chance the flu will peak in December based on activity so far.

While this season may peak on the early side, the most unusual aspect is that it's being driven by an influenza B strain. This isn't necessarily good or bad, just unusual.

Type B is one of three types of influenza viruses that infect humans—A, B, and the very mild C. (There's a fourth type, D, but so far it mainly seems to infect cattle.) Most flu seasons are driven by type A viruses, the kind you've probably heard about the most. Type A viruses are identified by numbered Hs and Ns, like H1N1 and H3N2.

Viral code

The Hs and Ns refer to hemagglutinin (Ha or H) and neuraminidase (Na or N), respectively, which are both viral molecules that hang on the outside of viral particles. Basically, Ha helps viral particles invade human cells in the respiratory tract and Na allows newly formed viruses to burst out of human cells and invade more (for more details, see this explainer). But, because they jut out from a viral particle, Ha and Na also help our immune systems identify flu virus. This essentially triggers an arms race.

Type A viruses can swiftly mutate and rearrange the molecular makeup of Has and Nas, making them difficult if not impossible for our immune systems to recognize. That's where the numbering comes in. There are 18 Ha subtypes and 11 Na subtypes known, creating 198 possible combinations. To make things more interesting, type A viruses are promiscuous—they infect humans, many mammals, and birds. This gives them a lot of opportunities to swap their Has and Nas and come up with exciting combinations. Occasionally, extremely dangerous combinations can spark pandemics, such as the deadly H1N1 "swine flu" that spread worldwide in the 2009-2010 flu season. The ever-morphing nature of type A viruses is the main reason why flu can be so deadly and why we need to get different flu shots every season.

Type B viruses—which are dominating this flu season so far—don't do any of this. Their Has and Nas genetically "drift" relatively slowly. Type B viruses also only infect humans and, oddly enough, seals, giving them fewer opportunities to mingle and rearrange themselves. Since type B viruses were first spotted in the 1940s, they have never been linked to a pandemic.

Instead of a soup of numbers, Hs, and Ns, type B viruses are mainly identified by their lineage. In the 1980s, geneticists noted that type B viruses seemed to have split into two distinct, evolutionary lineages based on studying their Has. They dubbed the lineages B/Victoria for a reference stain isolated in Australia and B/Yamagata after a reference strain isolated in Japan.

Type B viruses typically account for about a third of all of the flu disease burden. For many years, researchers had the impression that they were relatively mild flu strains, given their slow evolution and limited host-hopping. But recent studies have found that they can cause severe disease and sometimes cause the bulk of flu-related deaths in a season. Type B viruses are also detected most often in children.

Latest data

According to the latest surveillance data from US surveillance programs, around 60 to 70 percent of the flu viruses analyzed from patients this flu season have been type B viruses. Of those, about 97 percent tested were in the B/Victoria lineage. Over the last few weeks, the proportion of B/Victoria strains among the flu-positive cases has been increasing.

It's unclear why B/Victoria is surging or what that surge means for the rest of the flu season. CDC spokesperson Scott Pauley told Ars over email that flu is difficult to predict and that it's simply "too early to make any kind of assessment about the potential severity of the season."

We should also note that cases of type B viruses have been relatively low in recent years and almost non-existent in the 2018-2019 flu season. This may mean that there's less immunity in the population overall.

Some good news is that early testing suggests that most of those B/Victoria strains now roaring back are B/Colorado/06/2017-like (Victoria lineage), which are covered by this year's flu vaccine.

Otherwise, very small samplings of the type A viruses popping up this season—H1N1 and H3N2 viruses—suggest that they're nearly all similar to the A/Brisbane/02/2018 (H1N1)pdm09–like virus and the A/Kansas/14/2017 (H3N2)–like virus, which are both covered by this year's flu vaccine.

Get vaccinated

However, as CDC's Pauley noted to Ars, "there is relatively little laboratory data on the characterization of viruses collected since October, so it's not possible to make conclusions about how well the vaccine will work at this time."

There has been some concern that the flu vaccine recommended for the 2020 flu season in the Southern Hemisphere includes different H3N2 and B/Victoria lineage viruses than the 2019-2020 vaccine for the Northern Hemisphere. But again, flu seasons are notoriously difficult to predict, and it's still too early to say what will happen.

Regardless of how well-matched this year's vaccine will be to circulating flu viruses, getting a flu shot is critical, Pauley notes to Ars. The vaccine can spare you entirely and prevent disease spread or, at least, dampen the severity of the infection, potentially saving lives. "Flu vaccination is the best way to reduce the risk from flu and its potentially serious complications," Pauley says.

So far in the US, there have been six confirmed pediatric deaths from flu this season. Overall, the CDC estimates that there have already been at least 1.7 million flu cases nationwide this season, leading to at least 16,000 hospitalizations and 910 deaths. Southern states have been hardest hit.