The 2017-2018 flu season has been brutal and, according to a new report from the Centers for Disease Control and Prevention, isn’t winding down anytime soon. However, there is a marked change in this second wave of the flu: While the epidemic has predominantly been caused by the influenza A H3N2, that strain is no longer behind the majority of flu cases. It’s influenza B that Americans now need to watch out for: a strain that can be just as severe for adults and more dangerous for children.

In an evaluation of American flu cases in the week leading up to March 17, the CDC found that the number of influenza A viruses declined and influenza B viruses became more frequently reported. That week approximately 58 percent of all laboratory-confirmed cases of flu were caused by B-strain viruses. CDC spokesperson Kristen Nordlund told CNN that it’s not uncommon to see a wave of influenza B during a season were A H3N2 has been the dominant virus but “unfortunately we don’t know what the influenza B wave will look like.”

There are four types of influenza viruses: A, B, C, and D. It’s A and B that knock down humans during the winter. And while influenza A viruses are broken down into different strands, like H1N1 and this season’s H3N2, influenza B viruses are a homogenous group. In the 1970s, scientists discovered that this group had diverged into two lineages called Yamagata and Vicotria. Influenza B mutates two to three times slower than type A and has only been found to infect humans and seals.

For decades there was a misconception that B was less hazardous than A. However, scientists discovered in 2014 that flu B viruses can cause equally severe outcomes and clinical characteristics. In a study published in Clinical Infectious Diseases a team of CDC scientists explained that among hospitalized adults, being infected with A or B resulted in comparable numbers of patients sent to the ICU and similar proportions of deaths of hospitalized adults. Influenza B, doctors determined in 2012, can be more fatal in children and often causes death more quickly than seasonal H3N2.

This second wave comes five months after this flu season first proliferated in the United States, spreading to all U.S. states save for Hawaii. In January the CDC announced that the flu virus had directly or indirectly caused a proportion of deaths considered “above the epidemic threshold.”

The good news that the 2017-2018 U.S. flu vaccine protects against H1N1, H3N2, and the B/Vicotria lineage. Getting the flu shot is a good move for everyone, and if you haven’t got yours yet the time to do so is now — the CDC says it’s possible for people who’ve gotten one strain to get another strain in the same season. Just because winter is over, doesn’t mean that the flu season is as all.