Flu cases are ticking up, what you need to know about influenza B

As fears of a "tripledemic" die down, health officials say they are seeing an uptick of a particular strain of the flu: influenza B.

According to data from the Centers for Disease Control and Prevention, the percentage of tests that are positive for influenza B has increased from 0.12% the week ending Jan. 7 to 0.36% the week ending March 25.

Meanwhile, over the same period, the percentage of tests positive for influenza A -- the most common flu strain -- has declined from 8.58% to 0.58%.

Meanwhile, states are showing similar trends. In New York, as of the week ending March 25, of the 2,013 samples subtyped, 64.16% were identified as influenza B, state health department data shows.

Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital, told ABC News that it's not surprising to see an uptick of influenza B toward the end of the flu season.

However, due to few to no cases of influenza reported during the first two years of the COVID-19 pandemic, people may have forgotten how the flu normally behaves.

"We have to remember that, overall, we're returning to a somewhat normal respiratory virus season," said Brownstein, an ABC News contributor. "Influenza B is often later in the season. ... It's not a surprise at all that we're seeing flu B and, as part of our return to normal, we have to remember that flu still has an impact on population health."

Since the beginning of the year, hospital admissions for flu have been declining, from 12,883 the week ending Jan. 7 to 1,222 the week ending March 25.

Brownstein said the decline of other viruses as helped prevent hospitals from being overwhelmed but advised Americans to stay vigilant.

"Right now, the hospitals are benefiting from a little bit of a break," he said. "We do have more breathing room in our hospitals than we have in the past, and that's thankful, because that means that we can perform elective procedures, that means that our frontline health care workers are getting a little bit of a breather."

Brownstein added, "We still have COVID circulating, there's still other viruses that are still creating capacity challenges. A bad flu B peak could bring us right back to some of those capacity concerns."

There are four types of influenza -- A, B, C and D -- however, A and B are the most common and cause seasonal epidemics in people, what we refer to as "fu season."

However, flu activity across the U.S. remains in the "low" category and there Is currently no evidence that influenza B is more dangerous or more worrisome than Influenza A or any other strain.

"It just highlights the fact that, we still need to be vigilant about respiratory viruses, to being vaccinated against flu is still important," said Brownstein.

According to the CDC's weekly flu vaccination dashboard, about 48.3% of U.S. adults reported receiving a flu shot as of mid-February, higher than the 45.1% who reported doing so the same time last year.

Meanwhile, about 54.9% of all children in the U.S. are currently vaccinated against flu by the end of March, about the same as March 2022.

The federal health agency said flu vaccines are still very effective at preventing hospitalization and death. Flu vaccines administered this season reduced the risk of influenza A-related hospitalization among children by nearly three quarters and among adults by nearly half, the CDC said.

Brownstein said while it's never too late for someone who hasn't gotten the flu vaccine yet to be immunized, there will be availability issues as pharmacies and doctors' offices give out their final doses.

"You will be challenged by availability and, of course, the longer you wait, the further the value of that vaccine is diminished," Brownstein said. "The value keeps going down the longer you wait."

ABC News' Emma Egan and Youri Benadjaoudi contributed to this report.