Flu is making a comeback in the United States this year, with cases rising around the country even as the Omicron variant is surging, infectious disease experts say.
"We already are seeing significant increases in the amount of influenza occurring across the United States, especially with regards to the eastern part of the U.S. and the central part of the country," said Dr. Tina Tan, an infectious disease doctor at Ann & Robert H. Lurie Children's Hospital of Chicago.
All regions of the country have reported H3N2 influenza cases, and flu activity has reached moderate levels in at least nine states, according to the U. S. Centers for Disease Control and Prevention.
Experts are concerned that influenza could surge at the same time as Omicron, flooding hospital emergency rooms.
"Flu cases will likely continue to rise over the course of the next month, especially with the holidays coming up," said Matthew Binnicker, director of Mayo Clinic's Clinical Virology Laboratory. "That typically leads to more flu cases after the holiday gatherings.
"We're not at what I would call epidemic levels yet, very high levels, but we're tracking more comparably to what we would expect with a typical influenza season," Binnicker added.
Most flu that's been detected this season is the H3N2 strain, and most infections have occurred among children and young adults 5 to 24, the CDC said. However, infections among the middle aged and seniors have started to climb.
The 2020-2021 flu season essentially didn't exist. COVID lockdowns and preventive measures such as masking and social distancing kept flu cases at a historic low.
Flu season returns as COVID measures eased
"Here at Mayo Clinic in the Midwest we performed close to 20,000 influenza tests between mid-November and February of last year, and we didn't detect a single positive case," Binnicker said. "That's just unheard of. I can't recall another time in history that has happened, since testing's been available."
But all those COVID restrictions have relaxed now, leaving people more exposed to both the coronavirus and influenza.
For example, kids are back in school and they are known sources of viral transmission, Binnicker said.
People also are having more get-togethers, wearing masks less often, and traveling more both around the country and internationally, said Dr. Abinash Virk, a Mayo Clinic infectious disease doctor.
"We know influenza this year will be similar to our pre-COVID-19 pandemic levels. We don't know how high it's going to be, but that's something that we're watching," she said.
There also are troubling early signs that fewer people are getting the flu vaccine this year compared to last. In late October, the CDC reported a 15% drop in pregnant women getting the flu shot and a 6% decrease among children.
"What we have seen so far is that influenza vaccination rates have also been a little bit lower," Virk said. "Does that mean we will have more influenza this winter? We don't know. We think we may, but we don't know yet."
Early reports have also indicated there might be a mismatch between this year's flu vaccine and the most prevalent strain of influenza, but experts warned that the findings are preliminary and haven't been confirmed by the CDC.
"I have heard in that certain areas of the country where different places have run some analysis that there seems to be a mismatch with the H3N2 influenza A that's circulating, but there's no confirmed data on that," Tan said.
Binnicker said that the reports of a mismatch are "interesting, but I would also emphasize that even if there is a mismatch, it probably doesn't cause the vaccination to go from 80% or 90% effective to zero."
Flu's peak could come later
Virk agreed that even a mismatched flu shot will provide some protection, and urges people to get the jab as soon as possible.
"Influenza is just at the beginning of its curve heading upward," Virk said. "Generally when somebody gets an influenza vaccine it takes about two weeks for it to do its thing and give you some immunity. It's a good idea for you to get the vaccine even now."
The flu season typically peaks between mid-December and late January, Binnicker said.
However, that peak could come later this time around, given trends in virus transmission, Tan said.
"This has been a very unusual viral year in that many of the viruses are all occurring much later than they normally would," Tan said. "This is a time when influenza would occur, but we are having surges of Delta and Omicron. It's going to be very, very difficult to determine at least at this point when influenza is going to peak."
To help tamp down the flu, people should return to COVID protections, the experts said. Wear a mask indoors, avoid large gatherings, stay home and get your vaccinations.
"People need to get their flu vaccine and they need to get their COVID vaccine," Tan said. "These viruses are not playing around. They're going to take every single opportunity to cause an infection."
The U.S. Centers for Disease Control and Prevention has more about this year's flu season.
SOURCES: Tina Tan, MD, infectious disease doctor, Ann & Robert H. Lurie Children's Hospital of Chicago; Abinash Virk, MD, infectious disease doctor, Mayo Clinic, Rochester, N.Y.; Matthew Binnicker, PhD, director, Mayo Clinic's Clinical Virology Laboratory; Fluview, Weekly U.S. Influenza Surveillance Report, U.S. Centers for Disease Control and Prevention, Dec. 11, 2021