Flu hits 15-year high

According to CDC, last week 27 states reported "very high" flu activity, with lab tests turning up around 30% positive for influenza.

In addition, the rate at which people visited the doctor with flu-like symptoms increased to 7.8%, the highest it's been since the 2009-2010 season, and the flu hospitalization rate was at 64 per 100,000.

So far this flu season, CDC estimates there have been at least 24 million flu illnesses, 310,000 hospitalizations, and 13,000 deaths, including at least 57 children.

Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, said it's possible this year "could turn out to be an unusually severe flu season."

"Influenza activity first peaked around the turn of the new year — late December, early January. Activity then declined for several weeks in a row, which is usually a sign that the season is on its way out," Rivers said. "But then it really took an unusual turn and started to rise again. So activity is now at a second peak — just as high as it was at the turn of the new year. It's unusual."

However, Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security, noted that this year's flu season "is not out of the ordinary for a pre-pandemic [year]. People have kind of gotten lulled into having light flu seasons, so I think that's part of what's driving people to be kind of surprised by this. We haven't had ordinary flu seasons for a while, so it seems extraordinary."  

While the flu vaccine doesn't guarantee protection against illness, it does help prevent around 40% to 60% of infections, Dumford said, and people who are vaccinated are 40% less likely to be hospitalized than those who aren't.

"Certainly the lower the vaccination rate is, the more we will see severe influenza cases, the ones that land in the hospital, ones that land in the ICU, ones that end up in death," Adalja said.

So far, testing hasn't suggested that the H5N1 flu virus, which has been circulating in poultry and dairy cows, has been circulating widely in people and contributing to the high flu numbers. However, the more people who catch the flu, the greater the chances people could get infected with both the regular flu and bird flu, which could give the bird flu the chance to swap genes with the regular flu and turn into something more dangerous, according to Aubree Gordon, an epidemiologist at the University of Michigan School of Public Health.

"That is certainly a huge concern," Gordon said. "The danger with flu activity is that we have so many people that are infected with these seasonal viruses that it could increase the chance that you get a co-infection in a person with one of these seasonal viruses and H5N1, which gives the opportunity to generate a new virus that transmits really well from human to human. And that is one way you can get a pandemic."

COVID-19 levels remain mild

While flu levels are elevated throughout the United States, wastewater data collected by CDC shows that there has been less COVID-19 circulating than during the winter holidays and all the summer waves the program has tracked.

In addition, the COVID-19 hospitalization rate has stayed around half of what it was last year, and deaths have dropped as well. In December, around 600 people were dying each week of COVID-19, compared to around 2,000 at the same time the year before.

"This is definitely the mildest COVID winter," said Michael Mina, an epidemiologist and chief science officer for eMed. "In terms of hospitalizations, in terms of spread."

According to Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, one potential reason for the mild levels of COVID-19 is that the population still has some immunity from a large, later-than-normal summer surge of the disease.

In addition, this year's vaccine was a good match for the circulating variant and more people got vaccinated this year than last year. The virus also didn't acquire any mutations after the summer wave that would have allowed it to transmit faster or lead to more severe illness, epidemiologists said.

"The variants have been pretty similar," said Heidi Moline, a medical officer at CDC. "We haven't seen big viral changes."

It's also possible that "viral interference" could be playing a role, which occurs when the presence of one virus — in this case, the flu — pushes out other viruses, Gordon said. Some scientists believe this phenomenon is why rates of other respiratory viruses like flu were low during the early days of the COVID-19 pandemic.

"It's possible that viral interference is playing a role this year," Gordon said. "There's a lot of influenza circulating. It may generate some non-specific immunity — some nonspecific protection, which then prevents people from getting other respiratory infections, such as SARS-CoV-2 — sort of crowds it out."

Gordon added that it's not unexpected several years into a new virus for things to calm down. "You have two or three years of it being really bad," she said. "Usually the first year is the worst — as far as incidence rates and severity goes — and then it settles out."

Similar to the flu, COVID-19 is here to stay. And just like the flu, there will be good seasons and bad ones, and this winter could be on the low side of our new baseline, Gordon said.

However, unlike the flu, there will likely be more waves of COVID-19 outside of winter, though those waves have yet to fall into a clear pattern.

"There might be some good times, some bad times," Chin-Hong said. "So whether or not we'll get something later on? We have to have humility."