This year’s winter surge of influenza and COVID-19 has yet to arrive for most of the country, data published this week by the Centers for Disease Control and Prevention shows, with trends so far a fraction of what they were at this time last year. 

“COVID-19 activity remains low and is stable in most areas. Seasonal influenza activity is increasing slightly among children but remains low nationally,” the CDC said this week, in its weekly report on data about the two viruses.

The agency’s updates are usually published every week by Friday, but were delayed until after the weekend because of the Thanksgiving holiday.

A return to normal for flu?

By this time last year, data tracking outpatient visits to places like emergency rooms and doctor’s offices for the flu had already climbed above the agency’s baseline for the season. This year’s latest report suggests flu cases are only now starting to cross that threshold.

“We’re maybe slightly later than last year, and maybe quite a bit later than the year before, but those years were unusually early,” said University of North Carolina epidemiology professor Justin Lessler, a top expert in the CDC-backed network of disease forecasters.

Lessler said this year’s figures look “much closer to about where you’d expect a normal flu season to be” before the COVID-19 pandemic, when measures to curb the outbreak scrambled seasonal patterns of flu and other respiratory germs. 

Compared with last year, rates of weekly flu hospitalizations are around five times lower. Emergency room visits with flu from school-age kids have accelerated, the CDC’s data shows, but remain less than a third of what they were at this time last year. 

One big factor that could be affecting the figures this year is immunity in the population from the last two waves of influenza, alongside vaccination.

“We have had two fairly substantial seasons, which could be driving this delay. Of course the question is, does that mean we’re having a delay, or a milder season. That’s really kind of uncertain at this point,” said epidemiologist Shaun Truelove, associate scientist at Johns Hopkins University’s public health school.

Modeling of this year’s flu season gathered from different experts make different assumptions about what the country’s flu immunity looks like, as they try to play out different scenarios of how this year’s flu season might look like.

Truelove, who also helps lead the CDC-funded disease forecasting network, also thinks that this year could mark a “return to normal” for flu.

“That being said, flu is never consistent. We get early seasons and late seasons. So it’s hard to say exactly what that means, and everything’s a bit weird post-pandemic, in terms of scale and timing,” said Truelove.

Watching for another COVID-19 wave

Several COVID-19 trends that the CDC tracks also remain below what was recorded at this time in 2023. This is a contrast with previous summer waves of the virus, which have only dipped slightly into the fall before picking up again during the winter. 

COVID-19 emergency room visits are “low” or “minimal” in nearly all states, after this year’s late summer wave of the virus. Levels of the virus in wastewater are “minimal” in all regions, compared to “high” levels around this time last year. 

“Does that mean that there was enough immunity built up in that summer wave that we’re going to not see a winter wave? Does it mean the winter wave is going to come, but be a little bit later and maybe a little smaller,” said Lessler.

Both Truelove and Lessler said one major “data gap” making comparisons to previous seasons challenging has been the lapse in nationwide COVID-19 hospitalization data during the summer surge.

A pandemic-era emergency requirement for health care providers to report COVID-19 hospitalizations lapsed earlier this year, and only recently resumed under a new rule issued by the Centers for Medicare and Medicaid Services. 

Other factors muddying the figures include changes to how people test and seek care for COVID-19 infections.

Another big unknown is the evolution of SARS-CoV-2, the virus that causes COVID-19. Most circulating variants right now are a medley of closely related variants like XEC and KP.3.1.1.

“We don’t know, like we do for flu, what the average pace of COVID’s evolution away from our immune system will be, when it settles down,” Lessler said.

Early data released last month by researchers at The Ohio State University found XEC looked to be more infectious compared with the parent variant it shares upstream with KP.3.1.1, but not significantly more than its siblings. 

“I actually have thought it had settled down a bit, after this year. We’ll see what I think after the season’s done. But right now, I’m a little less sure,” he said.

Other respiratory germs on the rise

While COVID-19 and flu remain at unusually low levels nationwide, rates of at least other germs that spread through coughs and sneezes remain elevated.

CDC’s data on emergency room visits from the bacteria Mycoplasma pneumoniae, sometimes called “walking pneumonia,” have increased in recent weeks especially in babies. 

Cases in this year’s wave of pertussis, or whooping cough, are also continuing to accelerate in several states around the country.

The agency tallied 577 infections of pertussis nationwide for the week ending Nov. 23, around 33% higher than last week’s count. That’s higher than the peak of 2014’s record pertussis wave, which reached 326 weekly infections reported in late December.

Ohio remains the state with the most cases of whooping cough this past week, after 66 infections with pertussis were reported. 

The CDC’s “acute respiratory illness” metric, which lumps together levels of patients showing up in emergency rooms with a broad array of cough and cold illnesses including influenza and whooping cough, tallies 14 states with “moderate” levels including Ohio.