The flu virus, which all but disappeared in early 2020, is once again circulating in the United States. The Centers for Disease Control and Prevention reported about 2,500 cases from clinical tests nationwide for the week that ended Dec. 11. That number is typical for this time of year, but it also represents a level of cases that has not been seen since before the coronavirus pandemic.
New weekly flu cases in the United States
Source: Centers for Disease Control and Prevention
Note: Figures reflect specimens tested at clinical laboratories. Public health laboratories reported about an additional 500 cases in the week ending Dec. 11, which are not reflected in the chart.
It is a bad time for the flu to re-emerge, as the Omicron variant causes a surge in coronavirus cases and with travel expected to increase over the holidays.
“We have seen how surges in Covid-19 infections can overwhelm hospitals, and influenza infections could further stress health care systems,” said Sonja Olsen, an epidemiologist at the C.D.C. “If both viruses continue to circulate and increase in activity, the situation could get worse.”
The agency considers several factors when assessing influenza activity, including the number of overall patients who report respiratory illnesses, the number of long-term care facilities with at least one case of the flu, the share of flu tests that come back positive and the number of flu patients admitted to hospitals. In recent weeks, all of these indicators have gone up.
Since early September, the share of flu tests that have come back positive has risen to 3.5 percent from 0.05 percent. That positive rate is still below the prepandemic average, which may mean that there has been less flu in circulation in recent weeks than what was typical in recent years, Dr. Olsen said.
Positive flu tests
Source: Centers for Disease Control and Prevention
Note: Figures reflect specimens tested at clinical laboratories.
The average number of patients with the flu who are admitted to hospitals each day has also risen, to 243 from 43 in early September. For context, the average number of patients with confirmed cases of Covid being admitted to hospitals each day is about 8,200.
New hospital patients with influenza
Source: Department of Health and Human Services
In recent years, the flu season has peaked at an average of 12,500 diagnosed weekly cases, usually in late January or February, but most cases go undiagnosed. The C.D.C. estimates the number of annual flu cases over the past decade has been between nine million and 41 million.
It is not yet clear how severe this season will be, but the virus will most likely continue to spread in the weeks to come, Dr. Olsen said, and the flu strain that is now the most dominant in the U.S. has in the past been linked to more-severe flu seasons.
That dominant virus, which is a strain of H3N2, is not among the types of flu that this season’s vaccine specifically guards against. That means the flu vaccine may be less effective against that strain, said Richard Webby, a virologist at the St. Jude Children’s Research Hospital in Memphis. But, he said, it is still worth getting immunized.
“Even though it’s not a perfect match, and it may not provide you with that protection from getting infected, it’s still going to protect you from getting more severe disease,” Dr. Webby said.
Dr. Olsen laid out several possible scenarios for the coming flu season. Cases could peak later in the year than usual, or they could drop off sooner than expected. Or there is still a chance that cases remain lower than what was normal before the pandemic.
“Of course, we could also experience a more typical season,” she said. “It is too soon to know.”