The United States may be headed into a bad flu season, according to figures recently released by the Centers for Disease Control and Prevention.
As of the last week of December, “widespread” flu activity was reported by health departments in 46 states. More ominously, a second measure — the percentage of patients with flu symptoms visiting medical clinics — shot up almost to the peak reached at the height of the 2017-18 flu season, which was the most severe in a decade.
About 61,000 Americans died of flu that season, the C.D.C. said. (The original estimate of 79,000 was revised downward last year; the agency said the number changed as more death certificate information became available.)
This year’s flu vaccine may not be particularly effective against the strain of the virus now widespread in the United States, experts said. But even so, it’s worth getting the shot: people who are vaccinated fare better if struck by the flu than those who are not.
It is still too early to know how severe this season will be, said Lynnette Brammer, leader of the agency’s domestic influenza surveillance team.
Although many people are coming down with flu, the two chief indicators of severity — hospitalizations and deaths — are not yet elevated, she noted.
Deaths from pneumonia and flu are actually lower than normal at this time. But reports of hospitalization and death normally lag other indicators by at least two weeks.
The current season did begin unusually early. By late November, the flu had hit hard in the Deep South, from Texas to Georgia. The virus then broke out in California and the Rocky Mountain states, but was not widespread in the Northeast until recently.
That pattern echoes what happened in Australia, where winter runs from June through August. Flu came unusually early to the Southern Hemisphere in 2019. In seasons when Australia has a bad flu season, the Northern Hemisphere sometimes does, too.
In another important way, however, the United States is not following Australia’s lead. The A(H3N2) strain of influenza was dominant there last year, while most American cases this season have been caused by a very different strain, called B Victoria. (B strains are named for the cities where they were first isolated.)
B strain flus do not normally arrive until late in the season. But when they do, “they often impact children more than adults and older adults,” Ms. Brammer said.
The C.D.C. tracks the deaths of children individually, rather than making estimates, as is done for adults. Those over 65 are usually the group hardest hit by flu. Thus far this season, 27 children have died of flu — in 2017-18, 187 died — but pediatric deaths don’t normally start peaking until mid-January.
On the rise now is the A(H1N1)pdm09 strain, which is a descendant of the pandemic “swine flu” that first appeared in 2009 and then morphed into a seasonal flu.
H1N1 strains are usually the first to appear. They usually cause fewer hospitalizations and deaths per capita than B strains or A(H3N2).
Thus far, based on limited testing data, this season’s flu shot does not look like a good match for the B Victoria flu and may not be very effective, the C.D.C. said. But the shot does still appear to be well matched for the A(H1N1)pdm09 strain.
C.D.C. flu data relies on reports from doctors’ offices, clinics and hospital emergency rooms about how many patients come in with flu symptoms.
An even faster measurement of flu’s spread comes from Kinsa Health, which collects daily readings of fevers from up to two million users around the country who own its thermometers. The devices connect to smartphones and instantly upload readings to the company’s app.
Kinsa readings indicate that flulike activity peaked on Dec. 24 at a level just below the 2017-18 level — confirming what the C.D.C. found — and has since dropped by almost a third, said Nita Nehru, a company spokeswoman.
But even this week’s lower figure “is much higher than is typical of this time of year,” she added. It may bounce up again soon, now that students have returned to school from holiday vacations.
The company assumes that fevers lasting three or more days indicate flu rather than a common cold, said Inder Singh, the company’s founder.
The C.D.C. has not endorsed Kinsa’s methods, but the data does show flu patterns at least a week or two ahead of reports from medical clinics.
Thus far, almost none of the hundreds of samples tested by the C.D.C. have been resistant to Tamiflu or any other common antiflu drug. Those medications do not cure the flu; they only reduce the severity of an infection, and only if they are taken early.