The number of lab samples testing positive for influenza increased to 14.4 percent for the week that ended Feb. 18, up from 13.4 percent and crossing an important threshold for the CDC.
“The flu season has officially begun,” said Dr. Joseph Bresee, the chief of the epidemiology and prevention branch in the Influenza Division of the National Center for Immunization and Respiratory Disease at the CDC.
It is the latest start in 29 years, with the flu season starting in February only one other time, during the 1987-88 season, he said.
The cause of the late start is unknown but might be linked to immunization. The vaccine is identical to the previous year’s, and this year about 132 million doses have been distributed. More people had been vaccinated by November than the previous year, but the total number vaccinated remains unknown.
As vaccination levels increase, “we ought to see less disease” in the U.S., Bresee said.
Only two states – California and Colorado – are reporting widespread cases of flu and the Midwest seems to have the most flulike illness, according to the CDC FluView weekly report.
Georgia and South Carolina report local activity of flu cases, the second-lowest level of activity, and minimal flulike illness activity. Flu activity is expected to increase in coming weeks, Bresee said.
Another factor on the late start could be the mild winter, though links between weather and flu remain murky, he said.
“We do know that in cold, less humid times influenza viruses remain more viable for longer,” he said.
When the season will peak is still a guess – late peaks have come four times in March and twice in April in recent years, Bresee said.