By the second week of January, the Centers for Disease Control and Prevention (CDC) thought this flu season had already peaked. Cases of the disease were widespread around the country, but the overall numbers were not overwhelming: The hospitalization rate was about half as high as in 2014–15, the last severe flu season. CDC officials predicted there would be fewer deaths.
But the feverish masses kept growing. Now, for the third week in a row, flu activity remains widespread in 49 states, according to the latest CDC data. Some 6.6 percent of patients visiting the doctor now have flu-like symptoms, the highest rate since 2009. And the rate of pneumonia- and influenza-related deaths has suddenly rocketed up. This year’s death rate is now on pace to match or exceed that of 2014-15.
“We’ll expect something around those numbers,” Daniel Jernigan, the director of the CDC’s influenza division, told reporters during a telephone news conference last week.
There are a couple of stories here. One, this is turning into a really aggressive flu season. Two, CDC data is far from a perfect predictor of how bad a flu season will be, let alone how bad it already is.
The CDC bases its “Flu View” reports and predictions on physician records that report “influenza-like illnesses” among patients. That means there’s about a six-day gap between the ground truth and the CDC’s best understanding of it, according to Jeffrey Shaman, a professor of environmental health sciences and the director of Columbia University’s Climate and Health Program. “It is not an ideal stream of data,” Shaman says.