A Bad Flu Season May Be on the Way. Here’s What to Know.

New York City logged the first “very high” flu activity reading of the year, raising concerns about the season ahead. (Mario Tama/Getty Images)

Key Points

• New York City recorded its first “very high” regional flu activity, with hospitalizations up 44% the week of December 6.
• The predominant H3N2 flu virus has changed, potentially weakening vaccine protection, though shots still prevent hospitalizations.
• Nationwide, 38.1% of children and 40.4% of adults received a flu vaccine, with positive flu tests up 8.1%.

The U.S. has clocked its first “very high” regional flu activity reading of the year, at the start of what could be a rough flu season.

In a weekly update, the Centers for Disease Control said flu surveillance data showed “very high” levels of influenza-like illness activity in New York City the week ending December 6.

It’s the first “very high” reading for the CDC’s flu tracking network this year. Flu levels remain moderate or low across most of the rest of the country.

The spike in New York City could be a sign of a difficult flu season to come. New York City health officials say that the percent of people hospitalized with a flu diagnosis was up 44% the week of December 6.

Nationwide flu rates remain normal for this time of year, but there’s some concern that this year’s influenza shots, while still effective at preventing hospitalization due to flu, may have been a less-than-ideal match for the virus that’s making the most people sick.

According to the CDC, most of the flu viruses seen in the U.S. this season belong to the flu virus family known as H3N2. This year’s flu shots were designed to protect against H3N2, but the H3N2 virus has changed since the shot was designed.

The difference between the new H3N2 virus, called subclade K, and older strains isn’t big enough to set off an influenza pandemic. But it is big enough to potentially weaken the protection offered by the shot.

“In general, years in which the H3N2 subtype predominates tend to be worse flu seasons,” Stephen Morse, a professor of epidemiology at the Mailman School of Public Health at Columbia University, told Barron’s via email on Friday. “Subclade K appears sufficiently different immunologically that it would be much less deterred by either the current vaccine or immunity from past H3N2 infections.”

The World Health Organization said earlier this week that there’s no sign that the emergence of the subclade K viruses has led to more severe influenza infections, and that the shots should still help prevent hospitalizations. “Early estimates suggest that the influenza vaccine continues to provide protection against hospital attendance in both children and adults, even though its effectiveness against clinical disease during the current season remains uncertain,” the WHO said.

The CDC says that U.K. health authorities estimate this year’s shot offers 70% to 75% protection against hospitalization for children, and 30% to 40% protection for adults.

The impact on flu rates this season is not yet clear. Morse says that it’s reasonable to expect rising numbers of flu cases. The number of positive flu tests was up 8.1% nationwide the week of December 6, according to CDC data.

The CDC says that as of November 29, 38.1% of children had received a flu vaccine this year, lower than last year’s mark of 40.3% as of the same week. Among adults, 40.4% said they had received a vaccine, compared to 38.2% at the same time last year.