Flu on the rise in the US as season kicks off early

Flu season is off to an unusually early start in the U.S., with Southeastern and South Central states currently reporting the highest rates of infection in the country, according to new data from the Centers for Disease Control and Prevention (CDC).  

"We've noted that flu activity is starting to increase across much of the country," CDC Director Dr. Rochelle Walensky told NBC News Thursday (Oct. 13). 

The CDC monitors for probable flu activity in part by tracking the rate of doctor's visits for influenza-like illnesses in different states and jurisdictions, such as Washington D.C. "Influenza-like illnesses" include respiratory illnesses that involve a fever plus a cough or sore throat, so this metric may also capture illnesses due to the coronavirus SARS-CoV-2 and respiratory syncytial virus (RSV), for example. That said, sudden upticks in influenza-like illness provide strong hints as to where flu is spreading in the U.S.

Last week, between Oct. 2 and Oct. 8, most states reported relatively low but rising outpatient visits for influenza-like illnesses, according to the latest CDC Weekly U.S. Influenza Surveillance Report, released Friday (Oct. 14). However, Virginia and Louisiana reported a "moderate" rate; Tennessee, South Carolina, Texas, Georgia and New York City reported a "high" rate; and Washington, D.C., reported a "very high" rate.

Related: When should I get my flu shot? 

Nationwide, 1,322 patients were hospitalized with laboratory-confirmed influenza infections last week, hospitals reported to the CDC.

"So we know that this virus is now spreading out in the community already," Dr. William Schaffner, a professor in the Division of Infectious Diseases at the Vanderbilt University School of Medicine in Tennessee, told CNN Health. "It's gathering speed already. It looks to me to be about a month early," compared with prior flu seasons. 

Among the influenza specimens collected last week, more than 95% were influenza A viruses, the CDC reported. A small number of those specimens underwent further genetic analysis, and of that sample, most of the viruses were specifically found to be the influenza A subtype H3N2. 

The seasonal flu shot protects against four types of influenza virus, according to the CDC: the influenza A subtypes H1N1 and H3N2, and the influenza B Victoria and Yamagata lineages, which refer to branches of the influenza family tree. Influenza A(H3N2) viruses present the biggest challenge to the makers of the annual flu shot, mostly because the viruses mutate the fastest out of the flu subtypes included in the vaccine. Typically, flu shots are less protective against A(H3N2) than they are against influenza B and A(H1N1) viruses. 

It's too soon to know how this year's flu shot will fare against the circulating viruses, but the CDC has begun collecting data to help determine the vaccine's effectiveness. 

"While there are little data to date, most of the H3N2 viruses so far are genetically closely related to the 2022-2023 Northern Hemisphere vaccine virus," the CDC report states; that could be a hopeful hint that the vaccines will offer decent protection. "But there are some antigenic differences" — meaning changes in how the viruses look on their surfaces — "that have developed as H3N2 viruses have continued to evolve," the CDC noted. If dramatic enough, such changes can help viruses evade the immune system's vaccine-induced defenses.

Whatever strain is circulating, however, flu shots remain the best way to protect yourself and your community from influenza. "Vaccination helps prevent infection and can also prevent serious outcomes in people who get vaccinated but still get sick with flu," the CDC report states. "CDC recommends that everyone ages 6 months and older get a flu vaccine, ideally by the end of October."  

So far this season, "about 12 million flu vaccines have been given in pharmacies and in physician's offices," which is a slighter lower number than had been administered this time last year, Walensky told NBC News. "We do want to get people protected before they have influenza in their own communities," she said.