Doctors are urging Americans aged 6 months and older to get vaccinated against the flu after an unusual early rise in cases on the West Coast that virologists are still working to understand.
It could be the strain itself. The predominant flu strain this year is H1N1, which caused the 2009 swine flu pandemic and is now a standard part of the flu shot. But this year’s version of the virus has a new mutation called G155E.
People who have been previously exposed to H1N1, either through infection or vaccination, would only have some residual protection from the older versions, scientists say.
"What people are a bit worried about is that the H1N1 strain this year hasn't been seen by a lot of younger folk, so it may be more severe in them," said Dr. Jennifer Duchon, a paediatric infectious disease expert at Mount Sinai Children's Hospital in New York, adding that her hospital has started to see some flu cases.
The 2026-2027 flu vaccines, approved by the FDA in August, target H1N1, H3N2 — the dominant strain last flu season — and one influenza B virus.
While most children recover, flu can be deadly, especially for the very youngest and those with underlying health conditions.
Promotion of flu shots has been reduced under U.S. Health Secretary Robert F. Kennedy Jr., a longtime anti-vaccine activist who has engineered an overhaul of long-standing childhood vaccine schedules.
The American Medical Association recommends the shot for all people 6 months and older.
The vaccine's effectiveness fluctuates depending on how well it is matched months in advance to circulating flu strains. Over the past 10 years, it has been between 30% and 60% effective in reducing flu hospitalisations and deaths. Last year, an estimated 32 million Americans had the flu, 390,000 were hospitalised and 24,000 died.
Scott Hensley, a microbiologist at the University of Pennsylvania, said two of the three components of this year's vaccine appear to be very well matched to circulating flu strains.
However, the H1N1 mutation might affect antibodies in younger people, and with flu season picking up steam, "it is probably a good idea to get vaccinated early," Hensley said via email.
Geography changes
Typically, flu cases start in Southern states and move north. This year, it's coming from the west, said Caitlin Rivers, a Johns Hopkins epidemiologist who tracks pathogens in a weekly newsletter called Force of Infection.
“There's no clear reason to me why this would be the case,”Rivers said.
As last year was dominated by the H3N2, there could be an "immunological gap" because immunity to H1N1 virus is not as fresh. Or, it could be the new mutation, she said.
Jeanne Marrazzo, chief executive of the Infectious Diseases Society of America, said studies of antibodies from people exposed to the flu last year found they did not recognize this new mutation. That was especially true of antibodies from teens and young adults.
Marrazzo said the small studies were something to be aware of but not cause for alarm.
Falling shots, rising flu cases
Some 58 million American adults received a flu shot last season, according to data firm IQVIA, down more than 20% from the nearly 75 million in the 2019-20 season, the last season before the COVID-19 pandemic changed U.S. vaccination patterns.
Around half of U.S. children got flu shots last year, according to CDC data, compared with more than 60% in 2019-20.
Standard flu shots are made by GSK, Sanofi, and CSL's Seqirus unit. AstraZeneca makes a nasal spray inoculation called FluMist. A new mRNA vaccine from Moderna is also available.
Vaccine manufacturers say demand has been consistent with last year. Two pharmacists told Reuters vaccination rates have not picked up significantly.
The Centers for Disease Control and Prevention dropped its recommendation for the shot for most children at the beginning of 2025, a move a court put on hold.
CDC also pulled its ad campaign to promote flu vaccination in 2025, according to press reports.
Paul Offit, a pediatric infectious disease expert at Children's Hospital of Philadelphia, said he believes immunisation rates have been hurt by HHS' decision to cut back on promoting flu shots, as well as by statements from Kennedy himself.
"He has done what he can to discourage flu vaccination," Offit said.
When asked about public health messaging on flu shots this season, HHS spokesperson Emily Hilliard pointed to the CDC's "Respiratory Illness Season Toolkit" on the agency's website. It says vaccination is the most effective tool to prevent serious illness.
Flu activity is low but has been on the rise since late August, CDC data show.
Positive tests reported to the CDC by clinical labs last week hit a 10-year high for this time of year at 5%. On the West Coast, that figure was 10.7%, compared with 0.5% a year earlier. At peak flu, the rates are around 30%.
Numbers are also elevated in eight Southern states including Florida and South Carolina, according to Hopkins' Rivers.