Massachusetts could be headed for a sharp uptick in flu cases in the coming weeks, public health experts warn, as a newly mutated strain is surging in the United Kingdom and appears to be only a partial match to this year’s vaccine.
In the UK, flu season this year took off about a month earlier than usual, said Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center.
“Every winter is a different flu season,” Barouch said. “Some seasons are more severe than others, and in some seasons the vaccines are more effective. It is largely driven by mutations that occur every year in flu.” Those gradual changes, known as antigenic drift, can make vaccines less effective at preventing infection and contribute to a more severe flu season.
The culprit is a drifted version of H3N2, a subtype of influenza A that has circulated for decades. This variant, dubbed “subclade K,” carries several new mutations. UK leaders are warning of a “once-in-a-decade” winter surge as more young people test positive for the subtype. The variant has also been detected in Canada and Japan, where public health authorities declared a flu epidemic last month.
The latest Massachusetts Department of Public Health dashboard shows low flu activity.
Dr. Daniel Kuritzkes, chief of infectious diseases at Brigham and Women’s Hospital, hasn’t seen confirmation of the flu variant yet, but said there’s evidence of influenza presence in Massachusetts waste water.
“We’re just beginning to see the earliest uptick in cases. We don’t know yet which variant is predominating as flu season begins to pick up,” Kuritzkes said. “In England, the uptick started in October, two months earlier than usual, with a more rapid rise in cases.”
Dr. Larry Madoff, medical director of the Department of Public Health’s Bureau of Infectious Disease and Laboratory Sciences, said Massachusetts has not yet detected the subclade K strain but will begin sequencing for it.
There are two main strains of influenza, Madoff said: H1N1, which triggered the 2009 swine flu pandemic, and H3N2, seasonal flu, “which tends to affect older populations more and results in more hospitalizations,” he said.
“We don’t really know the effect of this new variant, and it seems to be associated with more cases [in England], but not so much with disease severity. It’s concerning that in many other parts of the world, like in Japan and China, it’s been a bad flu season,” Madoff said.
Low vaccination rates could also mean more flu cases this year.
Only 26 percent of Massachusetts residents have received a flu shot so far this season, according to the DPH tracker, the lowest rate in five years.
Last year at this time, 29 percent of Massachusetts residents had opted for the vaccine. In 2020, the rate was above 35 percent. “Anyone over the age of 6 months should get an annual flu vaccination,” Madoff said. “We should at least pick up to last year’s rate, and we’re doing everything we can to encourage vaccination for flu, COVID, and RSV, which are easily obtained and should be free of charge.”
Historically, patterns in the UK have often signaled what is ahead for Massachusetts winters, and this year is no exception, Barouch said.
The concern is that the H3N2 strain in England carries a cluster of additional mutations that scientists have not seen before, meaning this year’s shot is less able to prevent infections, although health experts still expect it to offer meaningful protection against severe illness.
“The impact of those mutations is still being studied, so we do not know for sure,” Barouch said. “But it is likely that those mutations will result in partial evasion of the current vaccine and a more rapid spread.”
Individual patients will not be able to tell which strain they have unless they are tested.
“What we might see on a population level, since the uptick of the flu season appears to be a bit earlier, is that there is a possibility it could peak at higher levels and be more drawn out. We do not know that for a fact, but it is possible based on overseas data,” Barouch said.
The Centers for Disease Control and Prevention’s latest FluView report shows flu activity still low across the United States, with influenza A accounting for the vast majority of lab-confirmed cases, “but that’s likely to pick up in the coming weeks,” Barouch said.
H3N2 is already present in the United States, though it remains unclear how much of that is the specific mutated strain seen in the UK, which requires genetic sequencing to confirm.
Barouch cautioned that the new strain is “not a reason for panic, but it is a reason why it would be good for people to get vaccinated sooner rather than later.”
It takes about two weeks after a flu shot for the body to build up protection, so doctors say this is an ideal moment to get vaccinated to be protected by Thanksgiving.
Dr. Kristin Moffitt, a pediatric infectious disease specialist at Boston Children’s Hospital, said subclade K spreads especially quickly among children.
“What we’re hearing from the UK is that the vaccine is still good at keeping young children out of the hospital, and I’m encouraging families to vaccinate against the flu, and everyone over the age of 6 months is eligible,” Moffitt said.
She said parents should be on the lookout for symptoms that go beyond those of a common cold — if their children are working harder to breathe, sleeping more than usual, or fussier than usual, or if they’re not eating or drinking well and showing signs of dehydration. “If so, be in touch with your pediatrician.”
Sarah Rahal can be reached at sarah.rahal@globe.com. Follow her on X @SarahRahal_ or Instagram @sarah.rahal.