“Every winter is a different flu season,” said Barouch. “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.
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 US, 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 US, 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.
Sarah Rahal can be reached at sarah.rahal@globe.com. Follow her on X @SarahRahal_ or Instagram @sarah.rahal.