New flu strain drives Massachusetts cases to ‘very high’ levels

The CDC estimated there have been at least 11 million illnesses, 120,000 hospitalizations and 5,000 deaths from flu so far this season. That rate involves at least a dozen child deaths, including three in Massachusetts. The state health department is also reviewing two possible COVID-19-associated deaths and one RSV-associated death in individuals younger than 18 years.

Here’s what to know.

Influenza-like illness visits and lab-confirmed cases have risen significantly in recent weeks.

The Massachusetts Department of Public Health’s weekly flu tracker, which was last updated on Wednesday, marks “very high” influenza illnesses in the state.

Hospitalizations linked to flu are climbing toward, or above, levels seen in recent years, including last season’s high rates.

In Massachusetts, the share of emergency room visits that resulted in a patient being hospitalized with the flu is 9 percent, up from 5 percent the week prior, and triple the rate from the same time last year.

Visits to health care providers for flu-like illness increased to 11.8 percent, up from 7.6 percent the week prior.

Each day last week, there were nearly 9,000 emergency department visits statewide, with approximately 25 percent of those related to acute respiratory illness, including flu, COVID-19, and RSV. Some regions in the state reported challenging hospital capacity levels exceeding 90 percent in medical-surgical units, largely related to influenza.

Physicians say this pace of increase is concerning and could continue past January or February, the typical peak months for flu. Last year, hospitalizations peaked during the week of Feb. 2, with 10.5 percent of hospitalizations attributed to the flu.

“It’s hard to say when our peak is going to be, and in some years, we see a second peak,” Dr. Vandana Madhavan, clinical director of infectious disease at Mass General Brigham for Children, said Monday.

There have been 32 influenza deaths this season in the state, 29 adults and three children, including one who died during Christmas Week. Last season, there were 470 deaths in the state, nearly double the previous year.

The surge is driven mainly by a variant of influenza A H3N2, known as “subclade K.” This strain has been spreading across the US and is thought to infect more people rapidly.

Among influenza A H3N2 viruses that underwent additional genetic characterization at CDC, 90 percent belonged to subclade K.

While experts are still assessing its severity, subclade K appears capable of evading some existing community immunity, which may be contributing to the high case numbers.

UK leaders warned in November 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 in October.

The newly mutated strain, which was surging earlier in the United Kingdom, appears to be only a partial match to this year’s vaccine.

Although the current vaccine may not be a perfect match for the dominant strain, health experts say the annual flu shot remains the most effective way to reduce the risk of severe illness. Data show only 34 percent of Massachusetts residents have received a flu shot so far this season, according to the DPH tracker. That’s lower than last season’s 40 percent vaccination rate.

“It does take a couple of weeks to get full protection from vaccination,” Madhavan said. “Now is the time, if you haven’t gotten vaccinated, to get vaccinated, to get some protection on board. And I would say that for COVID as well.”

As people travel more, declining immunization rates and more severe illness have increased the likelihood of catching it, Madhavan said.

Children are experiencing worse complications from the flu. Madhavan said she’s seeing more patients with acute necrotizing encephalopathy, “which can be a complication of influenza with changes in a child’s mental status, inability to speak, to eat, in some cases, breathing, needing support with walking, and just daily living.”

Typical flu symptoms include high fever, cough, sore throat, body aches, fatigue, and sometimes nausea or vomiting, diarrhea. Some clinicians have noticed fevers reaching 104–105 degrees in some instances. High-risk groups include children, the elderly, pregnant women, and immunocompromised individuals.

Madhavan recommends staying hydrated, stocking up on popsicles and Pedialyte, and using Ibuprofen and Tylenol, and honey for children older than 1 year as supportive medications.

At the moment, COVID-19 activity in Massachusetts remains relatively low compared with influenza, though officials caution that seasonal waves of different respiratory viruses can overlap. Vaccination and preventive behaviors protect against both.

Sarah Rahal can be reached at sarah.rahal@globe.com. Follow her on X @SarahRahal_ or Instagram @sarah.rahal.