For the second year in a row, New Hampshire and the U.S. experience rough flu season

Dr. Michael Calderwood, an epidemiologist at Dartmouth Hitchcock Medical Center in Lebanon, said this flu season has been “probably the third most severe season amongst adults” in recent history.

The U.S. Centers for Disease Control and Prevention estimates there have been at least 30 million cases of influenza, 370,000 hospitalizations for the illness, and 23,000 deaths across the United States this season as of March 28, the most recent data available. In New Hampshire, 71 people have died of influenza, according to the New Hampshire Department of Health and Human Services, including two children. The CDC currently rates New Hampshire as having “moderately severe” influenza levels. The state’s neighbor to the south, Massachusetts, is also categorized as “moderately severe” but Vermont has “minimal” levels and Maine has “low” levels.

The 2025-26 season ranks behind 2024-25, which Calderwood said was the worst flu season since 2011, particularly for children. In the 2024-25 season, 289 children died from influenza across the country. So far during this flu season, 123 children have died. He cautions that “the number is still coming in, so we don’t have the final estimate for this year.” The typical flu season runs from early October to mid-May.

Calderwood said December was particularly rough, with hospitals challenged to keep up. The peak this year was the week of New Year’s — two weeks earlier than last year’s peak — and it began to taper off after that. Most of that surge, Calderwood explained, was influenza A, but he said influenza B still circulates in March and April. Influenza B figures are about twice as high now as they were this same time last year.

There were 5,701 positive influenza B tests reported (82.5% of all influenza cases) across the U.S. in the week ending March 21 and 72,175 throughout the entire season by that point, per the CDC. Last year there were 3,397 influenza B tests (40.6% of all cases) the week ending March 22, 2025, and 32,218 through the season by that point.

Calderwood said this season has been especially bad for childhood influenza cases, which has taken a toll on schools.

“The thing that was strange this year is that the flu caused a lot more GI illness,” he said. “So schools were seeing a lot of kids go home with vomiting and diarrhea, more so than the typical respiratory illnesses.”

Even more concerning to him is that doctors are seeing an increase in influenza-associated acute necrotizing encephalopathy.

“It’s a big, fancy name,” he said. “But really what it means is brain swelling with a kind of immune response that goes out of control. And about 1 out of 4 people who come in with that die. And of those who survive, about two thirds are left with some degree of disability. And this is something that is vaccine preventable.” Calderwood believes newer strains are more likely to inflict this symptom.

Calderwood suspects the driver behind the back-to-back difficult flu seasons is low vaccination rates and a vaccine that wasn’t as well matched to this year’s newest strain of the virus. As of Feb. 22, an estimated 46.5% of U.S. adults reported receiving this year’s flu shot, according to the CDC. He said the nation’s epidemiology community has a goal of reaching a 70% vaccination rate by 2030.

“In 2024-2025, the data were that vaccination prevented 10 million symptomatic illnesses, 5 million medical office visits, 180,000 hospitalizations, 12,000 deaths,” he said. “So vaccines have a huge impact. At the same time, we saw in that season that we had a 5% decline in children that were getting vaccinated. And that’s a huge number when you think about that drop off.”

He strongly encourages parents to get their children vaccinated for influenza starting at 6 months, when they are first eligible for the shots.

“That’s where we’re seeing a huge amount of drop off,” he said. “And that’s the group that is at highest risk of death.”

But he recommends adults get vaccinated as well.

“We have very effective vaccines, very safe vaccines,” he said. “And so understanding that that protection is something that is worthwhile, both to protect yourself but also to protect those around you, particularly those who may have an age or immune system that means that they, you know, are not going to have as much protection.”

The elderly, infants, and those with medical conditions affecting their immune systems are at most risk for the most serious flu symptoms.

Calderwood believes that next year, epidemiologists will be better able to match the vaccine to the new strain of the virus.

The CDC has been rocked by mass layoffs since President Donald Trump took office in January 2025, but Calderwood said he’s “been impressed with the data that we continue to receive.”

“There are obviously certain areas where we no longer have this robust data,” he said. “COVID tracking is one of them. But actually, there’s a really kind of robust infrastructure for tracking respiratory viruses nationally, so our flu data, our RSV data, has continued to be something we can trust.”

As for his clinic in Lebanon, Calderwood said he’s seeing a mix of concern among his patients. Some are much more concerned, wear masks, and want to do whatever they can for prevention “and you have others that kind of scoff at the idea that we need to do anything for prevention. And really it’s quite divided.”