Maine is experiencing a late-surging flu season, with nearly twice as many reported cases when compared to the 2015-16 season.

Through April 1, Maine has had 4,188 positive tests for influenza viruses, compared to 2,360 for all of the last flu season. The season begins in October and ends the third week in May.

Half of Maine’s flu cases this season occurred in March, according to the Maine Center for Disease Control and Prevention.

At Maine Medical Center, the total number of “patient days” when people were treated for influenza increased from 75 in February to 200 in March. If one patient stays five days in the hospital for influenza, that’s counted as five “patient days.”

In March, three to 11 patients were being treated for influenza at any one time, according to hospital officials. Throughout Maine, there have been 586 hospitalizations through April 1, according to the Maine CDC. There were 274 hospitalizations at this point in the 2015-16 season.

While influenza is not a disease that’s officially reported to the U.S. CDC, there have been nearly 140,000 positive flu tests reported nationwide from publicly run laboratories so far this flu season, according to federal estimates. There were roughly 93,000 cases reported for the entire 2015-16 flu season.

Dr. Siiri Bennett, state epidemiologist for the Maine CDC, said there could be any number of reasons why positive tests in Maine have increased, including more testing of patients, the severity of the flu, types of strains circulating and effectiveness of the flu vaccine.

There’s still a lot of flu out there,” Bennett said. She said the state has improved its reporting methods over the past two years, so that may be one reason why positive test results have increased, while another factor is the flu vaccine.

The effectiveness of the flu vaccine is 48 percent for 2016-17, according to the U.S. Centers for Disease Control and Prevention, which is not as good as last year, when the CDC estimated a 59 percent effectiveness, but much better than the 20 percent vaccine effectiveness reported two years ago.

Scientists try to predict what types of strains will be most prevalent when developing a flu vaccine, and getting the right match is difficult.

“It’s always a bit of a guessing game,” Bennett said.

Still, those who get their flu shot, even if they do fall ill with the flu, tend to recover more quickly, research shows.

Also, if a milder strain of flu is circulating, there may be just as much influenza in a season, but fewer people going to see the doctor and getting tested. It’s also unknown how many people receive the flu vaccine, and how that may fluctuate from year to year.

When the flu will hit is yet another unknown.

“This is a late season, but it’s not unheard of,” said Dr. Dora Anne Mills, former Maine CDC director and a vice president at the University of New England. “There have been years where we peaked in April.”

Despite more reported cases, the severity of this year’s flu season doesn’t come close to matching the H1N1 pandemic in 2009-10. During that season, about 60 million people across the United States came down with the flu, causing nearly 13,000 deaths, according to the U.S. CDC. In Maine, there were 21 deaths during the 2009-10 flu season attributed to the H1NI flu virus. The reporting standards in Maine for flu were not nearly as comprehensive as during the past few years, so even though there were 2,200 positive tests for H1N1 influenza in 2009-10, it would be difficult to compare that season to the current one.

This season, 85 of the 100 flu outbreaks reported by institutions in Maine occurred in long-term care facilities such as nursing homes, the state CDC said.

There have been only eight reports of outbreaks at K-12 schools, and none at universities.

Janis Hogan, the Maine representative for the National Association of School Nurses and the school nurse for Camden Hills Regional High School, said nurses in Maine are reporting “widespread” flu, but it’s not reaching the level where data is being sent to the state. She said Camden schools offer a free flu vaccine every October.

“I do have students out with the flu, but we are nowhere near the 15 percent absentee level which requires a report to the Maine CDC,” Hogan said.

Don’t tell that to the Boston Red Sox.

The team has been ravaged by the flu, with many players missing games. The clubhouse was fumigated while the team was on a road trip, according to news reports.

Joe Lawlor can be reached at 791-6376 or at:

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Twitter: @joelawlorph