Public health agencies have reported fewer influenza cases in Maine and across the country this winter, but the 2015-16 flu season is far from over.

“So far, so good, but we can’t be too joyful just yet,” said Dr. Caroline Teschke, public health program manager at the India Street health clinic in Portland. Teschke said Wednesday she’s concerned that people are not taking the flu seriously this year. The number of flu shots administered at city clinics is 20 percent behind last year’s total for this time of year.

Only 37 people in Maine had tested positive for influenza through Jan. 2, according to the Maine Center for Disease Control and Prevention, but Maine is known to have flu outbreaks through April and the number of reported cases varies widely from year to year. There were 4,233 cases confirmed by testing in 2014-15, 2,029 in 2013-14 and only 167 cases in 2012-13. The Maine CDC collects data from October through May. Not all flu cases are reported, so actual numbers each year were likely higher.

“The only predictable thing about influenza is its unpredictability,” said Dr. Dora Anne Mills, a vice president at the University of New England and a former Maine CDC director.

Nationally, 2,459 people have tested positive for the flu, and most states were reporting a mild season so far. Outbreaks have been reported in New Jersey and South Carolina.

Each year, U.S. Food and Drug Administration scientists predict which strains of influenza virus are likely to be circulating and manufacture a vaccine to protect against them.

It’s an inexact science – the vaccine for 2014-15 was only about 20 percent effective, according to a February report by the federal CDC. About 125,000 influenza cases were reported nationally last season, 4,233 of them in Maine.

The federal CDC called 2014-15 a “moderately severe” flu season, but has predicted that this season’s vaccine will be more effective.

Teschke said she’s concerned that people are not bothering to get flu shots this season after hearing how ineffective last season’s vaccine was.

“People get nonchalant about it, think they don’t need a flu shot, and that’s when you do get it,” Teschke said. The India Street clinic will be offering free flu shots every Friday, starting Jan. 15, until it runs out of vaccine.

Flu strains detected in Maine so far this season are nearly a 100 percent match with those targeted by the vaccine. But Mills said other strains could start circulating later this winter or spring.

Maine CDC officials did not respond to requests for comment Wednesday.

Mills said no one knows why the flu is worse some years than others, or why flu tends to circulate during winter. She said it’s not related to the weather, because flu can be severe during mild and severe winters, and outbreaks also occur in areas with warm winters.

It is likely that multiple factors affect the severity of outbreaks, Mills said, including the weak winter sun and children being in close contact with one another in school for months.

“The classroom is like a petri dish, and the children come home at 3 p.m. every day, spreading viruses,” Mills said.

Ultraviolet light from sunshine helps produce Vitamin D in the human body, which boosts immune systems and in turn protects against viruses, Mills said. With sunlight weaker during the winter, Vitamin D production falls off, possibly making people more susceptible to influenza and other diseases, she said.

“So far we’ve had a milder season. But never take the flu for granted,” she said.