The flu has arrived early in Maine.

Flu season doesn’t officially begin until October, but the state Center for Disease Control and Prevention is already reporting four confirmed cases of the disease.

Two are Maine residents – one from Cumberland County, one from Hancock County. One is an international visitor tested in Androscoggin County, and one is an out-of-state resident tested in Cumberland County. None of those patients was hospitalized.

“It is coming,” state epidemiologist Dr. Siiri Bennett said. “We are just trying to let people know the vaccine is now available. It’s the best way to protect yourself.”

Long-term data was not available from the CDC on Friday, but the start of the 2016 season is the earliest in at least a decade based on archived news releases and past media reports.

From 2006 through 2015, the state reported the first case of seasonal flu as early as October and as late as January. In both 2014 and 2015, that report noted five positive tests for the flu in the second week of October.

The CDC circulated a public health advisory this week warning about the early start to the flu season in Maine, but Bennett said Friday that it is not unusual to see small sporadic outbreaks during the month of September.

“The federal CDC is telling us there have been a number of localized influenza outbreaks around the United States so far, but activity is very low,” she said.

Bennett urged anyone older than 6 months to get vaccinated sooner rather than later. A flu shot takes about two weeks to become effective, she said.

Influenza – commonly known as the flu – is a contagious viral infection in the respiratory system. Different from a cold, the flu usually comes on suddenly and lasts for less than two weeks. Symptoms include a fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches and fatigue. To prevent its spread, public health officials recommend washing your hands, covering your cough and staying home from work if you feel sick.

The state CDC has not yet begun to publish the weekly surveillance reports that run from October to May each year – the typical flu season. Last year, Maine’s flu season peaked in March and was comparatively mild with 2,360 total cases. The previous year, the number was 4,239.

While many cases are mild, the disease can at times be fatal. The CDC data shows one child who had not been vaccinated died during the 2015-16 season.

Across the country, there were more than 45,000 positive flu tests last year. While the federal CDC also has not yet begun its weekly updates, it reported there have been 105 positive tests for the flu across the country in the last three weeks.

Dr. Stephen Sears, clinical adviser to the Maine Public Health Association, agreed Mainers shouldn’t wait to get their flu shots. In particular, he noted Maine’s preponderance of older people.

“As a group, when we get the flu, we get really sick and really miserable,” Sears said. “But when an older population gets the flu – and these are people who might have underlying illnesses that can be exacerbated by the flu – they’ll get into much more trouble.”

Vaccines for children also are essential, he said, because youngsters spread illnesses so readily at school and in their families. Sears said he couldn’t speak to the cases identified thus far in Maine, but he attributed the early reports to international travel and better technology for testing.

“The flu is a disease that is around the world,” Sears said. “In South America, for instance, this is their spring. That means it’s the ending of their winter, so they still could have the flu.”

Four different strains of the flu are expected this season, but the state’s advisory noted that the vaccine that is now being administered “is likely to offer good protection” against all four. Both Medicare and most private insurance pay for flu shots, which are widely available among physicians and pharmacies.

Laurie Martens, a physician’s assistant at InterMed in South Portland, is among the providers already offering vaccines for patients.

“It seems early, but we start vaccinating in September because we know the flu season can start at any point,” she said.

The flu vaccine is effective for the entire flu season.

While the flu arrived early, it’s too soon to tell if the virus will be more or less widespread than usual.

“How this will play itself out is still to come,” Sears said. “But what it really says is, get your flu shot now.”