CHICAGO — When Dr. Diane Chaney arrived for her morning shift at the University of Chicago Medicine’s emergency department on Monday, there were nine patients from the overnight shift waiting for treatment.

By late morning, 36 patients, most with flu symptoms, were waiting.

Across town, doctors at Rush University Medical Center have seen 203 flu patients since Nov. 5, compared with 119 patients for the entire flu season last year.

“We are coming to the point where we are running out of testing supplies,” said Dr. Ed Ward, an expert in emergency and internal medicine at Rush, a teaching hospital.

Similar scenes are being played out in emergency departments across the country, including in Maine, as the United States grapples with the earliest flu season in a decade.

“The emergency rooms are quite full and it’s clear that the annual flu epidemic is in full swing,” said Dr. Brian Currie, medical director for research at Montefiore Medical Center in New York.

According to the U.S. Centers for Disease Control and Prevention, the proportion of people visiting their doctor for a flu-like illness has climbed from 2.8 percent to 5.6 percent in the last four weeks. That compares with 2.2 percent during last year’s mild flu season and a peak of 7.7 percent during the 2009 H1N1 flu pandemic.

Dr. Daniel Lucey, who tracks global flu activity at Georgetown University Medical Center in Washington, said people naturally reflect on the past year’s flu season as a measure for the severity of flu, making this season appear all the worse by comparison.

Even so, he said, “it’s an objective fact that flu viruses are circulating earlier and more widely this year than most years.”

Lucey and others say it’s not clear why the flu arrived earlier than normal in the United States this year.

Although each season is unique, flu activity generally starts to pick up in December, peaks sometime in January or February and peters out by late March or early April.

One risk of an early flu season is that it arrives before people have had a chance to get vaccinated.

That may be especially true following last year’s very late and mild flu season, which may have led some to think they had more time. Experts, however, say it is not too late to get a flu shot.

“We strongly encourage people to get vaccinated, and we’d like them to do that as soon as possible,” said Dr. Michael Jhung, a CDC flu expert.

The predominant strain of flu this year is an influenza A virus called H3N2, which has accounted for 76 percent of the reported viruses.

“We know H3N2 has been associated in previous seasons with pretty severe illness, particularly among the elderly. That could be a contributor to why we are seeing such high levels of activity right now,” Jhung said.

Currie of Montefiore suspects the H3N2 strain also may pose more of a risk for younger people, who “don’t have as much experience with that strain.”

Fortunately, flu experts did a good job last spring of predicting which of the flu strains circulating in Asia would be most likely to cause disease in this flu season. As a result, the CDC says the current batch of flu vaccines appear to be a good match for this year’s flu.

So far, 91 percent of flu viruses analyzed by the CDC match flu strains contained in this year’s vaccine.

“The viruses that we’re detecting are like the viruses that are in the vaccine, so there should be good protection,” Jhung said.

Earlier this year, manufacturers estimated they would make 137 million doses of vaccine, and by early December, the CDC had vaccinated an estimated 112 million people, suggesting vaccine coverage is fairly high so far, Jhung said.

Even if vaccinated, people may still get the flu. Experts, however, say the vaccine will help reduce the severity of the illness, and may help reduce the number of deaths, which can range from a low of 3,000 to a high of nearly 50,000.

Dr. William Schaffner, an infectious diseases expert at the Vanderbilt University Medical Center in Nashville, said even in healthy young people, the flu vaccine was only 70 to 80 percent effective. And in older adults, the vaccine only protects about 60 to 65 percent of those who receive it.

But it is still the best defense against flu.

“Study after study after study shows that the influenza vaccine reduces the risk of getting sick and getting any of the other significant complications of the flu,” Schaffner said.