The flu has started spreading earlier than usual in some parts of the country, prompting concerns that cases could soon erupt as millions of Americans travel and gather for Thanksgiving and other upcoming holidays.

The warning signs come as a form of the virus, called H3N2 subclade K, has set off massive outbreaks in Canada and the U.K.

Dr. Cameron Wolfe, an infectious disease specialist at Duke University School of Medicine in Durham, North Carolina, said he’s noted a sharp increase in flu cases over the past few weeks.

“Typically, I might see one or two cases a week at this point,” Wolfe said. “At the moment, we’ve had two, three, four per day coming in.”

Alicia Budd, head of the Centers for Disease Control and Prevention’s domestic influenza surveillance team, said: “We certainly do expect we’re going to see influenza activity increasing over the next several weeks.”

The majority of flu circulating so far this season in the U.S. is the H3N2 strain, according to the CDC. H3N2 flus tend to cause more hospitalizations and deaths in older adults.

An early uptick in cases, Wolfe said, can create unique risks for this age group.

“If you’re seeing a really early flu season, you run the risk that some nursing homes may not have finished their vaccine deployments. Maybe a hospital hasn’t fully vaccinated its staff,” Wolfe said. “The community in general may not be as protected as it might have been if a virus turns up in January, for example.”

While flu activity is considered low overall, the CDC’s latest flu report shows cases are on the rise. The agency estimates that at least 650,000 people have had the flu so far this season. Around 7,400 people have been hospitalized and 300 have died, according to the report.

The uptick in cases and emergency room visits are occurring mostly in children, Budd said.

Last year, 280 children in the U.S. died from the flu. It was the deadliest flu season ever recorded for children, aside from the 2009-10 H1N1 pandemic.

No pediatric deaths have been reported so far this season. Still, the early rise in flu activity is unsettling for doctors who spent last winter watching the virus sweep through schools, day cares and, consequently, hospitals.

“It feels like the calm before the storm,” said Dr. Buddy Creech, a pediatric infectious disease specialist at Vanderbilt University Medical Center in Nashville, Tennessee. “We may have a difficult influenza year.”

The H3N2 strain is known for its ability to mutate easily. The subclade K form of the strain popped up over the summer with a handful of mutations. It emerged after the makeup of this season's flu shots had been set.

Budd, of the CDC, said the agency is studying how well the flu vaccine matches this latest flu strain. Still, she encouraged people to get the shot.

The majority of flu circulating so far this year is an A strain, called H3N2, according to the CDC. H3N2 is known for its ability to mutate easily.

This particular H3N2 variant, referred to as subclade K, popped up over the summer, after the makeup of this year’s flu shots had been set. It’s already caused massive outbreaks in Canada and the U.K.

Budd, of the CDC, said the agency is studying how well the flu vaccine matches this latest flu strain. Still, she encouraged people to get the shot.

“Even in years where the strain in the vaccine may not be as similar to the strains that we’re seeing circulating, we know that the vaccine can help prevent severity of illness,” Budd said.

Doctors across the U.S. are closely monitoring increases in flu cases.

“Does it turn on our antenna a little bit more? Yes,” said Dr. Daniel Varga, chief physician executive at Hackensack Meridian Health in New Jersey. “It makes us be even more vigilant at looking at the traditional monitoring we do locally.”

Dr. Melanie Kitagawa, medical director of the pediatric intensive care unit at Texas Children’s Hospital in Houston, said it’s still not too late to get a flu shot.

“The more people we can get vaccinated prior to the start of the flu season, the better it’s going to be,” Kitagawa said.

This article was originally published on NBCNews.com