As flu season gets underway, global health experts are increasingly worried about a new strain of the virus that popped up in June — four months after the makeup of this year’s flu shots had been decided.

The new strain, a version of H3N2, is causing outbreaks in Canada and the U.K., where health officials are warning about the early wave that’s sending people to the hospital.

“Since it emerged, it’s rapidly spreading and predominating in some countries so far in the Northern Hemisphere,” Dr. Wenqing Zhang, head of the World Health Organization’s Global Respiratory Threats Unit, said Wednesday during a media briefing.

The version of H3N2 that’s circulated worldwide this year “acquired 7 new mutations over the summer,” Antonia Ho, a consultant in infectious diseases at Scotland’s University of Glasgow, said in a media statement. That “means the virus is quite different to the H3N2 strain included in this year’s vaccine,” she said.

The U.K. is heading “into what looks set to be a cruel winter, with flu cases being triple what they were this time last year,” the head of the U.K.’s National Health Service, James Mackey, said last week.

It’s picking up in Canada, too, said Angela Rasmussen, a virologist at the University of Saskatchewan. H3N2 is generally thought to cause more illness and be worse for older adults than other strains. Japan is also experiencing an unusually early and harsh flu season that’s “unprecedented,” Rasmussen said.

Japanese news outlet Nippon TV reported that as of Nov. 4, flu cases in Tokyo had surged to nearly six times the level seen at this time last year, according to the Japanese Ministry of Health, Labor and Welfare. More than 2,300 day cares and schools in the country were at least partially closed because of the outbreak, the outlet reported.

“These are not good signs,” Rasmussen said.

Is this flu strain already in the U.S.?

H3N2 is an A strain of influenza. While there are plenty of anecdotal reports of people testing positive for flu A across the country, the insights stop there.

The Centers for Disease Control and Prevention hasn’t provided a detailed, national report on flu activity since Sept. 26 because of the government shutdown.

Even if it reopens in the coming days, Dr. William Schaffner, an infectious diseases expert at Vanderbilt University Medical Center in Nashville, Tennessee, said that the “hollowing out” of the CDC in the form of massive layoffs could further delay flu data collection and analysis. Earlier this year, the Department of Health and Human Services announced it would cut up to 10,000 full-time jobs in the public health sector.

“We are going to be dependent on state laboratories and academic laboratories for these investigations and report them out,” Schaffner said. “The information will not be as comprehensive, centralized and as quickly analyzed and communicated from the CDC as we have had in previous years.”

Will this year’s flu shot be helpful?

Global health officials and drugmakers select which viral strain to include in fall flu vaccines for the Northern Hemisphere each year in February, based on which types are circulating in the Southern Hemisphere. This year’s flu shots protect against three strains of influenza, including two types of influenza A and one type of influenza B.

The annual flu shot doesn’t prevent people from getting infected with the flu. It’s mainly used to lessen the severity of the illness. Last year, the vaccine was up to 55% effective in keeping adults with the flu out of the hospital.

On Tuesday, health authorities in the U.K. published preliminary evidence that this year’s vaccine is up to 40% effective in preventing hospitalization among adults.

Schaffner encouraged people to get the shot anyway.

“All of the data over previous decades shows that even if there is not a close match, use of the vaccine continues to prevent hospitalizations, intensive care unit admissions and continues to help keep people out of the cemetery,” he said.

This article was originally published on NBCNews.com