Hospitals are seeing three times as many people with the flu symptoms this winter, and it seems a new strain of the virus is the problem.

The Yamagata variant doesn't make people as sick as the A strain, but between them the two types of flu are causing problems.

Public health doctor Jill Sherwood says rates this year are running at about three times what they were last year.

She suspects the dominant strain of the flu virus has mutated.

"It doesn't make people as sick as the A-strain. We've seen more people in hospital with the H3N2 than the Yamagata but we are seeing more of both of those.

She says it's not too late to get a flu shot.

"We're certainly not over the worst," she said. "We're in the middle of it and it could get worse. And people can still be vaccinated, so it's worth considering that if they haven't been vaccinated."