(AP) -- European health officials are warning that the swine flu outbreak that appears to be spiking in Britain could soon spread to the rest of the continent.
The annual flu season struck the U.K. early this year, with cases surging last month and doubling almost every week.
The predominant strain infecting people is swine flu, which was responsible for the 2009 pandemic. Unlike most flu viruses, swine flu mostly affects people under 65 and many of its victims are previously healthy younger people with no underlying problems.
In Britain, a vaccine shortage is forcing authorities to resort to leftover shots from last year's swine flu pandemic. Some hospitals have been forced to cancel elective surgeries to accommodate extra flu patients.
Health officials say the European continent should pay attention to what's happening in Britain.
"What starts in the West of Europe tends to move East," said Angus Nicoll, director of influenza coordination at the European Centre for Disease Prevention and Control. "It's very likely this same pattern (in Britain) will progress to the rest of Europe though we simply do not know if the impact on hospitals will be as high."
Nicoll said some countries with particularly high vaccination rates - Finland, Norway, Oslo and Sweden - would likely dodge a severe flu season. He said other countries, including France and Germany, should step up their vaccination programs and possibly prepare their hospitals for an imminent wave of flu cases.
"It's not clear if flu in the rest of Europe will be as intense, or if there was a perfect storm of bad weather (and virus circulation)," Nicoll said.
According to Britain's Royal College of General Practitioners, there were about 99 cases per 100,000 people last week, down from 124 cases per 100,000 people in December. Nearly 800 people with flu are being treated in intensive care units and 50 people have died - almost all of the deaths were in people who were not vaccinated.
Ian Jones, a flu expert at the University of Reading, said the British numbers were higher than typical flu seasons but not yet at epidemic levels.
Swine flu symptoms include fever, muscle aches, a sore throat and diarrhea, though most people will recover without needing medical treatment.
Some experts blamed the British government for not advertising its annual flu vaccination campaign. A renewed effort began last week, but some experts say that's too late. Flu appears to be peaking now, but it takes at least about two to three weeks for people to develop full immunity after getting the vaccine.
"Dropping the ad campaign has directly affected the uptake of the flu (vaccine), especially because there is a certain complacency with regard to seasonal flu," said Tarit Mukhopadhyay, a vaccine expert at University College London.
According to the World Health Organization, the swine flu virus detected in Britain is similar to the pandemic strain and no changes have been identified to suggest it is more lethal or transmissible.
Charles Penn, a WHO flu expert, said swine flu appeared to be the main virus spreading in Europe so other countries might soon experience an outbreak similar to Britain's.
"Any countries where (swine flu) is the predominant virus should be prepared for more incidents of infection in younger people than normal," he said.
Worldwide, flu sickens up to 5 million people and kills as many as 500,000 people every year.
The flu season has also begun in North America, but the prevalent strain in the U.S. and Canada appears to be a different flu virus, H3N2, with swine flu only accounting for about 10 percent of cases.