British health officials say swine flu cases rose by about 8 percent last week, with about 84,000 new cases.

The week before, experts guessed there were approximately 78,000 new cases. But the numbers are based on indicators like how many people report flu-like symptoms and call the national flu hotline, and there is a large margin of error.

In a statement issued Thursday, the Health Protection Agency said swine flu is mainly striking children under four. Globally, officials have reported children make up a disproportionate number of those hospitalized or killed by swine flu.

Last month, Britain began vaccinating people most at risk against swine flu, including health workers and people with underlying health problems.