The World Health Organization has said that 2 billion people could eventually be infected with swine flu worldwide. Most cases are mild and require no medical treatment. More than 77,000 cases, including 332 deaths, have been reported worldwide.

Other countries including Australia, Japan and the United States initially tried to contain swine flu by giving out Tamiflu widely, but dumped the strategy within weeks.

Still, Britain's top medical officer defended the country's earlier approach.

"We've been fighting this pandemic very aggressively," Sir Liam Donaldson said during a press conference. "We're unapologetic about that."

Donaldson said Britain probably had the world's largest stockpile of Tamiflu. The antiviral can alleviate swine flu symptoms and shorten the course of illness by about a day if patients take it within 48 hours of getting sick.

Earlier this week, health officials reported the first instance of Tamiflu resistance, in a Danish patient who had been taking the drug. Experts worry that if Tamiflu is given out widely — as per Britain's earlier approach — that could make it easier for the virus to develop resistance.

Burnham said people with swine flu symptoms should check health services websites or call flu hotlines to get help before seeing their doctors. He said patients should stay at home and have friends pick up drugs for them from designated community centers.