Swine Flu: Worst is over

Cordova said the outbreak appeared to have peaked in Mexico between April 23 and 28 and fewer people had admitted themselves to hospitals with serious flu symptoms in the last few days.

"The evolution of the epidemic is now in its phase of descent," he told a news conference in Mexico City, where millions of people heeded government advice to stay at home.

"There is evidence that we are going downward."

U.S. officials said they were encouraged by the developments in Mexico but added it was too early to relax.

Scientists are still trying to assess how the new virus behaves and compares to regular seasonal flu strains, which kill between 250,000 and 500,000 globally every year.