US health authorities have welcomed news that the spread of swine flu has slowed in the past month, saying it gave people plenty of time to ensure they are vaccinated.

While the "flu is going down, it's far from gone. And flu season lasts until May. Only time will tell what the rest of the season will bring," warned Thomas Frieden, director for Centers for Disease Control and Prevention.

He said the slowing spread of the disease "leaves a window of opportunity for people to be protected by getting the vaccine. The flu virus is unpredictable. We can't be sure of what will happen in the future."

The CDC polled about a dozen top specialists in the field worldwide and about half said another wave of infection was likely while half said that was not the case, he told reporters.

Frieden said in the last similar pandemic from 1957-1958, "there was a large surge in cases at the beginning of the school year then a waning of cases, and then in December, January, February, there was a big increase in the number of people who were severely ill or who died.

"We don't know if that will happen this year," he added, stressing: "We do know that the vaccine is the best way to protect yourself."