As swine flu has slowed in the past month, US health authorities on Tuesday welcomed it, saying it gave people plent 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."
In the week ending November 21, 32 US states reported an increasing number of H1N1 flu cases compared to 43 the week earlier, and 48 in October, CDC data show.
But despite a fall in reported rates of infection nationwide, the number of visits to health care providers for H1N1 treatment remained high, the CDC said.
The number of pediatric fatalities also remained high with 27 new H1N1 deaths in the same week, taking the total deaths among children to 198 out of a total of 1,224 since the virus emerged in April in the United States.
Frieden said mutations of the H1N1 virus detected in Norway should be watched, but need not be a cause for alarm.
"With the increasing amounts of vaccine available it is a window of opportunity for protection," he said, after stocks of the vaccine fell short when it was first made available in October.
"There are now nearly 70 million vaccine doses available. And we're seeing that more people are getting vaccinated. More people are getting protected. And as that happens, it's harder for the virus to spread," he stressed.