The new H1N1 swine flu is estimated to have infected about 800,000 people in New York City in the spring, a top U.S. health official said on Sunday, citing a study due to be released later this week.

Dr. Thomas Frieden, who heads the U.S. Centers for Disease Control and Prevention, said surveys suggested the virus was widely spread around the city. Frieden was New York City's health commissioner before taking the top CDC job in June.

"In New York City where we had a lot of H1N1 this last spring the estimate is about 800,000 people, about 10 percent of New York City residents, got infected with the flu," Frieden said in an interview with C-SPAN television aired on Sunday.

"That's a lot of people."

New York City health department officials say the full study is being finished and will be released within days.

Frieden said there had been a twenty-fold variation in influenza infections around the country. "We expect that some places will have more flu. Some places will have less," he said.

Swine flu has infected well over 1 million people in the United States, and is now the CDC's No. 1 priority. Other research also shows that older children and young adults are by far the most likely to be infected with the new virus.

The World Health Organization predicts a third of the world's population will eventually be infected.

The virus is still circulating and most health experts expect a resurgence in the northern hemisphere's autumn as temperatures cool and schools, traditional breeding grounds for infection, reopen after summer holidays.

Detailed reports on outbreaks can help health officials prepare for epidemics in their communities.

Every year, seasonal flu infects between 5 percent and 20 percent of a given population and kills between 250,000 and 500,000 people globally. Because hardly anyone has immunity to the new H1N1 virus, experts believe it will infect far more people than usual, as much as a third of the population.

It also disproportionately affects younger people, unlike seasonal flu which mainly burdens the elderly, and as a result may cause more severe illness and deaths among young adults and children than seasonal flu.

Chicago health authorities said last week that the pandemic H1N1 flu infected 14 times as many children as adults over 60 there, and also disproportionately affected blacks and Hispanics.

WHO said pregnant women and people with asthma, diabetes and heart diseases are at special risk of severe complications of death from H1N1 flu.

Some countries are reporting that as many as 15 percent of patients hospitalized with the new H1N1 pandemic virus have needed intensive care, further straining already overburdened healthcare systems, WHO said on Friday.

Companies are preparing vaccines against H1N1, which will be given in addition to the regular seasonal influenza immunization.