Some 21.5 percent -- more than 261,000 residents -- of Allegheny County's 1.2 million population probably was infected with the H1N1 virus in 2009, according to a study published online by the University of Pittsburgh's Center for Vaccine Research.

School-age children were hardest hit (45 percent) and those in their 70s the least (5 percent).

But it appears there won't be another wave this spring, said one of the study's authors on Monday.

"We don't know if there will be a third wave or not," said Pitt microbiologist Ted M. Ross. "However, the transmission rate is quite low with this virus, so it looks like it has affected all the people this season that it is going to affect."

Bruce Dixon, director of the county Health Department, agreed. "If you take the estimated percentage of those who were ill and take the older population that had exposure in the past and those who had the vaccine in this year, and add them all together you have such a large number protected that the likelihood of disease spread is remote."

Dr. Ross said people who haven't yet been vaccinated against the illness known as swine flu should be, but Dr. Dixon felt differently.

"I think it's an independent decision," he said. "Do I think it's important to do it? No, though small, there are risks. I'd use a lot of judgment before I'd get the vaccine." He said there has been a case of Guillain-Barre syndrome among people vaccinated and some allergic reactions in people with allergies to eggs, a component of some vaccines.

But Dr. Ross noted, "Other parts of the country are still experiencing H1N1 even if Pittsburgh is not," he said. "Atlanta has had cases in the last week. ... It moves around."

The flu season generally ends in late March or early April.

The purpose of the study was to determine the amount of H1N1 antibodies in the blood serum among Pittsburgh-area residents, according to the decade of their birth. Such antibodies could come from either infection or vaccination, but the researchers planned the study to measure the prevalence of people who contracted the virus naturally.

"We don't know that for a fact," Dr. Ross acknowledged. But they tried to avoid examining the blood of vaccinated people by using blood samples collected before the vaccine was available in Pittsburgh.

The team gathered 100 anonymous samples per age group between mid-November and early December from UPMC Presbyterian, Magee-Womens Hospital of UPMC and Children's Hospital of Pittsburgh.

"The incidence of vaccine would be quite low in this group of people because the vaccines did not arrive until early to late November," Dr. Ross said. "And most hadn't been vaccinated or have time to become positive [if they had]. That's one reason we have the period of time we do. ...

"The second wave [of H1N1] occurred in November, [actually] late October, early November," he added. "The vaccine wasn't really available to everybody until around Christmas. So we feel very confident this [outcome] is because of infection, not vaccination."

The ages of the schoolchildren hardest hit by H1N1 were 10 to 19, and the second hardest hit were about 1 month old to 9 years old.

Those results were not a surprise to Dr. Ross.

"Children are easily susceptible," he said. "They're in tight quarters in schools and day cares."

The results of the group ages 80 to 89 was particularly interesting, suggesting to the researchers that some had residual antibodies related to the swine flu of 1918.

"Only 5 percent of the 70s group had antibodies, but it was 26 percent for those 80 to 89," Dr. Ross said. "It leads to a suggestion that those people born in the 1920s might have been infected with the 1918 virus and like viruses of that era."

The researchers' control samples were from healthy adults 18 years to 24 years, 6 percent of which contained antibodies against swine flu.

They also extrapolated the Allegheny County numbers for the nation as a whole and estimated that at least 63 million Americans, of more than 305 million residents, became infected with H1N1 in 2009.

Results were published on the Public Library of Science site dedicated to the rapid sharing of scientific results and ideas related to the H1N1 epidemic, under the heading PLoS Currents: Influenza.

Dr. Ross noted the World Health Organization and Centers for Disease Control and Prevention have suggested to vaccine manufacturers that they include the H1N1 vaccine in the seasonal vaccine for the next flu season. "So there won't be an additional vaccine," he said. "They anticipate this virus will still be around."

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