During most flu seasons, the group that gets top priority for vaccinations is the elderly.

But if the new H1N1 flu comes back in force this fall, it might be better to vaccinate children first, experts said at a news conference yesterday.

In the early stages of the epidemic this spring, the new flu strain has caused "explosive outbreaks" among schoolchildren who have no immunity to it, said David Fleming, public health director for Seattle and King County, Wash., which reported 430 confirmed cases through May.

While the impact of the H1N1 flu, or swine flu, is still evolving, "a critical target group for vaccinations is going to be younger children and particularly those with underlying health problems," said Dr. Fleming, who spoke at a news conference sponsored by the Trust for America's Health, a national public health advocacy group.

There are two other reasons why it might make sense to vaccinate children first, experts said.

First, there is preliminary evidence that people older than 50 may have some protection against the virus. Even though the H1N1 strain is new, parts of the virus are related to strains of the flu that dominated infections in the United States before 1957.

Dr. Joe Bresee, chief of epidemiology in the influenza division of the Centers for Disease Control, said in a phone interview this week that lab tests have given "some indication that people over 50 years old seem to have more cross-reacting antibodies against this bug."

And so far in America, Dr. Bresee said, fewer than 5 percent of those who have died from H1N1 infections have been over 65, compared with more than 90 percent of deaths recorded from a typical seasonal flu.

The second reason for giving initial vaccinations to children is statistical, says Dr. Kathleen Sullivan, chief of allergy and immunology at Children's Hospital of Philadelphia.

"There's a pretty powerful argument for giving priority to children, in that kids are little virus factories and tend to infect a larger number of contacts than adults do," she said yesterday in an interview from a conference in Ireland.

Some European nations have found that by vaccinating children first, they limit the spread to adults in their schools and their families and thus cut the number of overall workdays lost to the flu, Dr. Sullivan said.

Dr. Thomas Inglesby, chief operating officer of UPMC's Center for Biosecurity, who also participated in the Trust for Health news conference, said there is no evidence yet that children get the new flu strain more easily than other types of flu or that it makes them sicker.

But schools are always hotbeds of new flu epidemics, he said, "so it stands to reason if you could find a way to get influenza vaccine to kids, you could slow the infection rate down."

And despite the early signs that older people may have some immunity to the new flu strain, he noted that those with such underlying health problems as lung disease, diabetes and asthma are still at risk.

"In my view," Dr. Inglesby said, "the three groups that are in that high priority zone for getting early vaccinations would include people who work in the health care system, people who have an underlying condition that will make it likely they'll have a very serious illness with flu, and kids."

The CDC report on the H1N1 flu this week shows about 11,000 cases and just 17 deaths in the United States, but Dr. Fleming noted that is "a huge underrepresentation of the actual number of cases that have occurred."

He estimated there already may have been 200,000 H1N1 cases in America this spring.

In Allegheny County, there have been 22 confirmed H1N1 cases and six probable ones, county Health Department spokesman Guillermo Cole said yesterday. If the same ratio holds up here as in the rest of the nation, that means there could have been more than 500 cases in the county already.

How many people will get sick this fall will depend not only on the availability of a vaccine for the H1N1 strain, but also on what shape the epidemic takes in the Southern Hemisphere over the summer months, when it is winter in such flu incubation spots as Australia and New Zealand.

The experts all expect H1N1 to reemerge this fall, but don't know if it will play second fiddle to the regular seasonal flu, dominate it, or simply be layered on top of it.

Which of those scenarios arises could determine how much strain there will be on the health system, they said.

Federal officials currently estimate that if there is a pandemic, it could send 45 million people to doctors and clinics, and put 800,000 to 900,000 people in the hospital, Dr. Inglesby said.

But if the H1N1 strain and the regular seasonal flu target different groups of patients, the number going into hospitals could soar into the millions, he said.

With its relatively older population, Western Pennsylvania may actually get some protection from the new flu, but that is impossible to know yet.

Widespread flu outbreaks have great regional variations, Dr. Inglesby said.

The last time America had a major flu pandemic, in 1957, "it wasn't like a wave that started one place and then moved in a line. It was more like popcorn, showing up all over the country."