By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - The pandemic of swine flu may be hitting a peak in the United States, health experts said on Friday.

But they stressed that influenza, especially a pandemic, could hit several peaks in a single season. They said weeks or months more of disease could be expected.

"We are beginning to see some declines in flu activity around the country but there is still a lot of influenza," the Centers for Disease Control and Prevention's Dr. Anne Schuchat told a news conference.

"It is still much greater than we would normally see this time of year."

Separately, a team at Quest Diagnostics analyzed 142,000 flu tests and found a similar pattern, with tests showing a decline in flu-like illness since October 27.

"The number of specimens that tested positive for 2009 H1N1 influenza dropped in all age groups since late October, with the exception of those 65 years of age and older," the company said in a statement.

"Our data provides encouraging signs that the 2009 H1N1 pandemic virus isn't spreading as aggressively now as it did in September and most of October," Quest's Dr. Jay Lieberman said in a statement.

"This is encouraging but it doesn't mean that they still shouldn't be concerned about influenza. It's not over," Lieberman added in a telephone interview.

"There can be multiple ups and downs over the season. We have many weeks ahead of us where disease is going to be circulating," Schuchat said.

KISSES AND VIRUSES

The U.S. holiday travel season is about to start, with millions expected to travel for the Thanksgiving holiday next Thursday -- a time when many Americans visit family -- and December holidays after that.

"Some people say, well, all the kids get together with their grandparents and that's a lot of exchange of warmth and love, but a little exchange of viruses, too," Schuchat said.

CDC estimates at least 22 million Americans have been infected with H1N1, with 3,900 deaths.

In the past week, 21 U.S. children have died from H1N1, bringing the total of pediatric deaths in the pandemic to 171. Schuchat noted those were just confirmed cases -- CDC estimates more than 500 children have died since April.

She said CDC was still struggling to get vaccines up to desired levels. More than 54 million doses have been ordered or are available to order.

The original hope had been to have 20 million doses a week rolling out by now, but only 11 million doses were produced, packaged and shipped in the past week.

CDC officials also said they were investigating four cases of H1N1 resistant to Roche AG's antiviral drug Tamiflu at Duke University hospital in North Carolina. "All four patients were very ill with underlying severely compromised immune systems and multiple other complex medical conditions," Duke said in a statement.

Health experts are looking for any sign that H1N1 is mutating into a drug-resistant form. Last year, the seasonal version of H1N1, a distant cousin of the pandemic strain, developed resistance to Tamiflu, the main drug used to treat flu, but GlaxoSmithKline's rival inhaled drug Relenza still worked well.

British health experts are investigating a similar cluster of Tamiflu resistance, also among severely ill patients.

(Editing by Peter Cooney)