WASHINGTON (Reuters) - U.S. health officials strengthened their recommendations for seasonal flu vaccines on Friday, saying all children aged 6 months to 18 years should be immunized -- especially because of the H1N1 flu pandemic.

The seasonal vaccine provides little or no protection against H1N1 swine flu, but immunization will help prevent people from being infected with both at once and can help minimize the effects of the pandemic on schools, workplaces and the economy in general, health experts say.

“Vaccination against seasonal influenza should begin as soon as vaccine is available and continue throughout the influenza season,” Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention told reporters in a telephone briefing.

“At this point, 83 percent of the population is recommended to get an annual flu vaccine,” she said. “Unfortunately, only about 40 percent of the U.S. population received the flu vaccine last year.”

Last year the CDC “encouraged” all children to be vaccinated. Now it “recommends” this -- advice that does not have the force of law but that can affect what states and insurers do.

On Thursday U.S. Food and Drug Administration officials said they would work with companies and the National Institutes of Health to quickly test experimental H1N1 vaccines, with the aim of getting a vaccination plan underway as soon as possible.

Schuchat said H1N1 was still circulating.

“We are continuing to see transmission here in the United States in places like summer camps, some military academies and similar settings where people from different parts of the country come together,” she said.

“I think this is very unusual to have this much transmission of influenza during the (summer) and I think it’s a testament to how susceptible people are to this virus.”

TIP OF THE ICEBERG

The CDC said 43,771 cases of H1N1 influenza had been officially confirmed, with 302 deaths.

“But ... that’s really just the tip of the iceberg,” Schuchat said. “We believe there have been well over 1 million cases of the new H1N1 virus so far in the United States.”

She said the CDC would no longer report cases and was working on better ways to estimate how many people had been infected.

The pandemic spread globally in less than two months and has infected people in 160 countries, killing 800 people, the World Health Organization said. The WHO numbers do not include the latest CDC count.

Schuchat said there is no indication the virus is any worse in one country than another.

“There are differences in reporting. In some places, we’re hearing about only the severe cases. In other places, we’re hearing about illness that’s in the community,” she said.

She declined to call the pandemic “mild” and noted that people had died and many others had spent weeks in hospitals, sometimes on ventilators.

She said the CDC was also watching for more cases of seizures. The agency reported on Thursday on four children who suffered seizures from H1N1 infection but who recovered.

Schuchat advised against summer camps offering the antiviral drug oseltamivir -- Roche AG and Gilead Sciences Inc’s Tamiflu -- to prevent infection among children.

“At this point we’re strongly recommending them for treatment rather than for prevention,” she said.

To prevent flu, the drugs should be reserved for people at high risk of complications who have been in close contact with a known case, she said.