The reason for the urgency: Some past pandemics were preceded by “herald waves” of a flu strain that surfaced at the end of one flu season, only to return with far greater consequences the next flu season.

“We are mindful that pandemics of influenza have sometimes come in waves,” Dr. Anne Schuchat, the U.S. Centers for Disease Control and Prevention’s interim deputy director for science and public health program, said during a Tuesday afternoon news conference. “The very severe 1918 pandemic had a moderate herald wave in the spring and a much more severe second wave in the fall. So that very terrible experience of 1918 is in our minds.”

Some estimates have placed the worldwide death toll from the 1918 outbreak — often referred to as the “Spanish Flu” — as high as 40 million people.

“We are really on a fast track, over the next to eight to 10 weeks, to learn as much as we can as this virus heads to the Southern Hemisphere [where flu season is just beginning] and to strengthen our planning for the surge of illness that we expect to experience here in the fall,” Schuchat added.

Scientists will be looking to see if the H1N1 swine flu virus mutates or becomes resistant to antiviral medications, or is more easily spread among people, she said.

Schuchat said there’s no way to tell now if the H1N1 virus will be more virulent when — and if — it returns to the Northern Hemisphere with the approach of winter. “Whether it will dominate among the seasonal flu viruses or whether it will disappear is not predictable right now,” she said.

To date there have been 7,927 confirmed and probable cases of infection in the United States, the CDC reported Wednesday. Most of the cases have been mild and patients have recovered quickly.

The CDC was reporting 11 deaths linked to the swine flu, and all of the victims had underlying health problems before they were infected.

The World Health Organization said Wednesday that 48 countries have reported 13,398 cases of infection, including 95 deaths, most of them in Mexico, where the outbreak began.

The CDC said last week that progress was being made toward the development of an H1N1 swine flu vaccine, with two promising candidate viruses for use in such a shot. And U.S. Department of Health and Human Services Secretary Kathleen Sebelius said Friday that the federal government was allocating $1 billion to the search for a swine flu vaccine.

In the United States, most cases of the swine flu continue to be no worse than seasonal flu. Testing has found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza, according to the CDC.

The CDC says some older people may have partial immunity to the H1N1 swine flu virus because of possible exposure to another H1N1 flu strain circulating prior to 1957. So far, 64 percent of cases of swine flu infection in the United States have been among people aged 5 to 24, while only 1 percent involves people over 65, officials said last week.

This is a great idea in my opinion.

* Need to vaccinate earlier for seasonal flu

* Eighth death reported in U.S.

* New H1N1 strain still affecting younger people more

U.S. health officials said on Wednesday they are considering starting the vaccination campaign for seasonal flu earlier this year to make room for a possible second round of shots against the new H1N1 flu.

The United States also reported its eighth death from the new swine flu virus, in a patient in Arizona.

“If possible we do want to have an earlier rollout of seasonal vaccine,” Dr Daniel Jernigan of the U.S. Centers for Disease Control and Prevention told reporters in a telephone briefing.

He said the CDC would work with manufacturers of influenza vaccines and a committee of doctors that advises on vaccination policy to see if that would be possible and desirable. Flu vaccination usually starts in September in the United States and peaks in November.

Vaccine experts agree it would be better to launch a second round of vaccinations against the new H1N1 strain instead of trying to add it to the seasonal flu vaccine or replacing one of its three components with the new H1N1 virus.

“The production of seasonal flu vaccine is nearly complete,” Jernigan said.

On Wednesday, the CDC reported 5,710 confirmed and probable cases of the new H1N1 strain in 48 states, with eight deaths. Globally, more than 10,000 cases have been confirmed with 80 deaths, most in Mexico.

Jernigan said it is clearly still affecting younger people more in the United States — 40 percent of the 247 people who have been hospitalized with serious illness from the new H1N1 strain are aged 19 to 49, he said.

Seasonal influenza puts 200,000 Americans in the hospital every year and is a factor in 36,000 deaths, but 90 percent are over 65 or have chronic diseases such as asthma and heart disease that make them more vulnerable. With the new H1N1 flu, Jernigan said, 70 percent of hospitalized patients have an underlying condition.

AGE 50 ADVANTAGE

Only 13 percent of people who have been hospitalized with swine flu are over 50, Jernigan said, and he said tests of blood among the general population may eventually show whether people born before 1957 may have been infected with an older H1N1 strain that was close to the swine flu strain. They may then have some lifetime resistance to the new strain, he said.

In 1957, a new flu strain called H2N2 caused a pandemic and became the predominant strain of flu circulating globally. There is a seasonal H1N1 strain that still circulates but it is only a very distant relative of the new swine flu strain.

Seasonal flu is also still infecting people but Jernigan said 78 percent of U.S. cases tested for influenza are the new H1N1 strain. The CDC and state health departments are only doing targeted testing and say the true number of flu cases across the country is likely more than 100,000.

Jernigan said many flu patients are getting antibiotics — which prevent so-called secondary infections — but not enough are getting the antiviral drugs Tamiflu and Relenza, which are known to help symptoms.

Tamiflu, or oseltamivir, is made by Roche AG (ROG.VX) and Gilead Sciences Inc (GILD.O) while the inhaled drug Relenza, or zanamivir, is made by GlaxoSmithKline (GSK.N) and Biota Inc (BTA.AX).

The new flu seems to be about as contagious as seasonal flu, Jernigan said.

He was unable to say why so many more people are infected in the United States than in countries such as Britain, which reports 102 cases, but said there may have been more U.S. travelers to Mexico and more time for the virus to spread.