Sept. 10 (Bloomberg) -- Swine flu vaccinations planned to start in October will come too late to prevent the first wave of fall infections, said researchers who did a study on the best way to tame the pandemic.

If at least 70 percent of the U.S. had been vaccinated against swine flu this month, the spread of the disease could have been slowed to that of a mild seasonal flu, according to a study published today in the journal Science. If widespread vaccination starts next month, that may still blunt the impact of the swine flu pandemic this winter.

A targeted program that starts in children and reaches 70 percent of the population would be the most effective way to curb swine flu, according to the research led by Ira Longini, a professor of biostatistics at the University of Washington. Pandemic flu strains often cause more deaths and hospitalizations for several seasons after the first wave.

“There is some uncertainty when the peak will occur,” said Longini, in an e-mail today. Vaccination in October “would still be valuable for future population level protection. There could be a second winter wave.”

The study found children are especially important because during a school outbreak each child infects an average of 2.4 people, the researchers wrote.

Vaccines in Production

More than 20 manufacturers are planning to produce swine flu vaccines, and medical trials for safety and effectiveness have already started in people, the authors wrote.

Vaccines are slated to arrive in mid-October, a time health officials say could be the peak of outbreaks, and may not provide immunity until November, too late to hold off outbreaks triggered by students returning to schools.

“If the peak is in late October, the vaccinations, which will probably cover 15 percent of the population, will be too late to mitigate the first fall wave,” Longini added in a telephone interview today. About half the population will contract the swine flu virus in the first wave, so October vaccinations may still protect the other half against later waves, he said.

The median age of those with the pandemic virus has been 12 to 17 years, according to the World Health Organization in a statement on July 24 citing data from Canada, Chile, Japan, U.K. and the U.S.

45 Million Doses

The U.S. will receive 45 million vaccine doses by mid- October, and 20 million more each week after that until the full order of 195 million doses from five companies has been received, said Bill Hall, a spokesman for the U.S. Department of Health and Human Services on Aug. 19. Some cities, such as New York, will provide free swine flu vaccinations at schools, fair grounds and union halls.

Swine flu, known as H1N1, may infect 30 percent to 50 percent of the U.S. population, according to a planning scenario released Aug. 24 by outside advisers to the White House.

About 30 percent to 40 percent of the flu transmissions occur when one member of a household infects another, today’s research report said. Another 20 percent of transmission takes place in school and the rest happen in other settings, such as the workplace, according to the report.

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net.

Last Updated: September 10, 2009 14:00 EDT