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Swine Flu Shots May Come Too Late to Stem Fall Outbreak: Study

Scientists' projections have infections peaking in mid-October, ahead of vaccine delivery

THURSDAY, Sept. 10 (HealthDay News) -- The height of this year's fall H1N1 swine flu outbreak is expected in October, but a planned vaccine may arrive too late to stop it, a new study suggests.

The first batch of the new H1N1 vaccine, totaling only 45 million doses, is not expected before mid-October, according to the U.S. Centers for Disease Control and Prevention. Clinical trials are still underway, and on Wednesday the U.S. National Institutes of Health announced it had just begun its trials of the vaccine in one high-risk group, pregnant women.

So, a vaccine arrival date of mid-October may be too little, too late. Because even if a vaccine were available in time, 70 percent of the U.S. population would need to be vaccinated to stop the spread of this pandemic virus, the study authors projected.

"If the H1N1 flu follows the pattern we are expecting we should see a sizable outbreak in the U.S. starting in early September and probably peaking in mid- to late- October," said lead researcher Ira Longini, a professor of biostatistics at the University of Washington in Seattle.

A vaccine is the best way to stem the epidemic, Longini said. "However, in the current situation we could come up short," he said. "It looks like we will be too late if everything stays the same."

The CDC has already reported that the number of H1N1 cases is already on the rise in the United States, particularly in the Southeast where school opened early, and on college campuses.

But infections with the H1N1 virus continue to be mild, producing symptoms typical of run-of-the-mill seasonal flu.

Although the vaccine may not be all that helpful to curb a fall epidemic, it is still important to get vaccinated, Longini said. That's because the H1N1 flu is expected to linger in the population for at least 20 years, with new outbreaks occurring this coming winter and spring, and for years to come.

The report is published in the Sept. 10 online edition of Science.

For the study, researchers created a model to estimate how fast the H1N1 flu might be transmitted from person-to-person. The model showed that for a vaccination program to effective, 70 percent of children aged 6 months to 18 years would have to be vaccinated first, as well as people in high-risk groups. Those groups include pregnant women and those with chronic illnesses, those with immune system disorders, such as people undergoing chemotherapy, and health-care workers.

Longini noted that it may take two doses of the vaccine, given three weeks apart, to provide full protection. This means that it would take about four to six weeks before the vaccine would create enough antibodies to the virus to protect a person from getting the H1N1 flu, he said.

Since the virus spreads especially fast in schools, vaccinating school children is essential, Longini said. According to the study, a single infected child can be expected to pass the virus on to an average of 2.4 other children at their school.

While other strategies, such as social distancing and antiviral medicines are partially effective in slowing the spread of flu, vaccination is the most effective way of controlling a pandemic influenza outbreak, Longini said.

Vaccination increases immunity throughout the population, which slows the spread of infection, which in turn reduces overall illness, hospitalizations and deaths, the researchers noted.

In predicting how fast the H1N1 flu virus spreads, the researchers first estimated how many people one person with the virus will infect, and came up with a range of between 1.3 to 1.7 people. At a 1.6 infection rate, the pandemic could infect a total of 2.2 billion people worldwide over a year. That's an overall illness rate of 32 percent of the entire population of a city or country, the researchers explained.

Dr. Pascal James Imperato, dean of the School of Public Health at SUNY Downstate Medical Center in New York City, said, "this study confirms that children and young adults are the primary transmitters of the H1N1 swine influenza virus and that vaccination will mitigate the spread of the disease."

However, Imperato is concerned that the model created by the researchers to make their estimates is flawed and the swine flu's eventual spread could be even greater.

"The difficulty with these estimates is that many cases of H1N1 swine influenza are sub-clinical or so mild as to escape reporting to health officials," Imperato said. "Thus, the degree of spread could be much greater than these estimates would indicate."

In related news, a British study published Sept. 10 in the journal Nature Biotechnology appears to confirm that the H1N1 virus infects cells much deeper in the lungs than does the regular seasonal flu.

The difference comes in the H1N1 virus' ability to attach to a receptor lying on the outside of cells typically found at a greater depth in the lung. This could explain the more severe illness experienced by certain patients infected with swine flu, the study authors said.

More information

For more information on H1N1 swine flu, visit the Flu.Gov.

SOURCES: Ira Longini, Ph.D., professor, biostatistics, University of Washington, Seattle; Pascal James Imperato, M.D., dean and distinguished service professor, School of Public Health, SUNY Downstate Medical Center, Brooklyn, N.Y.; Sept. 10, 2009, Science; Sept. 10, 2009, news release, Imperial College London

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