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Swine Flu May Have Infected 63 Million Americans PDF Print E-mail
Tuesday, 23 February 2010 10:00


Bloomberg - Swine flu may have infected at least 63 million people in the U.S. last year, according to a study in Pittsburgh, where almost every second schoolchild probably caught the pandemic virus.

(Click here to view the "Flu or False" presentation for more information on this topic.

Blood tests on Pittsburgh residents found 45 percent of people aged 10 to 19 years had antibodies against the new H1N1 flu strain. About 22 percent of people across all groups developed immunity to the virus by early December and a quarter of those born in the 1920s may have already had protective antibodies before the pandemic resulting from prior flu infection, researchers at the University of Pittsburgh found.

The findings, reported online yesterday in the Public Library of Science, suggest a fresh wave of swine flu infections isn’t likely unless the virus mutates or people become more susceptible to infection. A World Health Organization advisory panel is holding a teleconference tomorrow to discuss whether the first influenza pandemic in 41 years has peaked.

“With current estimates of seroprevalence and continued increases in population due to vaccination, a significant change in viral antigens or a change in population immunity would be required for further disease spread,” Ted Ross, associate professor of microbiology at the university, and colleagues wrote. “We cannot rule out the possibility that geographical pockets of limited immunity may be present in which a third wave may yet occur.”

Symptom-Free Cases

At least 15,921 people have died from swine flu as the fast-moving pandemic spread to 212 countries and territories since its discovery in North America in April, the WHO said in a Feb. 19 statement. The global tally underestimates the actual number as many deaths are never tested or recognized as influenza related, the Geneva-based agency said.

In yesterday’s study, researchers looked for infection- fighting antibodies against the 2009 pandemic flu strain in 846 anonymous blood samples collected in November and early December from people in southwestern Pennsylvania’s Allegheny County ages 1 month to 90 years. The tests identified people who caught the virus, including those who didn’t develop a fever, cough or other flu-like symptoms.

The researchers compared the results against tests on blood samples collected in 2008, of which 6 percent contained antibodies that protected against swine flu, probably as a result of infection from a related influenza strain.

Children and adolescents in the 10- to 19-year age group had the highest prevalence of swine flu antibodies, while 29 percent of blood samples from children younger than 9 years tested positive. Residents in the 70- to 79-year age group had the lowest prevalence rate of 5 percent.

When the researchers extrapolated their findings across the county’s 1.2 million residents, they found swine flu antibodies in 21.5 percent of people, including more than 70,000 school-age children.

“Extrapolating these results further to the entire US population, we estimate that 63 million persons became infected in 2009,” the authors wrote.

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