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Cold, Flu, & Cough Health Center

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1 Swine Flu Shot Enough?

Single Swine Flu Shot Gives Immunity in Early Tests
WebMD Health News
Reviewed by Louise Chang, MD

Sept. 10, 2009 - Adults are immune to swine flu after just one shot of vaccine, early test results suggest.

It's a surprise finding. Most experts expected that two shots of the vaccine -- given three weeks apart -- would be needed.

Now vaccine supplies may go twice as far, and may start working twice as fast as expected, suggests University of Washington researcher Kathleen M. Neuzil, MD, MPH, chairwoman of the flu vaccine working group of the Advisory Committee on Immunization Practices (ACIP), the independent panel that recommends vaccine policy to the CDC.

"On the basis of these data, it would be appropriate to begin vaccination with the use of one dose," Neuzil writes in an editorial accompanying a report of the study rushed into online publication by The New England Journal of Medicine.

Neuzil, a pediatrician, notes that children may still need two doses of the vaccine. But she says vaccine supplies "should not be held in reserve to be used for a second dose."

The finding comes from a clinical trial of the CSL H1N1 swine flu vaccine in Australia. Some 40% of the 195 million doses of swine flu vaccine purchased by the U.S. will be made by CSL, although other makers' swine flu vaccine is expected to be equally effective.

In the Australian study, Michael E. Greenberg, MD, MPH, and colleagues gave the vaccine to two groups of adults, one group age 18 to 50 and the other age 50 to 64. Half of the subjects in each group got a 15-microgram doses of the vaccine -- the same dosage being prepared for U.S. vaccines. The other half got a double 30-microgram dose.

Of the 120 volunteers who got the lower dose, 116 -- 96.7% -- developed at least the minimum level of anti-flu antibodies considered to be protective.

"The robust immune response to the H1N1 vaccine after a single dose was unanticipated," Greenberg and colleagues note. "Much of the current global pandemic planning is predicated on previous experience that two doses of vaccine are required to elicit a protective immune response in populations that are immunologically naive to a new influenza strain."

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