Single H1N1 Swine Flu Shot for Kids?

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Australian Study Shows That 1 H1N1 Shot May Protect Kids; CDC Disagrees

By Daniel J. DeNoon
WebMD Health News

Reviewed By Louise Chang, MD

Dec. 21, 2009 -- Can kids get by with just one dose of the H1N1 swine flu vaccine?

Yes, an Australian study suggests. No, says the CDC -- they'll still need two doses.

Australian researchers report that a 15-microgram dose of H1N1 vaccine -- double the dose approved for U.S. kids under age 3 but the same dose given to older kids -- raised anti-H1N1 antibodies to protective levels in more than 90% of children ages 6 months to 9 years.

But the CDC warns parents not to act on this information, noting that that in other studies, kids needed two doses of the H1N1 swine flu vaccine for protection.

Terry Nolan, MBBS, PhD, of the University of Melbourne, Australia, and colleagues gave kids either 15-microgram or 30-microgram doses of the vaccine. In the U.S., kids under age 3 get 7.5-microgram doses and other kids and adults get 15-microgram doses.

Each child got a second shot three weeks later. That second shot wasn't necessary, Nolan and colleagues suggest. The first one raised anti-H1N1 antibodies to protective levels in 92.5% of kids in the 15-microgram group and in 97.7% of the 30-microgram group.

The second dose gave protection to 100% of the kids. There were no serious adverse reactions.

"Our findings suggest that a single-dose 15-microgram vaccine regimen may be effective and well tolerated in children, and may have positive implications for disease protection and reduced transmission of pandemic H1N1 in the wider population," Nolan and colleagues conclude.

CDC flu experts Anthony Fiore, MD, MPH, and Kathleen Neuzil, MD, MPH, disagree in an editorial published along with the Nolan report in the Dec. 21 early-release issue of the Journal of the American Medical Association.

"It is premature to assume that only one dose will be needed to provide adequate protection for all young children based on these data," Fiore and Neuzil argue.

Why? The CDC researchers note that:

• The "protective" level of antibodies seen in the Nolan study is the level at which only 50% of people will be protected.
• Infants and kids not previously vaccinated all need two doses of any flu vaccine for protection.
• A significant percentage of the kids in the Australian study (up to a third of those over ages 3 to 9 years) had pre-existing antibodies to H1N1 -- suggesting that before entering the study, they may have had a symptomless swine flu infection that primed their immune responses.
• Anti-H1N1 antibody levels in kids who got just one dose of the vaccine were 30% lower than those seen in adults given one dose of the vaccine.
• The kids in the Australian study were healthy, and the vaccine is expected to be somewhat less potent in kids with chronic medical conditions, who are at highest risk of severe swine flu complications.

Nolan and colleagues, however, question the two-dose approach and suggest that a single, larger dose might be a better strategy to more quickly protect communities against H1N1 swine flu.

SOURCES: Nolan, T. Journal of the American Medical Association, published online Dec. 21, 2009.

Fiore, A.E. and Neuzil, K.M. Journal of the American Medical Association, published online Dec. 21, 2009.

News release, Journal of the American Medical Association.

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