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Linda Wadsworth of the University of Maryland, left, draws blood from Ethan Houley, 11, of Annapolis, Md., right, as Infectious Diseases Specialist Dr. Julia Hutter, second from left, and Gina Parsons of the University of Maryland, second from right, look on at Annapolis Pediatrics in Annapolis, Thursday, Aug. 27, 2009. (Susan Walsh/Susan Walsh/The Associated Press)
Linda Wadsworth of the University of Maryland, left, draws blood from Ethan Houley, 11, of Annapolis, Md., right, as Infectious Diseases Specialist Dr. Julia Hutter, second from left, and Gina Parsons of the University of Maryland, second from right, look on at Annapolis Pediatrics in Annapolis, Thursday, Aug. 27, 2009. (Susan Walsh/Susan Walsh/The Associated Press)

Kids will require two H1N1 shots Add to ...

Multiple vaccines. Multiple tear-filled encounters with a needle.

Parents of children under 10 could face an eventful influenza schedule this year as the first U.S. trial of swine flu vaccine in kids shows the youngest will require two H1N1 shots, on top of as many as two seasonal flu doses.

But there's good news for children 10 and over: They appear to need just one dose of the pandemic vaccine.

The findings, released Monday by the U.S. National Institute of Allergy and Infectious Diseases, come as countries prepare to administer the H1N1 vaccine and worry that global supplies of the drug may be limited.

The results for older children, along with earlier studies showing that healthy adults will also need one dose, means that there could be enough of the new drug to go around.

Anthony Fauci, director of the NIAID, said the H1N1 vaccine is "strikingly similar" to the seasonal flu shot. With seasonal flu, young children who have never received a shot must also receive two doses to be adequately protected.

"The initial results are encouraging. As we had hoped in children, the 2009 H1N1 influenza vaccine is acting just like the seasonal flu vaccine," Dr. Fauci told reporters yesterday.

The trial in the U.S. involved unadjuvanted vaccine - one without a chemical booster. Canada plans to use a smaller amount of vaccine with an adjuvant, which increases production. As a result, it remains unclear whether the U.S. dosage regime will work in Canada.

The country has ordered 50 million doses from GlaxoSmithKline for all Canadians. The vaccine will be rolled out in mid-November at the latest.

The Public Health Agency of Canada said the U.S. data will be used in evaluating the safety and effectiveness of the vaccine. The agency said GSK's clinical trials, using the adjuvanted vaccine that Canada has ordered, will in time show whether one or two doses is required in children.

The U.S. study involved 600 subjects between the ages of six months and 17 years.

Researchers found that 76 per cent of those age 10 and older who received a single shot had a strong enough immune response after eight to 10 days to assume they would be protected.

Only 25 to 36 per cent of the younger age groups had a strong response, indicating that they need a second shot. The two doses would be 21 days apart.

Young children didn't respond well after one dose because they have had less exposure to influenza and their immune systems are not as strong as those of adults, Dr. Fauci explained.

Anne Matlow, director of infection prevention and control at the Hospital for Sick Children, said parents may be reluctant to see their children receive multiple shots. But this virus has disproportionately affected younger people, unlike seasonal flu that mainly burdens the elderly.

"I'm a big advocate of vaccines," Dr. Matlow said. "If you can prevent something, why would you not want to? That's my philosophy."

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