(26-09-2009) It may take two doses of vaccine to protect against swine flu instead of the one dose used in a regular flu season.

According to scientists, it takes the immune system a while to ramp up to fight unique types of influenza. Since we've never been exposed to this brand-new virus, formally called the 2009 H1N1 strain, our bodies will act like those of young children. When children under age 9 get their first flu vaccine -- the kind that protects against regular winter flu -- one dose doesn't spark as much immune protection as the doses they'll get over the next years. So those first-timers are given two shots, a month apart, for good protection.

Indeed, Chinese officials say they're about to approve vaccines that seem to protect against swine flu in one dose, results that the World Health Organization calls encouraging although experts say more results are needed from other vaccine makers. Different companies make different formulations.

In the U.S., scientists insisted that studies of swine flu vaccine include two doses for everyone, about three weeks apart, so they could measure the booster effect before making a final decision on whether one dose or two is best. Only in the last few days have children begun getting their first dose.

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