U.S. trials of swine flu vaccines demonstrate that only one dose is needed to immunize people under the age of 65. In addition, the immune response is very rapid, with people being protected only eight to 10 days after vaccination.

This means that people can be protected much earlier than was thought. The U.S. would like to vaccinate 160 million Americans by the beginning of December. This includes children, young adults, pregnant women, and people with asthma, diabetes, or heart disease. As is standard, children who have never been vaccinated will get a double dose.

Officials would like to get the vaccine out as soon as possible since the flu is spreading rapidly and earlier than normal in the season. Now it looks like it may start by mid October.