To the surprise of many scientists, preliminary results from a clinical trial of a vaccine for pandemic H1N1 influenza in Australia suggest that one dose of vaccine might be sufficient to provide protective antibodies, a finding that could double the number of available doses and greatly reduce the logistical problems associated with giving two doses.
The results, reported online Thursday by the New England Journal of Medicine, confirm results from a Chinese study of a similar vaccine.
Unlike the Chinese vaccine, however, the Australian one will be used in the United States, accounting for about 20% of the domestic supply.
Researchers had feared that two doses of vaccine for H1N1, referred to as swine flu, would be required because most people younger than 65 have little or no residual immunity to closely related viruses.
Seasonal flu vaccines, in contrast, work with only one dose because people have some residual immunity from exposure to other influenza viruses and vaccines. The exception is young children receiving their first dose of flu vaccine; they require two doses.
Dr. Michael E. Greenberg of Australia's CSL Ltd. and his colleagues studied the company's vaccine in 240 subjects, half younger than 50. Ninety-five percent of those who received a 15-microgram dose of antigen -- the standard dose used in seasonal flu vaccines -- developed a protective antibody response within 21 days after vaccination, and usually within eight to 10 days.
No deaths or serious adverse effects were observed, only tenderness and pain at the injection site and some headaches.
Elsewhere, MedImmune of Gaithersburg, Md., said it would ship as many as 5 million doses of its H1N1 vaccine, which is inhaled, by the end of September.
MedImmune researchers had found that the strain of virus it uses for its vaccine grows faster than that used by other manufacturers, speeding up the testing and manufacturing process.
The vaccine, called FluMist, is not approved for use by pregnant women and people with underlying medical conditions.