The majority of the seasonal flu vaccine has already been shipped ahead of flu season in the Northern Hemisphere, but candidate vaccines against pandemic H1N1—which had killed nearly 1,800 people as of August—are just entering clinical trials. Health officials predict that the first doses should be available, at the earliest, in September.

“The hope is that we will have vaccine available in time to still have a substantial impact in terms of preventing cases during what would normally be the peak season,” says Art Reingold, head of the epidemiology division at the University of California in Berkeley and a member of the World Health Organization's Strategic Advisory Group of Experts on Immunization.

The planned clinical trials of H1N1 vaccine, which will involve about 30,000 people in at least nine countries, will provide short-term safety data and help health officials decide whether one or two doses will be needed, how large each dose needs to be and whether the seasonal flu shot should come before, after or alongside the pandemic vaccine. Seven of these trials are being sponsored by the US National Institute of Allergy and Infectious Diseases (NIAID) to test inactivated H1N1 vaccines from Sanofi-Aventis, the world's largest flu vaccine manufacturer, and Australia-based manufacturer CSL Biotherapies. Other companies, including Novartis, GlaxoSmithKline and Baxter, are conducting their own trials as well.

One manufacturer, China-based Sinovac, already has preliminary results. On 18 August, the company announced that data from a trial of 1,600 people in Beijing suggest that its vaccine against H1N1 is safe and that one dose will be sufficient. As this issue of Nature Medicine went to press, the company had not yet specified the dose needed to raise an immune response.

Even with manufacturers working round the clock, vaccine supplies might outstrip initial demand. Growing the vaccine virus in chicken eggs—the conventional method of flu vaccine production—is a time-consuming process, and the seed strain for H1N1 has been growing more slowly than anticipated. A new method involves growing vaccine virus in cell culture, but this process has not been approved in the US. Anthony Fauci, head of NIAID, says that cell-based vaccines would only be used “if we run into an emergency where we don't have enough of the egg-based vaccines.”

Although manufacturers of the inactivated H1N1 vaccine are reporting lower than expected yields, MedImmune, a subsidiary of AstraZeneca, is getting higher than expected yields of its live nasal mist vaccine—about 80 doses per egg compared to 1 or 2 doses per egg for the inactivated vaccines. “There is a possibility that a much larger quantity of the live vaccine will be available early. And this could be used very advantageously by immunizing healthy schoolchildren and working adults,” says W. Paul Glezen, a flu expert at Baylor College of Medicine in Houston, Texas. “Those are the groups that spread the virus in the community.