A shot against pandemic flu: vaccines would play pivotal role in response.

Mass vaccination should be the linchpin of the U.S. response to an influenza pandemic, new computer simulations suggest. Other measures, including treating people with antiviral drugs, closing schools, and restricting travel, could slow the spread of the virus but would be unlikely to halt an outbreak of a highly contagious flu, say the government-funded researchers who conducted the simulations.

In another development, doctors announced last week that, for the first time, an experimental vaccine appears to protect some people against the H5N1 avian flu (SN: 9/10/05, p. 171). Other vaccines are undergoing tests against that lethal virus, which currently doesn't spread among people. But scientists predict that the avian-flu virus could someday give rise to a fast-spreading strain against which people would have less immunity than they do to a typical winter flu.

In the computer study, Timothy C. Germann of Los Alamos (N.M.) National Laboratory and his colleagues simulated how a pandemic flu would spread among 281 million U.S. residents. Even if vaccination gives people only partial immunity to a pandemic strain, widespread flu shots would be the single most effective response, the researchers report in the April 11 Proceedings of the National Academy of Sciences.

Use of antiviral drugs would reduce the scale of the outbreak if the virus weren't highly contagious. But if the flu spread readily, demand for the drugs would quickly outstrip the nation's supply.

Closing schools, restricting travel, and imposing other "social-distancing" measures being considered would slow the outbreak, potentially buying time for scientists to tailor a vaccine that has maximum efficacy against the pandemic strain, the researchers predict. But by themselves, those measures would have only a small effect on the number of people eventually infected, Germann says.

In the new vaccine study, 54 percent of the volunteers who got two high-dose shots a month apart had a strong immune response, which scientists expect to be protective against avian flu. Lower doses were less effective at stimulating immunity, and a single shot of any dose produced little measurable effect.

Ensuring that each vaccinated person gets a second shot might be difficult during a pandemic, says John J. Treanor of the University of Rochester (N.Y.) Medical Center. He led the vaccine study, which included 451 volunteers. It appears in the March 30 New England Journal of Medicine (NFJM).

The high dose needed to trigger a strong immune response suggests that the world's vaccine manufacturers probably could produce enough vaccine for only 75 million people a year, comments Gregory A. Poland of the Mayo Clinic College of Medicine in Rochester, Minn. For each year's winter flu worldwide, vaccine makers can produce enough doses for 900 million people, he notes in the same issue of NEJM.

Treanor says that his team and other groups are testing alternative vaccination methods, including adding ingredients that might stretch the number of effective doses.

For their simulations, Germann and his colleagues assumed that 250 million doses would be distributed in the United States during a 6-month pandemic. The Department of Health and Human Services currently has enough of the experimental vaccine to treat 4 million people at the highest dose used in the study.