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It was back-to-school time for much of the U.S. this week, as millions of students bustled into classrooms to start the new year. But compared with school years past, this academic season has been decidedly more fraught, since it marks what could be the full-scale return of the H1N1 influenza virus.

If previous flu pandemics are any measure, we may see spikes in infection once school gets under way. Kids in classrooms are major spreaders of infectious disease; they get sick, infect one another, then bring the disease back home. That's why officials are trying to get the new H1N1 vaccine tested and ready for use as soon as possible — the longer America's schoolchildren go unprotected, the bigger the H1N1 pandemic could become.

A new study in the Sept. 10 edition of Science makes the case for widespread and speedy immunization, suggesting that doing so could stifle the pandemic. A team of researchers led by Ira Longini, a biostatistician at Fred Hutchinson Cancer Research Center, used data from earlier H1N1 outbreaks this year in the U.S. and Mexico to model how the pandemic is likely to unfold this fall. The team found that by first vaccinating children, then adults, until 70% of the U.S. population is covered, officials would be able to all but stop the pandemic.

The problem is that, according to the model, vaccinations would have to begin by mid-September — but the first batch of the vaccine isn't due until October. "Clearly the best strategy is to vaccinate schoolchildren first," said Longini. "The problem is the supply of the vaccine." That means the disease could begin to spread, and quickly, before protection from vaccinations could take hold.

The Science study also provided the first estimate of the person-to-person transmissibility of H1N1. The researchers calculate that every person infected by the virus will go on to infect 1.3 to 1.7 other people on average. (That number will probably be higher for schoolchildren — in one outbreak at a private school in New York City in the spring, each sick student infected 2.4 classmates.) As flus go, that makes H1N1 more transmissible than most — on par with the moderate 1957 Asian flu pandemic — which makes it particularly important to get a large chunk of the population vaccinated early.

Vaccine trials are currently under way in the U.S. and around the world — and the early findings suggest that the immunization is safe and effective. In the U.S., the National Institutes of Health recently opened trials to pregnant women and children to test the safety and efficacy of the shot, and so far, the vaccine appears suitable for those groups.

There's more good news from the international front. In the New England Journal of Medicine on Thursday, researchers in Australia and Britain reported the early findings of their H1N1 vaccine studies. Preliminary data from the Australian trial showed that 21 days after getting one shot, 96% of the 240 trial volunteers ages 18 to 64 generated an impressive amount of antibodies to the virus. The results were "unanticipated," according to the authors; health officials had expected that people would need two doses of the vaccine for full protection because H1N1 is a new flu virus to most of the population.

It's not clear yet how much immune response is necessary to protect against infection or illness, but if the trials' final data confirm that one dose of vaccine is sufficient, it could mean that twice as many people as expected could be immunized in mid-October, when the U.S. government intends to make available the first batch of 45 million doses. The Australian researchers stress that while their results are encouraging, they are also incomplete; they have not compared the response in these vaccinated volunteers to a group of unimmunized controls, so it's still too early to decide whether it's safe to do away with the second inoculation.

In the U.K. study, scientists explored another question — whether the current vaccine doses would still be effective if they were halved, by being diluting with an adjuvant (currently, the U.S. government has not approved any vaccines with adjuvants, but some have been approved in other countries). The answer, it seems, in another early look at the data, is yes. Even a single dose of the watered-down version of the vaccine produced enough antibodies to protect against infection in 80% of the 175 people studied.

If borne out, the results suggest that more vaccine may be available for more people and that more people could be protected against H1N1 sooner. If only a single dose is required, that means vaccines could yield protection within three to four weeks after inoculation, instead of the six to eight weeks it would take if two doses were needed.

The question now is whether we can test and produce vaccine faster than H1N1 can spread. Act quickly, and we might be able to stifle the disease in its earliest stages. But even if the vaccines arrive too late to stop H1N1 from spreading rapidly now, they can help build population-level resistance to the disease going forward. The Science study estimates that without vaccines, the virus could infect as many as 2.2 billion people worldwide over the course of the year. "The virus will be with us for many years to come in many forms," said Longini. "It's important to start building up resistance now."

• Alice Park and Bryan Walsh