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LISA MULLINS: I'm Lisa Mullins. This is The World. Millions of Mexican children returned to school today for the first time since late April. The schools had been closed in an effort to control the spread of the H1N1 swine flu. That virus has now spread to at least 30 countries, and the World Health Organization today said that it's still watching the situation to see if a sustained pandemic is inevitable. Meanwhile, a new WHO study suggests that the number of people infected in Mexico likely runs in the tens of thousands. And the analysis suggests that the virus could cause a pandemic similar to the severity of the 1957 Asian flu. Christophe Fraser is the lead author of the study. He's also an epidemiologist at Imperial College in London. I think one of the key questions, Dr. Fraser, about this virus has been just how deadly it is. I mean, at first, the reports from Mexico suggested it was killing people at a very high rate. You have gone back, calculated the fatality rate. What did you find?

CHRISTOPHE FRASER: Well, I mean, I agree it's a key question. It's also one of the hardest ones to answer. What one needs to understand about the situation in Mexico is the relatively high burden of research or disease anyhow. And also that a lot of the surveillance really focused on serious hospitalized cases when the virus was first discovered. What was unknown was the amount of infections that were out there in the community that didn't require hospitalizations – and we estimated that around 23,000 people by the end of April were infected.

MULLINS: According to your research, what is the fatality rate of this virus?

FRASER: The fatality rate we estimate in Mexico – our best estimate is about not .4 percent. That was really based on the basis at the end of April. The situation has changed a little bit since then, but most of the uncertainty is coming actually not from really being able to reliably ascertain the cause of death amongst those who did die during that period.

MULLINS: So it's spreading, would you say, at a rapid rate but killing people at a relatively low rate?

FRASER: That might be one way of looking at it. I mean, that's consistent with what happened in 1957. We were almost doubling in all cases, in the order of one week to 10 days, for the epidemic to double in size.

MULLINS: That was the Asian flu in '57?

FRASER: And that's our estimate as to what's happening now.

MULLINS: When you look back right now as to how this swine flu, this H1N1 virus, began, your analysis seems to support the evidence that it may have begun over the wintertime in a town called La Gloria in the state of Veracruz?

FRASER: That's right. The outbreak started around the 15th of February, and the outbreak was finished by the time the virus was discovered at an international level, so it's not been possible to confirm all the other cases. So the cause of the outbreak within that village again is very consistent with what we know about influenza. It's a very small village with about 1500 inhabitants, of which 600 were infected. And we're all quite thrilled there were no fatalities within that village.

MULLINS: And one final question. For those who felt at this point that the potential of a pandemic was over, what do you say to them?

FRASER: Well, I would say that I mean, there are still uncertainties. I think the key uncertainty is really over the severity of the disease among cases, rather than whether this is going to continue to spread. So we would expect it probably to spread southwest more than northwest -- and the other thing we don't know is how this is going to compete when it starts missing those seasonal influenza viruses. Will the seasonal influenza virus be more transmittable and out-compete them? Will they mix together or will this virus just spread independently due to lack of cross-immunity? Because there were uncertainties, but it certainly looks like we will expect the virus to continue spreading for some time to come.

MULLINS: Christophe Fraser is an epidemiologist at Imperial College, London. His study on the pandemic potential of the H1N1 swine flu virus is being released today by the Journal of Science. Thanks a lot, Dr. Fraser.

FRASER: You're welcome.

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