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LISA MULLINS: With swine flu crossing oceans and borders, scientists are trying to figure out just how big a threat it poses globally. Marc Lipsitch is an epidemiologist at the Harvard School of Public Health. His work includes the study of pandemics. So the big fear, Marc, is that this outbreak of swine flu could spark a worldwide pandemic, endangering millions if not billions of people. Is there any evidence that that could happen?

MARC LIPSITCH: It is still a real possibility, but it is definitely too early to say that that is certain. The evidence so far of human-to-human spread in Mexico and outside of Mexico suggests that this is a virus that can transmit between humans. And it is a different virus from those that are circulating now, although not as different as pandemic strains usually are when circulating viruses. So it's difficult to tell, but I think there's reason to expect that this will continue to transmit, but the number of cases will go up where it already is, and the cases will appear where it isn't yet – or where they haven't appeared yet. The real question is how severe this will be. The 1968 pandemic, the last real pandemic that we had of influenza was overall no more lethal than a normal year of seasonal influenza. The 1918 pandemic is well known because it was much worse. And it's very hard to tell whether this virus is a particularly severe virus or not. And the reason it's so hard to tell is that it seems to be severe in Mexico. But we don't know how many people are having mild illness there, and it seems to be mild in the United States, but the number of cases is so small that we might just not have had any severe cases yet.

MULLINS: So how do we make sense of the inconsistency?

LIPSITCH: This reflects the problem that we often have in the early phases of an epidemic, which is that cases in Mexico were ascertained because people were being hospitalized with severe illness. Nobody was looking in Mexico – they are now – to find out whether thousands or tens of thousands or more non-severe cases in Mexico, which might not have come to attention, just as the American cases might not have come to attention except that we already knew about the severe cases in Mexico. So it may be that we're seeing the tip of the iceberg in Mexico in a good sense, meaning that we're seeing the severe cases. A large fraction of those people have gone onto die, “large” being several percent. And there may well be quite a lot of non-severe cases in Mexico, in which case the overall severity of this disease is not as bad as we think. If that's the case, then that can help to explain what's going on in the United States, where we've seen only a handful of cases – and maybe there just haven't been enough to even see the severe cases yet. The alternative would be that there's something different, that the United States cases for some reason are really different in kind from those in Mexico, and that most cases in Mexico are as severe as we know about, and most cases in the United States are as mild as the ones we know about. I think it's really too early to say, but given the number of cases and the way they were detected, the leading hypothesis still is that there are probably lots of cases in Mexico that we don't know about and that are milder.

MULLINS: Which would be good news?

LIPSITCH: Which would be good news.

MULLINS: There are countries that have instituted travel restrictions to Mexico. European Union officials are urging people to avoid any kind of non-essential travel to both Mexico and the United States. In your view, is that a wise idea or not?

LIPSITCH: Recommending against travel to an area with widespread disease, which is parts of Mexico at this point, makes sense from a personal perspective and also from the perspective of slowing the spread of the virus. It's not going to make a large difference in the amount of time it takes the virus to get to the rest of the world because the volume of travel is so great. If this is a virus that can spread widely, which it's appearing to be, then reducing travel by 50 or 90 percent isn't going to slow the spread very much. Discouraging travel to the United States at this stage is
probably quite premature, because the risk of exposure to an average person is going to be miniscule. That could change if the epidemic spreads, but I would then guess that if it spreads to be widespread in the United States, it probably also will have spread to other countries outside North America.

MULLINS: All right. Marc Lipsitch, Professor of Epidemiology at the Harvard School of Public Health. Thank you.

LIPSITCH: Thank you.

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