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Harvard epidemiologist Marc Lipsitch, an expert on pandemics, has advised the U.S. government on the possible trajectory of the H1N1 influenza virus. He recently spoke with us about what we might expect from the virus this year.

Public School Insights: Do you have any predictions for the kind of flu experience we can expect in the coming months?

Lipsitch: I think that the first thing to say about any predictions about flu is that they are probably wrong. The goal is to be as close to right as possible, but everybody who spends time working on flu becomes very quickly humbled, not making predictions even about the near-term future.

Having said that, I think that the current situation clearly shows that this virus can spread very effectively even when it’s not the usual flu season of December, January and February. I think that given the precedent of prior pandemics and the evidence we have about how well this virus spreads, it’s likely that the early indications of rapid takeoff in August and September will continue and that we’ll have a growing epidemic that will probably peak sometime in October. That will then come down and most the activity will be over by the end of the calendar year, if I had to guess. That is exactly what happened in two of the three twentieth century pandemics, and this flu is showing signs of an early fall takeoff now.

An implication of that timeline is that the largest proportion of the vaccine being manufactured will not be available until much of the activity is already over. Therefore, there will be a real need to focus prevention and treatment efforts on the people at highest risk of a severe outcome.

The good news so far is that the virus causes a mild infection in the large majority of people. I think data that have come from the Southern Hemisphere, especially Australia and New Zealand, in the last three or four weeks have really underscored the idea that this is probably not going to be as severe as some of us had previously thought.

One of the reasons why everyone’s uncertain about how bad this will be is that it is very hard to count the number of cases of this flu. Nobody really knows how many there have been. So when you know how many people have died, you don’t know whether that is out of lots of cases or out of a few cases.

The best guess, I think, is that this flu will be very mild in the elderly, as it has been up to now, and that it will be severe in a number of people with preexisting conditions. The CDC has identified a list of the groups at risk, although they haven’t described the magnitude of the risks yet. Pregnant women are clearly one of those groups, and people with certain underlying health conditions are other groups.

This flu will also be very severe in a small number of people who don’t have known preexisting conditions but are healthy, mainly adults. We don’t know why that is, but it seems to be happening so far.

Public School Insights: There has also been a lot of talk about the susceptibility of younger people, particularly children. Is that a pattern you are seeing?

Lipsitch: Children are certainly very susceptible to getting infected. And because they are the most likely to get infected, they have had the highest rate of hospitalizations and certainly the largest rate of cases identified.

The good thing is that they also seem to be the age group where this flu is generally the mildest. If the burden of disease shifts from being mainly in children to more in adults, we could see an increase in the average severity of the illness. But so far children seem to be more likely to get it if they are exposed, and they are also more exposed because they are in schools.

Public School Insights: Not to get into more alarmist territory, but there are some people who have speculated about the possibility for the virus itself to become more severe. Is that science fiction?

Lipsitch: It’s not science fiction. It’s what most people think happened in 1918.

We are trying to imagine the future based on three data points from the past—the three pandemics that are fairly well documented from the twentieth century. They are all very different from one another, so the best expectation is that this will be different still.

In 1918, what probably happened was a mutation of the flu virus. Some people think something else happened. In any event, there was a major increase in the severity of the flu between the spring and the fall.

The reason to have relatively little concern about this in the short run is that there is just no evidence that the virus is changing right now. And unlike in 1918, when the influenza virus was not even known, there is very extensive tracking of the virus now. There are efforts to genetically sequence and test biological isolates of the virus to track any possible changes. I have heard there is no evidence of any kind that this virus is becoming more severe.

Public School Insights: Does our current experience offer a training opportunity for governments and institutions like schools, so that we can prepare ourselves for what could happen in future years?

Lipsitch: I think so. I think we will learn a lot from this.

One thing we have learned is that it is really difficult to tailor a response based on exactly how severe the virus is. We learned this in the spring. If we knew how bad it was, there was pretty broad agreement about what should be done. But we didn’t know how bad it was, especially at the very beginning of the epidemic in late April and early May.

There was, in some sense, a reaction that would have been too strong if we knew then what we know now, but we didn’t know that yet. So it’s been an exercise in caution, and the next one will be different. This is about as mild a pandemic as has been observed, which will hopefully be true of future ones as well.

Public School Insights: So people who have accused educators and others of being too alarmist in their reaction to this flu don’t really understand the difficulty of predicting what is going to happen and the importance of being prepared for what could happen.

Lipsitch: Right. My colleagues and I wrote about this in The New England Journal of Medicine in May. Every decision that has been made in this pandemic had to be made before we had the quality of data we would like to have. But just like any other dynamic situation, you have to make your decisions based on what you know and what you realize you don’t know. So you do your best and you try to be conservative and do things that are appropriate in case it’s on the more severe end of what you think it could be.

I think the lesson of the 1976 swine flu was that you do things that are reversible whenever possible. You make your decisions and you pay the costs of those decisions, but you don’t make irreversible decisions until you really have to. I think that lesson has been really well learned this year as well.