DER SPIEGEL: You predict that up to 60 percent of adults could become infected with the novel coronavirus. Isn’t that alarmist?

Lipsitch: I don’t think so. It’s of course a projection and, like any projection, it could be wrong. But if you have a reproductive number of an infectious disease of around two, which seems to be the estimates that we’re getting right now (Eds: meaning that, on average, each infected person transmits the disease to two other people), then at a minimum, half the adult population needs to become infected before the spread can stop permanently. This is not an ungrounded estimation, but simply the basic math of epidemics.

DER SPIEGEL: Back in 2003, the World Health Organization warned that SARS could become a global epidemic. In the end, however, there were significantly fewer cases than the coronavirus has already caused as of today.

Lipsitch: Yes, but there was the risk that it could have become an epidemic. However, with SARS, almost all transmissions seem to have been in symptomatic patients, so you could control the spread more easily. That seems to be different with the virus that causes COVID-19. And our job as epidemiologists is to think about worst-case scenarios so we can spur the right actions to stop an epidemic.

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