The Coronavirus, by the Numbers

What if you’re not in public health, but are thinking about your own personal chances and what your behavior should be?

If you imagine you’ve got a reproduction number of two, each person’s infecting two others, on average. But some situations are more likely to spread infection than others. We’ve found for things like Covid-19, it’s close-knit interactions that seem to be most important.

What we need to think about — and what a lot of our modeling is certainly thinking about — is not just how much transmission is happening, but where is that transmission happening. If you’re going to change your behavior, think how to reduce those risky situations as much as possible.

If you were the average person, what would you pay attention to — in terms of the news and the numbers?

One signal to watch out for is if the first case in an area is a death or a severe case, because that suggests you had a lot of community transmission already. As a back of the envelope calculation, suppose the fatality rate for cases is about 1 percent, which is plausible. If you’ve got a death, then that person probably became ill about three weeks ago. That means you probably had about 100 cases three weeks ago, in reality. In that subsequent three weeks, that number could well have doubled, then doubled, then doubled again. So you’re currently looking at 500 cases, maybe a thousand cases.

I think the other thing that people do need to pay attention to is the risk of severe disease and fatality, particularly in older groups, in the over-70s, over-80s. Over all we’re seeing maybe 1 percent of symptomatic cases are fatal across all ages. There’s still some uncertainty on that, but what’s also important is that 1 percent isn’t evenly distributed. In younger groups, we’re talking perhaps 0.1 percent, which means that when you get into the older groups, you’re potentially talking about 5 percent, 10 percent of cases being fatal.

In thinking about social behavior and thinking about your interactions, the question should be, “How do we stop transmission getting into those groups where the impact could be really severe?”