Writing in The Economist, NAChristakis explains what we can learn from previous coronaviruses to help tackle covid-19
SEVEN VARIETIES of coronavirus infect humans: four give us the sniffles; one causes a deadly disease smouldering in the Middle East since 2012; and two erupted into full-on pandemics. The first caused SARS and it petered out quickly. The other causes covid-19 and it has hobbled the global economy. Why the difference?
But that is where similarities end. The virus behind SARS had intrinsic qualities that made it harder to spread and easier to control, compared to the one behind covid-19, which has overwhelmed the world. This is because of their respective fatality rates, symptoms, infectious periods and a nuance in their reproduction number. Consider these features in turn, for they explain why covid-19’s virus has been so destructive—and how we can best fight it.
Now imagine a second pathogen that again makes 20 seriously ill and kills two for every 1,000 people, but also infects a further 180 people making them mildly or moderately ill, but not killing them. Perhaps some of them are left seriously disabled, too. But the case fatality rate gets calculated as two deaths out of 200, for a mere 1%. The second disease seems much milder.
The period between becoming infected with a pathogen and showing symptoms is called the “incubation period.” This ranges from 2 to 14 days for covid-19 and is typically 6-7 days. For SARS, the incubation period is 2-7 days. Yet the transmissibility of a pathogen need not be the same for every person. The extent of the variation in R0 across individuals in a population, if any, can be quantified. And this has subtle but important effects on the course of an epidemic. The greater the variation or dispersion , the more likely an epidemic is to feature both super-spreading events and also dead-end transmission chains.
An epidemic with large variation in the R0 manifests itself with many super-spreaders and super-spreading events. This is what happened with SARS. It was estimated that four importations were necessary for one transmission chain to be initiated . For covid-19, it appears that the variation in R0 is lower than for SARS, so super-spreading events, though they do occur, are less important than the more frequent, humdrum chains of transmission. Yet again, this makes covid-19 harder to control.
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