One figure from the paper, reinterpreted below, depicts possible scenarios (the details would differ geographically) and shows the red trajectory of Covid-19 infections in response to “intermittent social distancing” regimes represented by the blue bands.
Social distancing is turned “on” when the number of Covid-19 cases reaches a certain prevalence in the population — for instance, 35 cases per 10,000, although the thresholds would be set locally, monitored with widespread testing. It is turned “off” when cases drop to a lower threshold, perhaps 5 cases per 10,000. Because critical cases that require hospitalization lag behind the general prevalence, this strategy aims to prevent the health care system from being overwhelmed.
The green graph represents the corresponding, if very gradual, increase in population immunity.
“The ‘herd immunity threshold’ in the model is 55 percent of the population, or the level of immunity that would be needed for the disease to stop spreading in the population without other measures,” Dr. Kissler said.
Another iteration shows the effects of seasonality — a slower spread of the virus during warmer months. Theoretically, seasonal effects allow for larger intervals between periods of social distancing.
This year, however, the seasonal effects will likely be minimal, since a large proportion of the population will still be susceptible to the virus come summer. And there are other unknowns, since the underlying mechanisms of seasonality — such as temperature, humidity and school schedules — have been studied for some respiratory infections, like influenza, but not for coronaviruses. So, alas, we cannot depend on seasonality alone to stave off another outbreak over the coming summer months.
Yet another scenario takes into account not only seasonality but also a doubling of the critical-care capacity in hospitals. This, in turn, allows for social distancing to kick in at a higher threshold — say, at a prevalence of 70 cases per 10,000 — and for even longer breaks between social distancing periods:
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