Maria Chikina and Wesley Pegden
Hiding infections in the future is not the same as avoiding them
A keen figure-reader will notice something peculiar in Kristof’s figure. At the tail end of his “Social distancing for 2 months” scenario, there is an intriguing rise in the number of infections (could it be exponential?), right before the figure ends. That’s because of an inevitable feature of realistic models of epidemics; once transmission rates return to normal, the epidemic will proceed largely as it would have without mitigations, unless a significant fraction of the population is immune (either because they have recovered from the infection or because an effective vaccine has been developed), or the infectious agent has been completely eliminated, without risk of reintroduction. In the case of the model presented in Kristof’s article, assumptions about seasonality of the virus combined with the longer mitigation period simply push the epidemic outside the window they consider.
For example, in our work studying the possible effects of heterogeneous measures, we presented examples of epidemic trajectories for COVID-19 assuming no mitigations at all, or assuming extreme mitigations which are gradually lifted at 6 months, to resume normal levels at 1 year.With no mitigations we see nearly 500,000 deaths relatively quickly.With mitigations which let up between 6 months and a year we still see nearly 500,000 deaths, just later.
Unfortunately, extreme mitigation efforts which end (even gradually) reduce the number of deaths only by 1% or so; as the mitigation efforts let up, we still see a full-scale epidemic, since almost none of the population has developed immunity to the virus.
In the case of Kristof’s article, the epidemic model being employed is actually implemented in Javascript, and run — live — in a users web browser. This means that it is actually possible to hack their model to run past the end of October. In particular, we can look into the future, and see what happens in their model after October, assuming mitigations continue for 2 months. In particular, instead of the right-hand figure here: