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1. I believe that if #SARSCoV2 is allowed to spread uncontrolled through an entire nation, it will be an unprecedented humanitarian disaster. Every possible measure should be taken to prevent this from happening. Yet in some countries this may be unavoidable.
2. Some nations may simply lack the economic resources, technological capacity, and political will to contain the virus until a vaccine can be developed.
Should this happen, I think it is important to consider how more modest control efforts could yield major benefits.
Should this happen, I think it is important to consider how more modest control efforts could yield major benefits.
3. Earlier today I posted a thread explaining the concept of epidemic overshoot as pertains to herd immunity.
To make sense of this thread, it's worth understanding that background. https://twitter.com/CT_Bergstrom/status/1251999295231819778
To make sense of this thread, it's worth understanding that background. https://twitter.com/CT_Bergstrom/status/1251999295231819778
4. The basic point is that when an epidemic spreads through a population, herd immunity will be reached when a fraction 1-1/R0 of the population is infected—but the epidemic will keep going well past that point. These additional infections are known as overshoot.
5. Of course there are major caveats to any of this theory: perhaps most importantly we don't yet know what fraction of recovered individuals develop protective immunity, nor how long this lasts. Herd immunity may not even be feasible. But even if it is....
6. The key observation is that compared to controlling the pandemic well below the herd immunity threshold, reducing the number of cases due to overshoot may require shorter, less painful control measures.
7. Reminder: I am not advocating this as a desirable control strategy. Where possible, I think the only option we have is to control the pandemic at low levels until a vaccine can be developed.
Still, it is worth thinking about how to save lives if this goal cannot be attained.
Still, it is worth thinking about how to save lives if this goal cannot be attained.
8. Recall how overshoot works. This graph, from the thread linked above, illustrates the epidemic curve for parameters that are often used to model #COVID19. With R0=2.5, the epidemic threshold is about 60% but with overshoot another 30% of the population is also infected.
9. If one can break the momentum that is driving the epidemic beyond the herd immunity threshold, one can reduce that overshoot substantially. Below, I've modeled a 30 day period of social distancing around the epidemic peak, that drops R0 from 2.5 to 0.3.
10. This 30 day lockdown period is far less onerous than that required to hold the virus in check until a vaccine is available but reduces the total fraction infected from 90% to 70% by eliminating much of the overshoot.
11. In my view, this is not a substitute for aggressive control and containment, because 60% of the population still becomes infected. In India, for example, this would cost about 19 million lives with a 2% infection fatality rate.
But the 30 day lockdown would save >5 million.
But the 30 day lockdown would save >5 million.
12. One of the agonizing aspects of pandemic epidemiology is that one has to consider situations that seem intolerable and that should be avoided at any possible expense.
Yet should such a situation come to pass, it is good to know what one can do to ameliorate the harm.
Yet should such a situation come to pass, it is good to know what one can do to ameliorate the harm.
13. Of course there are many open questions here. How does the efficacy depend on the timing and duration of the shutdown? How bad it is to miss by a few days—or a few weeks—with the timing of the shutdown?
14. To summarize, I believe we must do all we can to control and contain the #SARSCoV2 coronavirus worldwide until we have a vaccine.
But if we fail to do this regionally, there are still actions that can save millions of lives. We need to thinking of these now, in time to act.
But if we fail to do this regionally, there are still actions that can save millions of lives. We need to thinking of these now, in time to act.
