Next wave: Lockdown or selective quarantine?

Amnon Sashua, Sachs Professor of Computer Science at Hebrew University in Jerusalem, Israel, proposes a promising alternative to a pandemic lockdown: risk-based selective quarantine. The column below has been edited for brevity, and also appears in this week’s Chronicle of the Vermont State House. See Sashua’s March 26 column for more details. – Editor 

by Amnon Sashua

One could consider two models for handling the spread of Covid-19.

Risk-based selective Quarantine divides the population into two groups, low-risk and high-risk. Quarantine the high-risk and gradually release the low-risk population to achieve a managed herd immunity of that population. Allow the health system to cope with the expected number of severe cases. Given the herd immunity of the low-risk group, we can gradually release the high-risk population. The question is how to manage the release from quarantine of the low and high-risk populations in a way that will not overwhelm the health system. Or – 

Countrywide (or regional) lock-down can be imposed until the spread of the virus is under control. The lock-down could take anywhere from weeks to months. This is the safest route but does not prevent a “second wave” from occurring. Lockdown is currently the default model around the globe and naturally has a tremendous crippling impact on the economy. 

Covid-19 spread over time (in terms of ICU cases) when applying countrywide lockdown (model 3)

Covid-19 spread over time (in terms of ICU cases) when applying the proposed Risk-based model.

Risk-based Selective Quarantine Model: is it safe?

For the sake of concreteness, assume the cut-off age for ‘high risk’ is 67+ which represents the retired segment of society. The low-risk group is the remainder of society which is released to their daily routine while following certain distancing protocols that are aimed at slowing the spread, while keeping the economy un-disrupted to a large degree, but ultimately reaching a herd immunity level. At that point it is safe to gradually release the high-risk group from quarantine. 

The question is: how do we guarantee that the health system will not be overwhelmed during the spread of the virus in the low-risk group?

Even under a worst-case analysis, the health system is not likely to collapse. We already made a pessimistic assumption since we did not take into account the fact that not all of the low-risk population will get sick, and even those that will get sick will not get sick at the same time and will not need an ICU at the same time.

Lockdown just buys time – what’s needed is herd immunity

A full population-wide quarantine is not a solution in itself — it is merely a step to buy time followed by a more managed (non brute-force) approach. The managed phase is to create herd immunity of the low-risk group in a controlled manner while keeping the economy going. It is all about keeping the health system in check and not overwhelming its capacity to handle severe cases. Can estimates in advance, through sampling, show that the number of severe cases arising from the low-risk group would not overwhelm the system?

The risk-based quarantine model is not only beneficial from the point of view of economical sustainability. Among other selective quarantine ideas (like based on geography or contact-tracing isolating the infected and those around them) the risk-based approach has better chances of reducing the overall mortality rate. The reason is that the highest mortality is with the high-risk group which in this model is isolated.

When the high-risk group is released from isolation they would be facing a largely immune population thus naturally facing a very slow spread of infection with a good chance to weather the storm until a cure or vaccine is available. In all other selective quarantine models the high and low risk are equally susceptible to be infected so that even if the health system is not overwhelmed still the mortality of the high-risk group is likely to be higher than the risk-based model.

As a final remark, going out of quarantine is a choice, not an obligation. This is no different than people that are afraid of flights and decide not to go on an airplane. Families can decide to stay quarantined either as an extra safety measure or if some members of the family are from the high-risk group while the others are from the low-risk group.