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What Is the Optimal Length for a Quarantine?

The issue is one of acceptable risk. After 14 days, transmission risk is effectively zero. As quarantine shortens, risk goes up.

This article was first published in The Montreal Gazette.

Let me begin by saying that whether our COVID-19 situation is about to get worse is hard to predict. If experience is any indication, some people may ignore public health rules and gather for the holidays, which could lead to a surge of cases in January. Or, the current lockdown, and the additional measures set to begin on Dec. 25, may indeed serve as a “circuit breaker,” and the case count will fall.

Many people had been hoping to celebrate with others if they could effectively quarantine before and after Christmas. In fact, the government’s initial plans were for something along those lines, but fears grew that holiday dinners could become super-spreader events. Not everybody was in a position to be able to quarantine and there was also reason for doubt about how rigorously the rules would be observed by those who could do so.

The problem with COVID-19 is that you can be infected with the virus and not know it. You may feel fine today, go visit your family for Christmas dinner, test positive for COVID-19 three days later and realize in retrospect that you unwittingly spread the virus.

Quarantining is the way to prevent this. The idea is to stay away from others after possible exposure to COVID-19 until enough time has passed to be certain that you are not infected or infectious.

But based on the questions people ask me, it remains unclear to many people how long it is scientifically advisable to quarantine: two weeks (the period specified in Quebec), 10 days or just one week (which would have been allowed under Quebec’s original plan for Christmas)?

The answer relies on a balance of probabilities. Clearly, the longer you quarantine, and presuming you don’t develop symptoms, the less likely it is that you are sick. So if you attended an event on Day 1 where you might have come into contact with someone who had COVID-19, if you do not develop symptoms by Day 15 it is very likely that you are fine. There is no such thing as zero risk, but after a two-week quarantine without symptoms, the probability that you would still be infectious is extremely low.

Of course, staying at home for two weeks is hard for some people and quarantines are difficult to enforce. For that reason, there has always been some debate about whether shorter quarantines are sufficient. The U.S. Centers for Disease Control (CDC) did update its previous recommendations on this issue. Whereas it still recommends a two-week quarantine, it does suggest that 10-day and seven-day quarantines can be used in some circumstances.

The issue becomes one of acceptable risk. As quarantine shortens, risk goes up. After 14 days, post-quarantine transmission risk is effectively zero. After 10 days, the median transmission risk is 1.4 percent and after only 7 days it is 10.7 percent. Getting a negative COVID test at the end of your quarantine lowers that risk substantially. With a negative test before breaking quarantine, these numbers drop to 0.3 percent and 4.0 percent respectively. However, there is a margin of error to these estimates and the 4.0 percent transmission risk with a seven-day quarantine plus negative COVID test could be as high as 8.6 percent.

Staying at home for 14 days is undoubtedly hard, but breaking quarantine is undoubtedly dangerous. Shorter quarantines followed by negative COVID tests might be reasonably safe, but access to COVID tests and long waits might make this somewhat impractical. COVID tests can also sometimes yield false negatives depending on when and how they were taken.

The fact that guidelines differ slightly between quarantine and isolation makes the situation understandably confusing. There is, however, a simpler way to understand the inevitable trade-off. The longer you quarantine, the less risk you are to others.


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