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The CDC’s reported change to recommend the shortening of the quarantine period for people with COVID-19 exposure seems like welcome news. The rationale for this of course is that many more people may be willing to comply with a quarantine if the timeframe is shorter and that may result in an overall reduction of viral transmission.

While CDC’s new recommendations have been widely reported by the media,  the agency has as not yet put this on their website. Whether CDC has new data to suggest the scientific basis for changing their recommendation is unclear. 

Shortening the time of a quarantine during the height of the pandemic, in the absence of scientific data, is potentially risky. The World Health Organization has warned that doing so may be unwise. WHO cautions that even a slight reduction in the length of a quarantine may significantly raise the risk of viral spread. 

It may be that CDC will share new data that will address these concerns. But data published to date suggests reason for caution. The mean incubation period for COVID-19 appears to be 4.5-5.8 days. But even with a mean time under 7 days, in the context of millions of infected patients there are still large numbers that would experience longer incubation times. Approximately 2.5% of patients will develop COVID-19 more than 11 days after exposure. So even at 10 days after exposure, 101 out of every 10,000 exposed patients will develop active infection. With 150,000 new COVID patients each day in the US, this means that approximately 4,500 patients each day will have developed COVID-19 even 10 days after their initial exposure. While the number of people an infected person can infect is variable, some people with COVID can infect hundreds of other people. So 4,500 people can do a lot of damage and that means a quarantine period of less than 14 days represents the potential for significant additional viral spread. 

The international experience on quarantines is mixed. In September, France cut its quarantine time after exposure from 14 days to 7 days.  Whether it was related to this or not, France has seen a significant increase in infections since the new policy was put into place. China took a different approach and doubled quarantine time to 28 days, although foreign travelers to China are required to keep a 14 day quarantine upon arrival.