One of the general assumptions we’ve formed about Covid-19 is that when someone is infected, they’re no longer contagious after two weeks—hence the recommended 14-day quarantine. But this isn’t necessarily the case.
Following the first appearance of symptoms, the vast majority of people are unlikely to be infectious for more than two weeks. For people who develop severe symptoms, that period extends to 21 days. Now, the Chinese government has determined that as many as five percent of Covid-19 cases have an incubation period longer than two weeks, and is taking action accordingly.
Their precedent may be one to follow. At NYU Shanghai, for example, local authorities recently issued a seven-day exclusion rule. Anyone returning to NYU Shanghai, a satellite campus of New York University, from overseas—even after a standard 14-day quarantine—cannot enter campus buildings for an additional seven days.
While no specific study was cited in communications announcing the seven-day rule, the claim that some Covid-19 patients take longer than two weeks to develop symptoms isn’t without precedent. A study conducted by Chinese researchers early on in the pandemic and later published in New England Journal of Medicine pinpointed the incubation period between 0 to 24 days. For one man, government officials in Hubei Province reported in February, incubation took nearly four weeks. A preprint that analyzed more than 60 articles on the subject found the upper limit in some reports to be even higher—a maximum 34 days.
That a small percentage of people might take longer than two weeks to get sick after being exposed to Covid-19 isn’t exactly surprising. From the long-haulers who experience mild but debilitating Covid-19 symptoms for months on end to the long-term virus shedders who remain contagious far longer than most, the general assumptions we use to shape our understanding of this disease—as well as our defenses against it—are riddled with no shortage of outliers and exceptions.
What is compelling about NYU Shanghai’s new policy is that it speaks to the sort of precautions we might have to take once the bulk of transmission has ceased—a horizon more distant for some of us than others, but hopefully inevitable for all. With only 913 new cases and three deaths reported in the past month, China has come so close to containing Covid-19 that not one infection can be overlooked. The Chinese government is well aware that the current pandemic started with the infection of one person, and to eliminate Covid-19 for good means eliminating every single case.
The seven-day rule also targets faculty abroad for a reason. Most outbreaks in China now originate outside national borders, either from shipments of frozen foods or travelers returning from overseas. On November 6, another new policy went into effect—this one countrywide—that requires airline passengers flying into mainland China from other countries to not just quarantine for 14 days upon arrival and submit negative PCR test results beforehand, but include a negative IgM antibody test result as well.
As was the case with the seven-day exclusion rule, the science behind the additional requirement isn’t specified, but can be inferred based on more marginal clinical presentations of the disease. Presumably, the IgM test functions as a second layer of security—screening out, perhaps, those who initially receive a negative PCR test result only to test positive later, a rare but not impossible occurrence according to studies in Nature and JAMA.
At this point we can only speculate, and in the months to come we’ll be able to see if these strategies actually pan out. If they do, countries currently in the thick of the pandemic will know to follow suit once the worst is finally behind us. But for now, especially with winter holidays fast approaching, we can only continue to take extra precautions to keep ourselves and those around us safe.