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How long are coronavirus patients contagious for?

Containing the coronavirus crisis without proper intel is like shooting in the dark. Until we can find a vaccine or cure, we need to know everything there is to know about the virus’ behavior in order to properly manage resources and manpower. Otherwise, we risk living the rest of the year (and perhaps following ones) going from lockdown to lockdown.

Some important characteristics that should be precisely known include the mortality rate, the virus attack rate, the average viral load and the minimum infectious dose, the basic reproductive number (which tells you how many people a single individual can infect, or how contagious the virus is), as well as how long an infected person remains contagious.

This last bit is the subject of a new study by researchers at Singapore’s National Centre for Infectious Diseases and the Academy of Medicine, who tested 73 COVID-19 patients. Their analysis revealed that, on average, the virus could not be isolated or cultured after 11 days after the onset of illness — this was true despite viral RNA being detected with PCR tests.

Much confusion around positive tests and infectious cases

In other words, a person might still test positive despite signs of obvious recovery. However, just because there is still viral material in a person’s respiratory fluid doesn’t mean they’re infectious.

This explains a lot of very weird things we’ve seen in relation to this virus, such as people testing positive for weeks on end. Because of such cases, many have been worried that people can get sick with COVID-19 many times after recovery or that the human body cannot produce sufficient antibodies to become immune to the coronavirus.

Instead, this study suggests that people really do build immunity — it’s just that our PCR tests are so sensitive that they can pick up even tiny traces of the virus. What’s more, these viruses may not be able to replicate in other hosts 11 days after the onset of symptoms.

The implications are huge. This means that many protocols currently in place for managing COVID-19 patients need to be rethought or entirely discarded. For instance, in many countries, COVID-19 patients are not allowed to be discharged from the hospital until they have two consecutive negative tests.

In light of these findings, hospitals could discharge a patient after symptoms have disappeared and/or at least 14 days have passed from the onset of illness. This way, many valuable hospital beds can be cleared for new patients.

The findings are in line with other previous studies that investigated how long the individuals remain infectious. One study on COVID-19 patients from Hong Kong concluded that an infected person can pass the virus on to other people a few days before the onset of symptoms, with infectiousness peaking right when the symptoms appear, then declining over the next 7 days. These findings are supported by another study on Chinese patients which concluded that significant viral shedding likely begins two to three days before symptoms first appear and that the number of whole viruses that are expelled declines after people begin feeling sick.

Another study on Taiwanese COVID-19 patients offers hints about the infectiousness of the virus from another angle. The authors of the study tested people who were in close contact with the COVID-19 patients, finding that secondary cases arose within 5 days of the initial host’s onset of symptoms. Contacts made after this period did not result in new cases.

Finally, a study from Germany that cultured the virus from throat and lung samples found that the virus could replicate and infect other cells in the first week of symptoms, but none after day 8 — that’s in spite of the samples showing high viral loads in PCR tests.

A positive test doesn’t mean the patient is still sick or contagious

These studies all seem to suggest that viral replication quickly drops one week after symptom debut and that patients stop spreading viable viruses after the second week of illness.

They also speak volumes about the faultiness of RNA detection by PCR testing, which so many hospitals and countries see as a gold standard of COVID-19 outcome success. Instead, patients could be discharged based on other criteria as well, like the time elapsed since the onset of illness. This would help free up resources for new patients.

And, perhaps most reassuringly, the findings suggest that SARS-CoV-2 isn’t really some superbug. People who’ve tested positive again and again after weeks of illness probably aren’t getting reinfected after recovery. Instead, it’s just a matter of tests picking up residual viral material.

Hopefully, similar studies will soon answer how contagious asymptomatic cases are as well, as this is another important missing piece of the puzzle.

For now, this much seems very likely with regard to the behavior of the coronavirus: people can pass it on as early as a few days before symptoms appear, but no later than 14 days after the onset of symptoms, regardless of positive PCR tests.

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