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New research finds it could take a month for Covid-19 patients to clear the virus


Covid-19 patients may need to wait over a month before being retested to know whether they have cleared the virus, and one in five negative test results could be false, according to research published Tuesday in the British Medical Journal.

Dr. Francesco Venturelli of the University of Modena and Reggio Emilia and colleagues studied 1,162 patients in the Reggio Emilia Province of Italy who tested positive for Covid-19 using a polymerase chain reaction (PCR) test.

Patients were retested around 15 days after their first test, 14 days after their second and 9 days after their third. The researchers set these time intervals in accordance with European Centre for Disease Control and Prevention recommendations.

About 60.6% of the patients who recovered tested negative for Covid-19 by their first follow-up test. Another test confirmed that negative result in just 78.7% of these patients, which the team says suggests about one in five negative tests are false negatives. They say this could mean that many are still shedding the virus after testing negative and unknowingly passing it on to others. 

The researchers determined a patient had cleared the virus once they tested negative on two consecutive PCR tests. Overall, it took about 30 days from diagnosis and 36 days from the onset of symptoms for patients to clear the virus.

It took slightly longer for older patients and those with more severe disease to clear the virus. The length of time increased from 35 days for those under 50 years old to 38 days for those over 80. Non-hospitalized patients took about 33 days, while hospitalized patients took about 38 days.

By 34 days after patients first noticed symptoms, nearly 87% of them tested negative.  This suggests patients may need to wait a month or longer to determine whether they have truly cleared the virus, the researchers said.

The team notes that understanding the timing of viral clearance is key to determining testing strategies and ensuring people don’t have to spend unnecessary time in isolation. The say that postponing the follow-up testing of those who are no longer experiencing illness or symptoms could increase the efficiency and performance of testing strategies.

The US Centers for Disease Control and Prevention advises Americans that they don’t need a second coronavirus test. “Unless your illness required hospitalization, you can return to normal activities (e.g., work or school) after the passage of 10 days from the onset of symptoms and 24 hours from when any fever has subsided on its own (without the aid of any fever-reducing medications),” it advises.

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