People who've had COVID-19 should be swab tested again four or more weeks after symptoms first appear to minimise the risk of onward infection, suggests a large population based study in one of Italy's former coronavirus hotspots, and published in the online journal BMJ Open.

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This is because SARS-CoV-2, the virus responsible for COVID-19 infection, takes an average of 30 days to clear from the body after the first positive test result and an average of 36 days after symptoms first appear, the study findings show. And it's not yet known how infectious a person may be in the recovery phase, warn the researchers.

What's more, the findings indicate that the rate of false negative test results—whereby people are falsely reassured they no longer have active (shedding) virus in their bodies—is relatively high (1 in 5) in early convalescence, so putting them at risk of unwittingly passing on the infection.