COVID: Most people remain contagious after five days of symptom onset


A new study from the UK has found that most people who test positive for COVID-19 remain contagious five days after first seeing symptoms.

Researchers from Imperial College London followed a group of people who developed mild cases of COVID-19 to better understand when and for how long they remained infectious.

The study followed 57 people in various households who tested positive for COVID-19 and found that two-thirds of participants remained infectious five days after their symptoms began. In addition, a quarter were still contagious seven days after their first symptoms.

Published in the peer-reviewed journal The Lancet Respiratory Medicine on Thursday, the study found that one in five people were contagious before they started showing symptoms, suggesting that the majority of people infected with COVID-19 do not are not likely to be infectious until symptoms develop.

The study group underwent daily symptom reports and tests, including both PCR and lateral flow tests, more commonly known as rapid tests. Through this, researchers found that lateral flow tests are unreliable in detecting the onset of infection, but can be useful in determining when to shorten the quarantine period.

“Our study finds that infectiousness typically begins shortly after you develop symptoms of COVID-19. We recommend anyone who has been exposed to the virus and has symptoms self-isolate for five days and then use daily lateral flow tests to safely exit isolation when two consecutive daily tests are negative.” , lead researcher Ajit Lalvani said in a press release.

The study period took place between the pre-Alpha and Alpha variant waves in 2020 and the Delta variant wave in 2021. While the study predates the most recent wave of the Omicron variant and its subvariants, the lead researchers say the length of infectivity is likely similar to that of the other variants.

As in the UK, the majority of COVID-19 guidelines in Canada recommend people self-isolate for five days after showing symptoms and can end their quarantine after five days if their symptoms do not persist.

However, the authors of the new study emphasize the importance of following self-isolation guidelines, whether mandatory or not.

“There is no longer a legal requirement to self-isolate if you test positive for COVID-19 [in the U.K.] but most people still want to self-isolate until they are no longer infectious,” co-author Seran Hakki said in the statement.

“Our study is the first to assess the duration of infectiousness, using real evidence of a naturally acquired infection. Our results may thus inform advice on how to safely end self-isolation.