Covid-19: Most people are still infectious five days after symptoms begin

A comprehensive study has revealed that two-thirds of people with covid-19 are still infectious five days after symptoms begin, calling into question self-isolation advice

By Clare Wilson

Two-thirds of people with covid-19 are still infectious five days after their symptoms begin, according to the most comprehensive study yet of people catching the virus in real life.

The research also found that lateral flow tests – also called rapid antigen tests – give incorrect negative results about a third of the time when people are in the first days of developing covid-19, although they are much more accurate for detecting when people stop being infectious at the end of their illness.

Seran Hakki at Imperial College London and her colleagues studied people before, during and after they developed covid-19 between September 2020 and October 2021. They did this by asking those who had been in close contact with known cases to carry out frequent nasal and throat swabs and keep daily symptom diaries.

The swabs were tested by PCR and lateral flow devices, and any PCR-positive samples were subjected to a further assay that calculated how much infectious virus was in the mucus – the person’s “viral load”, which is seen as a key measure of infectiousness. This was done by incubating different dilutions of the mucus in a dish with cells vulnerable to infection.

Of more than 700 contacts of cases, 42 people had the beginning and end of their infectious period revealed by the tests. “We captured the moment they developed infection until they ceased to be infectious,” says Hakki.

About one in five of these people were found to be infectious, according to the viral load assay, before any symptoms began. Lateral flow tests gave an accurate positive result 67 per cent of the time early on in the illness. Negative lateral flow results shouldn’t be trusted if people have symptoms, says Hakki.

Towards the end of people’s illness, two-thirds were still infectious five days after their symptoms began, although they did have lower levels of infectious virus in their nasal and throat mucus. A quarter were still infectious after seven days. In later stages of the illness as people’s viral loads fell, lateral flow tests correctly gave positive results for 92 per cent of people who were infectious.

Advice in many countries, including the UK, is that most people can stop self-isolating after five days, although they should avoid meeting people who are more vulnerable to covid-19 for 10 days. Those under 18 are advised to self-isolate for only three days in England and Scotland.

“This shows beyond a shadow of a doubt that people are releasing infectious virus for far longer than is stated on current guidance,” says Stephen Griffin, a virologist at the University of Leeds in the UK, who wasn’t involved in the study.

However, the results may be slightly different for the BA.5 omicron variant that is currently prevalent, says Hakki. The study was carried out when people in the UK were being infected by the original coronavirus strain or the alpha or delta variants. A previous study found that omicron is linked with lower viral loads.

Adam Kleczkowski at the University of Strathclyde in Glasgow, UK, says asking people to isolate for longer would stop more transmission of covid-19, but would also have downsides in terms of keeping people away from work and school. “I personally think [five days] is a reasonable trade-off,” he says.

Susan Hopkins at the UK Health Security Agency says the guidance remains under review. “We know that most transmission in adults occurs three days before the onset of symptoms to five days after, and the infectiousness for omicron peaks within a shorter time frame than previous variants,” she says.

Journal reference: The Lancet Respiratory Medicine, DOI: 10.1016/S2213-2600(22)00226-0