Covid: how long are people infectious and how do isolation rules vary?
The US has announced it is cutting the recommended self-isolation time with Covid to five days. How long are people with Covid infectious for, and why do the rules vary between countries?
What are the rules for self-isolation in the UK?
Since 22 December, people in England who have tested positive for Covid have been allowed to cut their self-isolation from 10 days to seven if they have negative lateral flow tests on days six and day seven.
Fully vaccinated close contacts of someone who has tested positive do not need to self-isolate, but those who are not vaccinated have to isolate for 10 days.
In Scotland, Wales and Northern Ireland, the self-isolation period for those who test positive remains 10 days, although rules around close contacts vary between the nations.
How long is someone with Covid infectious for?
It has been clear for some time that people can be infectious from about two days before they develop symptoms.
Estimates for how long an infected person remains contagious vary, but as Harvard Medical School notes: “By the 10th day after Covid symptoms begin, most people will no longer be contagious, as long as their symptoms have continued to improve and their fever has resolved.”
Dr Julian Tang, a professor of respiratory sciences at the University of Leicester, said although the Omicron variant showed some escape from vaccine-induced and past immunity, he would expect the immune response to follow a similar timeline to that in previous variants.
However, Prof Lawrence Young, a virologist at Warwick Medical School, said it appeared that the time from infection to becoming infectious was shorter with the Omicron variant, at two or three days, and it was very likely that the infectious period was also shorter and probably affected by vaccination status. “This infectious period can occur before the onset of symptoms and last for between two to three days after you become symptomatic,” he said.
The UK Health Security Agency says the new approach for England “reflects latest evidence on how long cases transmit the virus for, and supports essential public services and supply chains over the winter, while still limiting the spread of the virus”.
Dr Richard Tedder, a member of the Clinical Virology Network, said the shift was a compromise, and was based on the assumption that people with two negative tests are unlikely to transmit coronavirus to contacts.
“On a balance of probabilities this is probably correct,” he said, although he suggested the situation may be different for some people, such as those who are immunosuppressed, and it may not hold if future variants are not picked up by lateral flow tests.
Young agreed. “The approach adopted in the UK is sensible based on seven days and two consecutive negative lateral flow tests. These tests are a great way to determine if you are infectious.”
How do the UK’s rules compare with those of other countries?
In France, people with Covid must self-isolate for 10 days, while in Germany the mandatory self-isolation period is 14 days unless a PCR test comes back negative or the local health office confirms release.
In the US, the Centers for Disease Control and Prevention has recently cut the recommended self-isolation period to just five days provided the person has no symptoms at that point.
“The change is motivated by science demonstrating that the majority of transmission occurs early in the course of illness, generally in the one to two days prior to onset of symptoms and the two to three days after,” the CDC said. “Therefore, people who test positive should isolate for five days and, if asymptomatic at that time, they may leave isolation if they can continue to mask for five days to minimise the risk of infecting others.”
Close contacts of positive cases are also recommended to isolate for five days, followed by strict mask-wearing, although those who have had their booster jabs or have recently completed the primary course of Covid vaccines do not need to self-isolate but should wear a mask around others for 10 days.
Tang said the approach was essentially following the same 10-day isolation cut-off that the World Health Organization proposed last summer. “But they are also taking into account the viral load drop-off after the first five days, with five days of masking to follow,” he said. Whether it was sufficient would depend, he said, on how conscientiously masks were worn during that period to limit further transmission as the viral load drops.
“It’s just another compromise as we learn to accept and live with this virus,” Tang said. “Different countries will approach this differently based on their local knowledge, studies, experience, and competing resources and priorities.”
However, some experts, including Young, have raised concerns over the shift in rules in the US, noting there is no requirement for a negative Covid test before ending self-isolation.
Dr Michael Mina, a former assistant professor at Harvard TH Chan School of Public Health and now CSO of eMed, noted that some people stay infectious for 12 days. “CDC’s new guidance to drop isolation of positives to five days without a negative test is reckless,” he tweeted. “I absolutely don’t want to sit next to someone who turned Positive 5 days ago and hasn’t tested [negative].”