The first study, published by Japan’s National Institute of Infectious Diseases on Jan. 5 found that patients with the Omicron variant of COVID-19 shed the virus for much longer after symptoms emerged. The study found that the amount of viral RNA was highest three to six days after symptom onset, with a marked decrease only after 10 days.

Another U.S. study, published on Saturday by researchers at Harvard University, found that 39% of people infected with the Omicron variant in the National Basketball Association’s COVID-19 testing program were still contagious after five days. 

Similarly, a study done by the  U.K. government itself through its Health Security Agency found that up to 31% of people would still be infectious five days after the onset of COVID symptoms.

Such findings have led to friction between proponents of shorter and longer quarantines. The move to shorten isolation periods is motivated by a need to ease crippling staff shortages in essential public services, as well as in retail and hospitality venues—a desire that regularly puts politicians looking to buoy the economy at odds with researchers who are focused on health statistics.

“What we’re going to do is run the risk of highly infectious people returning to work or school,” Lawrence Young, a virologist and professor of Molecular Oncology at Warwick Medical School, told Fortune.

“The bottom line with this is this is not a scientific change or policy that is based on scientific evidence at all. It is based on the need to get people back to work,” he said.

Testing’s role

The U.S. Centers for Disease Control and Prevention was met with sharp criticism after announcing its decision to cut isolation to five days without a negative antigen test.

CDC director Dr. Rochelle Walensky defended the decision, telling the New York Times that the recommendation was based on evidence that the majority of transmissibility lasts in the two days before symptoms and the two to three days after onset.

On the decision to omit negative testing, Walensky added that rapid tests are not reliable for determining contagiousness. The U.S. guidelines still recommend people wear a mask around others for another five days after testing positive.

Unlike the U.S., the U.K. requires a negative lateral flow or rapid antigen test in order to shorten isolation times. The London School of Hygiene and Tropical Medicine suggested that such testing could cut isolation time even further, arguing the number of infectious days could be reduced to almost zero by requiring two consecutive days of lateral flow tests regardless of the date of symptom onset.

Around the world

As the Omicron variant rushes through Europe, many countries are also shortening their isolation periods to ease staffing shortages, but few are going as far as the five days required by the U.K. and the U.S.

France, Ireland, Spain and Portugal have all decreased their COVID self-isolation rules from 10 days to seven in the face of rising cases and staff shortages. Germany is also considering cutting its 14-day isolation. Greece has shortened its isolation to five days.

Italy however, has chosen a different course, leaving its 10-day isolation untouched, but scrapping quarantine rules for people who have come into close contact with someone infected with COVID.

Some countries like Canada, have shortened isolation periods for people who have been vaccinated. But according to Young from the University of Warwick, “I haven’t seen any data that demonstrates any difference in the timeline dynamics.” However, he added, he “can understand why some countries will try and bring that in as a way of encouraging people to get vaccinated.”