For much of the pandemic, public health authorities have focused on the question of how to prevent getting Covid. Less attention has been paid to how you should behave afterwards. It’s no surprise that ordinary people feel uncertain. Even top epidemiologists admit the rules can look confusing – and our understanding of the science has changed since the virus first emerged in Wuhan, China.

In any pandemic, epidemiologists say that infected people should self-isolate for a period of time until they are no longer infectious. You pass a virus on by shedding virus particles in your breath. With time, the number of particles you shed goes down, making you less and less infectious.

But setting the exact length of that isolation period is more of a delicate balancing act than an exact science. If you set a window that’s too short, you risk fuelling cases by allowing infectious people to circulate in the community. If you set a window that’s too long, you risk disruption to society, with unsustainable staff absences at supermarkets, hospitals, schools, and police stations. It can take as long as a month before you totally stop shedding virus particles – but making people isolate for a month would have even more drastic impacts on society than those we’ve seen, policymakers think.

At the beginning of the pandemic, the World Health Organisation (WHO) advised a self-isolation period of 10 days from the onset of symptoms (or from a positive test, whichever is earlier). Most governments followed their lead. But that figure was more of a “straw in the wind” than an exact science, says Prof Lawrence Young of Warwick University Medical School. With little data available on Covid, it was based mostly on our knowledge of similar respiratory viruses.

But, over the last two years, epidemiologists have built a clearer picture. In the latter half of 2020, for example, a study of more than 700 Covid-positive patients in the Zhejiang province of China found that Covid patients are at their most infectious in the two days before and the three days after the onset of symptoms. Other research has supported the idea that most transmission of SARS-Cov-2 (the virus that caused Covid-19) occurs early in the infection cycle.

Some scientists are now questioning whether 10 (or even seven) days is truly necessary. “We know that people are most infectious from about 24 to 48 hours before they’ve developed symptoms to three days after they develop symptoms,” says Paul Hunter, a professor of medicine at the University of East Anglia, and occasional adviser to the WHO. But some people will still be infectious after this period, he stresses.

Governments are taking note. A week after the UK’s Health Security Agency cut the isolation window from 10 to seven days, the US’s influential Centers for Disease Control and Prevention (CDC) halved their recommended isolation time from 10 to five days, provided you wear a mask when around others in the five days afterwards. “The change is motivated by science demonstrating that the majority of SARS-CoV-2 transmission occurs early in the course of illness,” the CDC explained.

The suggestion that we now copy the US and cut the UK’s isolation time down to just five days is dividing scientists. Prof Hunter is broadly supportive of the idea. “Five days probably is enough,” he says. “That to me is a reasonable balance.”

But Prof Young is more cautious. “It’s a very worrying and risky thing to do,” he says. “If [the Government] is going to do that, I think there needs to be some way of mandating the use of lateral flow tests.”