blueprint for producing tests — the UK became one of the first countries to develop an accurate test for the presence of the virus in patients.

Britain quickly began to track down and trace potential carriers, but it did not follow the much-praised path of mass testing in South Korea, which was soon testing more than 10,000 people a day.

Britain also processed its tests in a different way to South Korea and other countries. Instead of using a network of public and private laboratories, the UK initially used just one lab — Public Health England’s Colindale facility in north London, which was processing about 500 tests a day this week.

As coronavirus began to spread around the world, the UK gradually enlisted more labs across the country, announcing on Marcha day later, the prime minister, Boris Johnson — who on Friday revealed that he had tested positive for the virus — claimed that daily testing would rise to 250,000. This week it has reached about 6,500 a day and officials said it would not reach 25,000 until the middle of April.

On Friday the government also announced that a new alliance of businesses, research institutes and universities would boost tests for frontline health workers, with hundreds to be done by the end of the weekend and numbers rising sharply next week.

So why did the government ease up on what initially seemed to be a concerted contact tracing and quarantining effort?

“There comes a point in a pandemic where that is not an appropriate intervention,” Jenny Harries, the deputy chief