Covid: How does mass testing work?

It is "critical" that people in areas where "surge testing" for the South African Covid variant is being carried out should stay at home, the health secretary Matt Hancock told MPs.

The government said it would "come down hard" on the strain, after 11 cases were identified with no obvious link to foreign travel - prompting fears it is spreading in the community.

On-the-spot doorstep tests, home testing kits and mobile testing units are being deployed in eight areas.

Over-16s who live or work in the following areas in Surrey, London, Kent, Hertfordshire and Walsall are being asked to take tests whether or not they have symptoms:

Positive cases will be analysed to see if they are caused by the South African variant.

It does not appear to cause more serious illness in the vast majority of people. But there are concerns it spreads more easily and vaccines may not work quite as well against it.

Health Secretary Matt Hancock said "finding every case" of the variant was the goal.

People in the targeted areas are being asked to talk to their employers about working from home and to stay at home even more. Some schools in affected areas have asked parents to keep children at home if possible.

Those in the affected areas who have Covid symptoms are being asked to apply for a test in the normal way.

Anyone who gets a positive result test must immediately self-isolate for at least 10 days. Their contacts will be traced by NHS Test and Trace, and must also isolate if told to do so.

Self-isolating means staying at home and not leaving even to buy food, medicines or other essentials, or for exercise.

People in England who fail to self-isolate after a positive test can be fined up to £10,000.

Anyone on a low income who has been told to self-isolate can claim a £500 payment from their local authority.

Several types of test have been developed but two have mainly been used - both involve swabbing the nose or throat.

The PCR (polymerase chain reaction) test

If you have been tested at a hospital, a drive-in centre or used an NHS home-testing kit, this is the test you are likely to have taken.

The PCR test involves swabbing your nose or throat, dropping the swab in a vial of liquid and sending it off to a lab for analysis.

It is regarded as the most reliable test, but it normally takes a day or longer to get a result.

People tested under the South Africa variant surge programme are being offered PCR tests.

The lateral flow test

This is a rapid test involving a handheld kit, that gives a result in about 20 minutes.

Like the PCR test it involves a swab from the nose or throat, which is dropped into liquid. The solution is then added to a test strip, which shows up a result in 20-30 minutes.

In a pilot scheme these weren't good at picking up relatively small amounts of virus, but did identify the majority of the most infectious cases.

Mass testing campaigns make it easier to find people who may be unaware they are infected.

It can also be used in more focused ways, including:

In a pilot scheme in Liverpool in November, everyone living or working in the city was offered a voluntary lateral flow test.

Cases there fell from 635 per 100,000 people in mid-October to 99 by mid-December.