Covid: How does mass testing work?

Urgent testing is being carried out in some parts of England, after cases of the South Africa Covid variant were found.

It's the latest instance of mass testing, which the government is using as part of the fight against coronavirus.

The South Africa variant is believed to be circulating in some areas of England.

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

Mobile testing units have been dispatched to the neighbourhoods, while home testing kits are also being sent to some areas.

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

While it does not appear to cause much more serious illness in the vast majority of people, there are concerns it can spread more readily and that vaccines may not work quite as well against it.

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

They can then be told to isolate and be prevented from spreading the virus.

Mass testing can also be used in more focused ways, including:

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

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.

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.

There have been some concerns about the reliability of lateral flow tests following an evaluation of a pilot scheme in Liverpool.

The tests weren't good at picking up relatively small amounts of virus but did identify the majority of the most infectious cases.

The government has largely paused plans to roll out rapid testing of close contacts in schools.

Health officials say the emergence of a new variant means the risk of missing infections has risen.

Regular testing of staff will meanwhile increase to twice a week.

It's not currently clear how testing of pupils will be organised once schools reopen.

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.

Anyone who gets a positive result to either 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 should order online groceries, or ask friends and family to get supplies and leave them outside the front door.

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 in a phone call from NHS Test and Trace can claim a £500 payment from their local authority.