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It is “critical” that people stay at home in areas where mass testing for the South African Covid variant is taking place, the government says.

At least 147 cases of the strain have been identified in the UK but it’s “very possible” it has spread far more widely, experts have said.

Many of these have no obvious link to foreign travel – prompting fears it is now spreading within the community.

Where is extra testing taking place?

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

Over-16s who live or work in the following places are being asked to take tests whether or not they have symptoms:

Surge testing in the ME15 postcode area of Kent, and the GU21 postcode area of Surrey, has now finished.

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

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

Why is surge testing so important?

Health Secretary Matt Hancock says “finding every case” of the variant is the goal.

The South Africa strain does not appear to cause more serious illness in the vast majority of people.

However, there are concerns it spreads more easily and that vaccines may not work as well against it.

A small study conducted in South Africa suggests the Oxford AstraZeneca jab – one of the main vaccines currently being administered in the UK – offers “minimal protection” against mild disease caused by the variant.

Although 147 cases have been recorded in the UK, the true number could be far higher. Those detected so far come from random sampling of between 5% and 10% of positive cases.

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Infectious disease expert Dr Mike Tildesley told the BBC “it’s very possible” the South Africa variant could already be quite widespread in the UK, and the “surge testing” programme “really needs to be effective” to halt its spread.

People in 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.

What happens if I test positive?

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.

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What are the different types of coronavirus test?

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.

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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.

How else is mass testing used?

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:

Regular testing in a hospital or care home to prevent outbreaksKeeping places open such as workplaces, schools and universitiesTargeting high-risk workplaces and hard-to-reach communities