Covid: What is mass testing and how does it work?

A mass coronavirus testing programme has been taking place in Liverpool.

There are now plans to extend it to other parts of England, and to students around the UK before they return home for Christmas.

How does mass testing work and what does it achieve?

Mass testing means asking everyone to be tested, whether or not they have symptoms.

The idea is to find healthy people who may be infected, but not yet displaying symptoms. They can then be told to isolate and be prevented from spreading the virus.

Currently, across most of England, people can only have a test if they already have symptoms. But during the Liverpool pilot, everyone living or working in the city is being offered a voluntary test.

Mass testing can be used in more targeted ways, including:

Mobile testing sites have been set up across Liverpool, in places like care homes, schools, universities and workplaces.

People might be invited by their local authority to come for a test but they can also book their own online, or whole families can just turn up in their car.

About 2,000 military personnel are helping with logistics and delivering the tests across a potential 85 sites.

More than 23,000 people have been tested in Liverpool since the pilot began, with 154 people receiving positive results. The city has a population of about half a million.

The scheme is now going to be rolled out to 67 more areas in England, including Nottinghamshire, Yorkshire, the North East and parts of the West Midlands, North West and London.

Plans have also been drawn up to ensure university students in England and Wales can go home safely for Christmas, after being tested. Plans for Scotland will be announced later on Wednesday.

NHS staff will also start being tested for coronavirus twice a week. Routine testing of health workers already happens in the areas hardest hit by the virus.

The Liverpool pilot offers two types of testing, both involving swabbing the nose or throat - a PCR (polymerase chain reaction) test, and a lateral flow test.

The PCR test is regarded as the "gold standard" by epidemiologists, but may take up to a day or longer to produce a result as the sample is sent off to a laboratory. It is, however, the one used most around the world.

The lateral flow test involves a handheld kit that gives a result - a bit like a pregnancy test - in about 20 minutes, without the need for a lab. Fluid from a nasal swab or saliva goes on one end, then a marking appears if you are positive.

Liverpool's public health director Matthew Ashton said the city has the capacity to carry out 50,000 lateral flow tests and 14,000 PCR tests a day.

Anyone who tests positive during the pilot must immediately self-isolate for at least 10 days. Their contacts will be traced by NHS Test and Trace.

All other people in their household must also self-isolate for 14 days from the day the person first became ill, or from the day of the test, if they have no symptoms.

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

People should order online groceries, or ask friends or family to help out by getting what is needed and leaving items outside the front door.

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

Anyone on low income who was instructed to self-isolate in a phone call from NHS Test and Trace should be able to claim a £500 payment from their local authority.

In September, Mr Johnson outlined his "moonshot" plan to control the virus with mass testing, possibly by next spring.

He said that "in the near future" he wanted to start using testing to allow people who tested negative "to behave in a more normal way".

Other countries have already introduced mass testing programmes.