Coronavirus: What are the challenges of mass testing?

The prime minister talked about his "Moonshot" plan, on Wednesday, to test millions of people every day using tests that can produce results in as little as 20 minutes.

He said they "would allow people to lead more normal lives" by giving them the all clear to go to the theatre or a football match after a negative on-the-day test result.

No virus would mean a free pass to mix freely with other people without the need for social distancing.

But just how realistic is that plan to test vast numbers of people regularly and very quickly?

The plan involves increasing current testing - available free on the NHS to anyone with symptoms - alongside developing new, rapid tests that are being tried out among the general public in Salford and in schools and universities in Southampton.

Some are swab tests of the nose and throat, others tests of saliva from spit in a pot.

But what they have in common is the results can be processed on the spot, without the need to send tests away to a laboratory.

They would be used to test:

But the technology the new tests are based on has not been proven to work yet.

In leaked UK government plans, mass testing is described as "top priority".

But Mr Johnson acknowledged it was "an ambitious agenda".

So what do scientists think?

Dr Nathalie MacDermott, infectious diseases expert from King's College London, says the idea of a rapid test is feasible but "we don't know how accurate and sensitive they are yet".

And then there are the logistics of getting them to people and collating the results.

"Tests could be organised by workplaces and schools," she says.

"But then you could have queues of people waiting outside for test results."

And how would that be managed if 70,000 people wanted to get into a stadium, for example?

Prof David Spiegelhalter, statistician from the University of Cambridge, meanwhile, tells BBC News there is a "huge danger" of false positives - people being a given positive test result when they do not actually have the virus.

"Mass screening always seems like a good idea in any disease," he says.

But tests are rarely 100% accurate.

With a 1% rate of false positives, testing the whole UK population of 60 million would see "600,000 people unnecessarily labelled as positive".

And this could create challenges if they and their close contacts had to self-isolate and take time off work or school.

Jenny Harries, deputy chief medical officer for England, admits implementing mass testing would be "quite challenging".

Rapid tests are already in the pipeline and could be ready in a "reasonably short time frame", she says.

But manufacturing, supplying and distributing these tests would be a major hurdle.

Regular asymptomatic testing has only just started in care homes after many months of the government saying it would happen.

Wider testing of people without symptoms isn't always useful unless it's carefully managed, because a negative test result doesn't necessarily mean 'not infectious'.

It may just mean someone is in the early stages of infection and the test hasn't detected enough virus to give a positive result.

Experts have suggested that giving people immunity passports on this basis could be dangerous.

Would it not be better to concentrate on sorting out the current NHS Test and Trace programme, which has seen people struggling to order tests or being sent long distances to get one?

The government says its labs have been very busy - but it has now made sure people do not have to travel more than 75 miles for a test.

Health Secretary Matt Hancock says opposing mass testing would be a mistake because it is an "incredibly important tool in our arsenal".

And in allocating £500m to the project so far, he says he is backing "all the horses" while supporting businesses and universities to deliver more testing themselves.

The aim is to turn Operation Moonshot into Apollo 11.