Asymptomatic testing for COVID

Studying COVID transmission by testing asymptomatic students at Cambridge University
18 January 2021

Interview with 

Ben Warne, University of Cambridge


a scientist at a lab bench with samples


Testing is critical for controlling the virus, but if someone doesn’t have any symptoms, which happens about half the time, they won’t get a test and their infection will be missed. This is a big problem, because even though they have no symptoms, these people can still spread the infection. One strategy is just to test everyone regularly, but we have only a limited number of tests we can reasonably carry out. One tactic, though, that might solve this, is to “pool” or mix together the swab samples from everyone in a household and do one test on all of them. A negative result means everyone in that batch is in the clear. And if you get a positive result, you then only need to test a much smaller group of samples to find out who the infected people really are. This is what they’ve been doing at the University of Cambridge over the last term and now they’ve released a report on how well it can work. Phil Sansom heard from one of the doctors behind it, Ben Warne....

Ben - Between 2 to 10 students, depending on the size of the pool, would each take a swab and they put it into the same test tube. And that test tube would then go to the laboratory. And we would be able to test all of those students in one go. And that way we were able to test up to 10,000 students with 2000 tests.

Phil - How long were you testing students for then? Was it the whole term?

Ben - So over nine weeks in total. 1-2 people in each household at the start of term. And then by the end, we were able to test everybody in that household on a weekly basis.

Phil - Did it pay off?

Ben - We think that it did. Testing capacity is one of the really key limiting things in terms of COVID-19, the way that it's managed in the UK and indeed internationally. And if you can test five people with one test, that's a really, really, really efficient use of your testing capacity that we were keen to try and optimise.

Phil - We also heard on the show that some students were so keen to share things, they actually shared a swab.

Ben - You hear stories where people took it to the next level, and they're actually sharing swabs. Err, not the best way of getting tested, possibly an okay way of sharing COVID.

Phil - How many asymptomatic cases did you end up finding?

Ben - Over the course of the programme, we found well over 200 cases of asymptomatic COVID-19 infection. And the reason that I sort of hesitate slightly when mentioning that is because they were asymptomatic at the time that they took their swab. We have gone back and interviewed those students again, to see if they went on to develop symptoms. And actually a significant proportion of them did. So they'd been pre-symptomatic when they had the swab, they went on to develop symptoms afterwards.

Phil - Do you have an idea of how many of the COVID cases you found were asymptomatic?

Ben - Yeah. And it's a question that we're all trying to answer. And if you look at the published studies, I remember reading the first published study from Wuhan and they equated levels of less than 1%. And then there are some more recent studies that equate levels above 90%. There are a number of studies that sort of publish a range in between. It's probably different depending on which population you're looking at. So it's going to be different if you're looking at university students versus school-aged children and - although I haven't got a number to quote to you today, we are working hard to try and find one. I think it is a really, really important question to answer.

Phil - So once you had this knowledge, did it help you tracking down the infections going on spreading through the university?

Ben - Absolutely. So in the first couple of weeks we identified a number of transmission events among people in the same household or the same block of accommodation. It spread between people who share the same course in some cases, or in some cases the same social activities. I'll give you one specific example though. In the second week of term, we saw quite a big outbreak in one of our blocks of college accommodation. And if you look at the genetic code, this virus is only found in weeks two and three of term. After that the accommodation block was locked down, it was closed. And we don't see that virus again later in the term. Although we don't necessarily understand all of the way that the virus is transmitted, we do know that some of the interventions we're putting in place do help to control it.

Phil - Ben, what about this programme really worked and has made you go, "Yes. I think schools, universities should be doing this when they reopen"?

Ben - The problem with testing is that there's no one size fits all solution to all scenarios. What we have here though is a programme which first of all, really utilises tests to the maximum capacity that we have available to us by using pooling. Secondly, we've, it's been a really successful programme in terms of students participating. So it was an entirely voluntary programme, but more than 80% percent of the students that were eligible were taking part. It's through this combination of using efficient testing and getting lots and lots of people engaged to participate in it that has the real public health benefit.

Phil - Should families be getting tests that are then pooled together to save time so that there can be more tests?

Ben - Yeah, it's a really, really good question. And I think that certainly there is an option of using pooled testing for households. There are two things that are really important. The first is that they all live together, which means that in the event of a positive test, that's the household of people that you isolate together. And secondly, it makes the logistics a bit easier. The practicalities of getting the test delivered to a household of people is straightforward. It's just their postal address.


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