As coronavirus cases rebounded to devastating levels in much of Europe and the U.S. late last year, one country got its outbreak under control with the aid of widespread testing. In October and November—after a brief lockdown—Slovakia tested a large percentage of its population in several rounds of mass rapid antigen testing. Unlike the gold standard polymerase chain reaction (PCR) tests, this approach does not require specialized lab analysis and can often return results in about 15 to 30 minutes, as opposed to days. (The U.S. Food and Drug Administration recently authorized several rapid COVID tests for over-the-counter use.)

Within one week, counties in Slovakia that had undergone two rounds of mass testing saw the prevalence of COVID plummet by 58 percent, researchers reported in late March in Science. And additional modeling suggested that case levels fell by an estimated 70 percent, compared with a scenario of unchecked growth at the rate seen before mass testing began. The effects of public health measures such as lockdowns and social distancing were not enough to explain the decline in cases—mass testing must have had a significant impact. While the approach may not work everywhere, the findings support population-wide rapid testing, when combined with other measures, as an important tool for combatting the pandemic.

Scientific American spoke with Martin Pavelka, an epidemiologist at Slovakia’s Ministry of Health, about the country’s mass-testing effort and whether this strategy could be applied in the U.S. and elsewhere.

[An edited transcript of the interview follows.]

How did this mass-testing program start?

It was basically just an idea. But because we’re a small country, it’s much easier to execute ideas. There are institutions that discuss it—there are, like, five epidemiologists in the whole of Slovakia [in charge of these decisions]. We sit down every Thursday, and, basically, we decide. All this happened within two weeks. We asked the army to organize it.

I don’t think the U.S. would be able to replicate this approach [on a broad scale]. I mean, more than 300 million people—it’s just too much. But for smaller countries—I could see this working in Estonia, I could see it working in Portugal, I could see it working in Ireland.

Why wouldn’t the strategy work for larger nations? Do you need staff with medical expertise to administer the tests? I mean, could you not have some kind of at-home test?

It was a massive campaign. I think this is a limitation in larger countries that have tried to replicate it. Austria tried it and couldn’t manage. In Slovakia, 40,000 army staff supported the whole intervention. We had 20,000 medical staff because it was a nasopharyngeal [deep nasal] swab, so you have to be trained to administer it.

There already are some at-home tests available in the U.S, but they’re not very widespread. They are just starting to come out. The market is changing. I think Siemens is now offering a home test kit in Europe [Germany and Austria].

How well has the mass testing worked in Slovakia?

The politicians thought it would solve everything. And I think it had a massive impact. But two to three weeks after the [initial rounds of mass testing], we had an uptick of cases again, which is expected because the intervention can’t last forever. It’s like a lockdown: the moment you open up the country, the cases will rise. The massive uptick was just before Christmas. In January we repeated the whole exercise, and we had [another] round of mass testing. We are writing a second study on this. We are still in the process of analyzing the data.

At this moment, in Slovakia, we have continuous [weekly] testing. All residents who are not isolating and who are not working from home are asked to get a test at least once a week. You get a certificate, and a negative certificate is required to enter workplaces. To go to the bank, the tax office, the post office, you need to show your negatives. And it has to be no older than seven days.

Our reproduction number [the average number of cases one infected person produces in a population] at the moment is about 0.8, so our cases are coming down. And this is in a situation where nearly 100 percent of all analyzed samples are the B.1.1.7 mutation [a variant of the coronavirus that was first identified in the U.K.], which has a much higher reproduction number.

What effect, if any, is vaccination having? What about other interventions?

In terms of vaccination, Europe lags behind. [As of early April], only 10 percent of Slovak residents have received at least one shot. So while, of course, there will be some effect..., this cannot explain, on its own, such a low reproduction number. There must be an element [from] the mass testing. That’s basically what we discussed in the study as well. The 58 percent decrease in prevalence of COVID is a combined effect of everything. It’s not possible to calculate, you know, “One third of this is the mass testing” or one fourth or one half. It’s just not possible. The closest we could get is the mathematical model we created. We showed, even if the lockdown was perfect, it could not explain such a rapid decrease in prevalence. So we were able to say, “There must be an effect of the mass testing.”

Just for comparison, the U.K. had a lockdown for the whole of November. It was just as strict as here in Slovakia…. All leisure activities were curtailed; people were asked to stay at home. It took the U.K. one month to [reduce cases] by 30 percent. We had a 58 percent drop in one week in Slovakia.

If a person in Slovakia or their family member tests positive, is the isolation or quarantine voluntary, or is there some enforcement?

Everything is voluntary here. The amount of policing we need ... is just not possible. But Slovaks are highly scared of the virus—much more than, let’s say, in the Czech Republic. In the first wave [here], I was very surprised that everyone started wearing face masks overnight. No one questioned that.

People are asked to self-isolate for 14 days, together with all their household members. This is the second most important finding of our mathematical model: we tried to find what it is, specifically, that makes the mass testing successful. We had two scenarios in our model. We had a scenario in which all the household members would quarantine if they had a positive case. And in this scenario, one round of mass testing was enough to bring the prevalence down by [the observed] 58 percent. Then we had a scenario in which only the infected person would quarantine, but the household members would not. In this scenario, we couldn’t find any measured effect. While our model says that one round of testing is enough to observe such rapid suppression of prevalence, we know there must have been an effect of lockdown in play, because not all households were isolating [to the extent] our model demanded.

There has been some debate over the accuracy of rapid tests. How accurate are they?

It really depends on the test. I think everyone gets confused. One antigen test is not the same as another antigen test. From our own experience, SD Biosensor’s Standard Q [antigen test] is really good. It’s easily picking out people when they are infected. There’s a Geneva [University Hospitals] validation study that looked at the CT [cycle threshold] counts, and for the CT counts [of 18–22], there is a 100 percent match with the PCR test. [Editor’s Note: CT is a measure of how much amplification is required to detect a virus using PCR tests. While CT is not standardized across tests, the lower the CT, the more virus is present, and the more infectious the individual is thought to be].

In our study, the specificity [a measure of how well a test detects true-negative cases] was almost 100 percent. Of course, we will have some false positives. Many scientists were scared [we would have] thousands and thousands of false positives; we are talking about hundreds. We have identified more than 50,000 infected individuals, so a couple of hundred false positives is worth it.

Under what conditions do you think rapid antigen testing would be most useful?

It’s especially useful if things get out of control. When you suddenly have an outbreak, or you have a huge superspreading event, and you get a very high growth rate, it is a perfect intervention. You basically put people into a one-week lockdown, and then you test them. You could test a state; you could test a city. And this brings things back to normal—you could decrease [cases] by 50 percent. And then you can bring back the usual test and trace, you can bring back the conventional methods, when the epidemic is at a more stabilized, more manageable level.

The second thing is: it does actually bring you a little bit more freedom. I mean, you can’t keep people in lockdown forever. They will experience COVID fatigue. People will find ways to get around the rules. From my own experience, [in Slovakia], we have officially closed gyms, but I know of at least three gyms in my district that are open. Still, our cases are going down. [So the testing] brings people at least some degree of freedom.

How will the rapid testing fit in as more and more people get vaccinated?

Vaccines are the ultimate solution. Mass antigen testing in Slovakia will eventually fade out. I can’t tell you when. Is it when one third of the population is vaccinated? Maybe half the population? I don’t know. It will be very hard to keep the restrictions or even ask people to test when you don’t have transmission because people are vaccinated. But even after people are vaccinated, where we could find a use for antigen testing is clusters of people who are not vaccinated. And you could just mass test everyone in the cluster.

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