The Government is going to select 60,000 people at random to take the COVID test: what we know from the seroprevalence study

Yesterday Salvador Illa, the Minister of Health, announced that they were going to start a large-scale study that would test some 60,000 people to "determine the percentage of the population that suffers from the disease." During three weeks, the ISCIII teams will visit 30,000 households randomly to the selected households with the help of the National Statistics Institute to obtain a representative sample of the country and learn about the size of the epidemic.

As explained by the minister, the study will begin next week and will be one of the elements to take into account to draw up contingency plans with which to safely exit quarantine, first, and prepare for future waves, later.With its pros and cons, it would be one of the first seroprevalence studies to be launched in the world and therefore has caused some confusion.

Above all, because this proposal coincides in time with the intention of many countries to launch "immune passports" with which to accelerate the start-up of the economy. But are we talking about the same thing? What differences are there with the Government's proposal and why in Spain will 30,000 households be chosen 'at random'?

Immune passports

From the moment it began to become apparent that the only viable response to the coronavirus pandemic was the temporary closure of entire countries, the experts realized that some way to start those economies would be needed while minimizing the risk of that the number of new, hospitalized and deceased cases grew again. The easiest way to do this was to identify people who were already immune to the virus.

Thus, countries such as Germany or Israel have already announced their intention to launch them and many others are studying it. The main problem is that, as the G7 leaders acknowledged a few days ago, the tests necessary to implement these certificates do not exist.

As far as we know, there are promising ideas and several researchers have been able to create effective strategies to identify antibodies in the laboratory, but we do not have the necessary technology to implement these types of immune certificates safely. No one does, in fact. Even countries like South Korea that has displayed enormous diagnostic potential recognize that this move is too risky right now.

What are the quick tests we have for, then?

For the past few weeks, the case of rapid tests has been part of the political vaudeville that surrounds the epidemic. First, there were the 340,000 rapid tests that the Government had acquired and which "turned out" to be flawed; Later we have seen that the same problem was repeated in countries like the United Kingdom. And it is that, in front of the generalized idea (and that the same Ministry defended) that these tests were going to serve to decongest the PCRs, the truth is that according to everything we know they are "presumption tests".

In other words, very fast tests, but not very sensitive: they only identify 50% of the positives. What are they for then? In the phases prior to the peak, they help to "clear" the mass of possible cases and evaluate the magnitude of the contagion. This is how they were used in Korea, for example. The problem is that "in very early stages it could give false negatives" because many of those infected have not yet produced antibodies.

During the acute phase (near the peak of infections) its usefulness is not very great either. At that time, most contagions are active and easily identifiable through PCRs (infinitely more reliable than presumptive tests). Adding a screen with these tests can be useful at some point, but it does not help to decongest the bottleneck of molecular testing laboratories (and can even worsen the situation by adding more doubtful cases).

In later stages of the epidemic (and while we find more accurate serological tests) they can be used in combination with other tests to perform seroprevalence studies, a mapping of the true extent of the epidemic. It is not ideal because, if there is no rush, waiting for better tests is more cost-effective. However, there are times when circumstances may advise launching them as soon as possible (such as in case new waves are feared).

What are each type of test for?

In the upper image you can see the current diagnostic protocols in which both PCR tests and serological and immunochromatographic tests are combined to try to compose a clinically reliable framework. In contrast to the PCR that tells us if the virus is physically in the body, we have the IgM that leads the rapid immune response and the IgG that is the one that leaves immunity in the long term: a good example of the complexity of the process.

What does the government want to do?

Markus Spiske

As explained by the Ministry of Health, it is the latter that is to be implemented in the coming days. A seroprevalence study will be started with rapid tests and PCRs. As explained in the previous section, at a methodological level it may seem early to carry out this type of study. Therefore, as I said, it is one of the first to consider. Italy is beginning to raise its own, but they effectively have days of "advantage" in the evolution of the epidemic.

Right now, for practical purposes, it's like doing an epidemiological survey with PCRs (only that rapid tests will help reduce the necessary molecular tests by around 7%). And, although it may not seem like it, it is a very important decision. As the Ministry has repeated, Spain currently has the capacity to do between 15,000 and 20,000 PCR tests per day. A study of these characteristics possibly adds about 13,000 more PCRs per week (which would have to coexist with those that are already done for clinical reasons). It is, therefore, a huge logistical challenge.

Why do it now if the picture of the epidemic may change in the future? First, out of fear of a new wave. Without knowing how many people have been infected by the virus, it is very difficult to design contingency plans that help us weather the storm with minimal impact. Secondly, because we have the material. Spain has an important stock of rapid tests that, except for these types of studies, are now of little use.

Image | ICS

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