The great seroprevalence test of COVID-19 in Spain has already begun: how it is going to be done and what it really is for

Yesterday the ENE-COVID began in Spain, the most important seroprevalence study that has been done so far in Spanish territory and that will be applied to 36,000 families chosen randomly among all the inhabitants of the country. How is the whole process going to be? What are they trying to find out and how important are the results in the fight against COVID-19?

This is how the seroprevalence test will work in Spain

The ENE-COVID, the seroprevalence test that has begun in Spain, will analyze between 60,000 and 90,000 samples of volunteer serum obtained throughout the national territory. The tests will be carried out at the provincial level with a number of people who vary between 900 and 6,000 depending on the city, and will collect information by age and sex. According to the Ministry, the participants will answer a short questionnaire and donate their serum to carry out tests to determine if they have antibodies against the virus.

These tests are done with a personal kit, at home or in a health center, in both cases assisted by qualified personnel. This first test will be a rapid analysis of the determination of antibodies in the blood by immunochromatography, which provides information to know if the person has been infected at any time.

To ensure the reliability of the results, they explain, it is recommended, although not mandatory, to obtain a serum sample in all patients who give their consent. These samples will be obtained with the same technique that is routinely used to perform blood tests, and will be analyzed using a more sophisticated and accurate serological technique than rapid tests. The analyzes will be coordinated by the National Center for Microbiology and there will be the collaboration of multiple hospitals throughout the country.

Together with these two studies, the participants will answer different questions about symptoms and possible sources of infection, through an epidemiological questionnaire that will be collected through a web application to facilitate the rapid collection of information. We also know that the tests will be repeated in 21 days, in a second stage, and in a third, another 21 days later. How exactly they will be carried out has not yet emerged, but it is assumed that they work in the same way to maintain the same evaluation criteria and respect the information observed on the evolution of the disease.

All households have been randomly selected and each of them will receive a phone call to inform their residents about the objectives of the ENE-COVID study and request their consent. They will also arrange a home visit, or an appointment at the health center, for a medical team to carry out the analyzes. Participation in the study is voluntary, but, they insist, the collaboration of all the people selected is very important so that the information from the study is a real photo of the situation.

What is a seroprevalence test and what is it for?

Prevalence is a statistic that indicates the proportion of the population that manifests a characteristic related to a disease. In this case, said characteristic would be the presence of the virus, roughly, and its quantity, which is called the load (of the virus). The tests show if the person has passed the disease, is passing it, or even if he can be a carrier even if he does not physically show it. The intention of the test is not to look for new infections but to understand how the disease has been distributed among the volunteers to extrapolate to the entire Spanish society.

In summary, a seroprevalence test gives data on the proportion of people who have passed or are passing the disease

The seroprevalence test is also used to observe how infections develop in a population and during a certain time. In this case, said moment will be the following months, coinciding with the de-escalation after confinement. One of the main objectives of the ENE-COVID is to obtain the first data from the markers that will indicate when to start the de-escalation.

The first information will be obtained in two weeks' time, as assured by the Government, which will allow the first decisions to be made on how to face the measures to return to normality: for example, if the virus figures in this first stage are higher than those stipulated so far by the tests routinely obtained in hospitals, this could mean reconsidering the de-escalation dates. It could also work in reverse and speed up the process.

In the following stages, experts will check how the disease evolves in Spanish homes. With epidemiological models adapted according to the data obtained, experts will be able to foresee how the virus will continue to evolve in our society. In this way, the de-escalation guidelines will be set in the coming months.

Prevalence, as a statistic, is a proportional number, without units, which can be seen as a percentage or as a number from zero to one since, after all, it talks about the number of people who have the disease compared to the total population. As this changes over time, hence the analyzes every 21 days, we can know if the measures adopted are working or we must intensify them again.

Rapid tests and seroprevalence: the problem to avoid

Immunological tests, also known as "rapid" tests for COVID-19, work by detecting people's antibodies with a simple test, similar to a pregnancy test. Antibodies are a series of particles that are produced as an immune response to a microbiological attack. In other words, they are only present when a person has been exposed to the virus. In addition, the amount of the same varies according to what situation it is in.

This is where we find the first problem: according to some studies, still in the preliminary phase, many of the people who have suffered from the disease generate antibodies, yes, but after a certain time they are so mild that they cannot be easily detected in a quick test . This could lead to false negatives. On the other hand, the tests to be used in the ENE-COVID, as reported by the Government, have an average sensitivity of 80% with a specificity of 100%.

These data indicate that, in a patient within the mean, there is an 80% probability that he will give the positive correctly and 0% that he will give a false positive. However, and as we said before, not all volunteers have to enter the "patient within the average" profile. In fact, it is impossible to know in advance if the volunteer falls within said profile a priori. This greatly increases the chance of giving a wrong result.

To correct this problem, volunteers are asked to allow them to do a second test with their serum. This is much more specific and responsive, reducing the error as much as possible. However, as we have said, this test is only performed when a patient gives his consent. Consequently, there is the possibility of failing the result if the volunteer does not want to have the second test done.

The second test requires a simple blood draw but this test, and its analysis, are only legal with the informed consent of the person. In short, the ENE-COVID faces an error problem if the participants are only analyzed by means of a quick test, something that can be solved with a PCR test that they can voluntarily access.

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