The results of the Seroprevalence test explained: why it is not good news to reach the new normal

The Ministry of Health yesterday released the preliminary results of the seroprevalence study, ENE-COVID19: only 5% of the population appears to have antibodies against SARS-CoV-2. What does this mean and what consequences does it have?

Seroprevalence report: what the data tells us

Only 5% of the population shows antibodies to the virus. This is interpreted as only 5% is immune, although the question is not so simple. But before we talk about that, let's go over the data. This figure is well below that necessary to consider the disease as stabilized, which is 70%. Since there is not yet a vaccine, the possibility of new virulent waves is expected, something we already expected.

The seroprevalence registered, pointed out from the Government, is almost 10 points below the estimates made in epidemiological models, such as that of Imperial College, one of the most prestigious and complete, which provided for a general group immunization, in Europe, of a 15% of the population. However, the prevalence of the disease, associated with immunization and the presence of antibodies, only reaches similar figures in the peninsular center.

The rest of Spain varies with figures ranging between 7%, at most; and 1% at least. The most affected provinces are Madrid, Segovia, Guadalajara, Soria, Cuenca and Ciudad Real. Another preliminary data presented in this first phase of the study speaks of the loss of smell among 43% of the respondents affected and a prevalence of 6.4% in urban centers with more than 100,000 inhabitants.

For reference, let's look at France. According to the seroprevalence reports obtained by the Pasteur Institute, it is around 10% in the French country. These figures, although higher than in Spain, are also considerably lower than expected. Overall, data from many countries point to low immunization, and this is not a good thing.

Why is it bad news?

In general terms, the expected seroprevalence is well below the estimates. With these figures we do not have group immunity. This limits the routes of transmission by which a disease can be transmitted and continue to spread to other people. It takes immunization of between 60% and 70% of people to achieve group immunity for a virus like this.

On the other hand, this report also gives us some disturbing information: either very few people have been infected or there is some immune issue that we are not understanding. It is still too early to resolve this question, but its consequences are clear: we are far from achieving group immunity that protects us from new epidemic waves.

It's still too early to be blunt

However, let's go back to the ENE-COVID19 data and recover a cautious point of view. Remember that you are analyzing a representative sample of the Spanish population. This means that they have taken several thousand volunteers at random (it was estimated that they could have 90,000, although the test has counted less than 60,000) to whom they perform rapid and slower serological tests, in two batches. These first results come from those first faster tests, carried out with an immunochromatography test (Orient Gene IgM / IgG, from the company Zhejiang Orient Gene Biotech). The latter have not yet been done.

The study will continue with two other samples taken (and two other batches) 21 days apart. This process serves to see how the disease evolves in Spanish society. But of course, we must bear in mind that it is still a sample. Experts have selected it so that it can be reliably extrapolated to the entire population.

Therefore, what we actually see is still imprecise data. Among other things, we still do not have complete information on the slowest but clearest analyzes of the serological tests that are done on the volunteers. These could change the preliminary percentage, since the PCR tests have been shown to have certain inaccuracies. Hence, an immunoassay test, probably an ELISA, is required to confirm the figures. In other words, it is still too early to settle a figure.

On the other hand, we must also wait for the analyzes of the following shots, since these will give us a real picture of the evolution of the disease, something that serves to understand its true epidemiology, and not so much seroprevalence as an isolated figure in the weather.

A small review of the coronavirus figures in Spain

Beyond the ENE-COVID19, we have other data obtained from the information collected in hospitals and health services. According to the figures to date, the case fatality rate for SARS-CoV-2 in Spain is 11.9%. This number is relative to the number of total infections, which exceeds 220,000, of the almost 50 million inhabitants in the country. This means 58 deaths for every 100,000 inhabitants.

For the defenders of "herd immunity" and not applying restrictions, confinements or other security measures: 5% of the Spanish population has passed COVID-19 and there are + than 27,000 deaths. For group immunity (60-70%), it would mean 324,000 - 378,000 deaths.

- Esther Samper (@Shora) May 13, 2020

If we extrapolated the same conditions as before, assuming that we really have 5% of the population immunized, a simplistic calculation yields between 324,000 - 378,000 deaths as a result of COVID. Of course, this simplification is not taking into account many factors, such as health availability, comorbidity, etc. Nor are we taking into account various social and biological factors. But, what is clear, is that the figures would be much higher than those registered. This brings us to an unavoidable comparison. Can we invest in another strategy, like Sweden's?

In search of group immunity: can we be a new "Sweden"?

Sweden is a very special case. Unlike other countries, his Government decided not to take drastic measures of isolation and to launch himself in search of group immunity, leaving the disease to prevail. As a result, the figures indicate a lethality of 12.4%, According to the statements of Dr. Anders Tegnell, responsible for the Swedish strategy against the coronavirus, next month they will achieve the expected group immunity, with 60% of the population immunized.

Can we compare it with Spain? If we return to the figures, although its fatality is 12%, the death toll per 100,000 inhabitants is 33, more than half that in this country, which multiplies by 10 the number of those affected. In other words, we see that the number of inhabitants can mask these figures.

On the other hand, there is also a notable cultural difference. Sweden is a country in which social distance is regularly cultivated. Finally, we do not know for sure if the figures they are working with are realistic or not, since they have not yet provided them. If we add that to the preliminary seroprevalence data we realize that it is not possible to make a comparison, at least for the time being.

In short, making a comparison with Sweden seems wrong, and not only for demographic and cultural reasons. The figures don't help either. Does that mean that it is not an example to follow, or that imitating Sweden would be bad? Neither. All it means is that we will have to go our own way towards group immunity.

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