The first results of the ENE-COVID-19 project, the national seroprevalence study of SARS-CoV-2 infection, have already been released. In this first round, the data obtained from the reading of the IgG band (immunoglobulin G) of the rapid serological tests applied to more than 60,000 participants between April 27 and May 11 are presented.
According to the first conclusions, the prevalence of anti-SARS-CoV-2 IgG antibodies in the population is 5%, very similar in men and women, although there is a lot of geographic variability. It is lower in infants, children, and young people, and remains fairly stable in older age groups. We are talking, therefore, that ” just over two million people would have had contact with the virus, ” as reported by the Minister of Health, Salvador Illa, at the press conference to announce the results. In addition, these preliminary data confirm what was already suspected: the long-awaited group immunity is a long way off.
Autonomous communities such as Ceuta, Murcia, Melilla, Asturias and the Canary Islands present prevalences of less than 2%, while Castilla-La Mancha and Madrid exceed 10%. In the provincial maps, the central grouping of provinces with prevalences equal to or close to 10% in the surroundings of Madrid stands out.
Study design
As we have already commented, these data come from the reading of the IgG band in rapid immunochromatography tests, which have the advantage of giving results at the moment but which, in general, are less sensitive than the tests that determine the antibodies by immunoassay. . In the first round, tests of this type were also used, but the results are not yet available, as their analysis takes more time. “The combination of the information provided by both techniques (correcting the lower sensitivity of the first with the results of the second) will maximize the representativeness and quality of the information”, indicates the preliminary report.
The study also includes an epidemiological survey with background information and risk factors related to COVID-19. 87% of the participants who report having had a positive PCR present IgG antibodies. In possible suspected cases, the prevalence increases with the number of symptoms. It is particularly high, 43%, in people who report anosmia (total loss of smell). Finally, 2.5% of the participants who did not report any symptoms had IgG antibodies.
What information does reading the tests give us?
The tests used are called indirect diagnosis: this means that they do not detect the presence of the virus, but rather the response of our body to said infection through the generation of antibodies. Therefore, a positive result in this type of test does not indicate that we are carriers of the virus , but that we have been in contact with it at some point (past or present). To know if we are infected, a direct test such as PCR must be carried out.
Why IgG? While IgM antibodies (immunoglobulins M) are usually part of the first response against attack, IgGs involve a much more specific and effective response. Furthermore, immunological memory, if established, is usually mediated by an IgG response.
In any case, it should be remembered that we do not know how long immunity against SARS-CoV-2 lasts. For SARS-CoV-1, one of the most similar coronaviruses that we know of, it is known to last around two years, according to an article published in Emerging Infectious Diseases magazine in 2007.
The project plans to carry out two more waves of tests. This information is what will be really useful, since it will allow us to know the evolution of the prevalence and the changes observed over time in the participants. In addition, as we have already commented, the results will also include the data obtained with the determination of anti-SARS-CoV-2 IgG antibodies measured by immunoassay.
All the report data can be consulted here
