Scarcely five percent of the Spanish population has developed IgG antibodies against SARS-CoV-2. Results from rapid serological testing of more than 60,000 people between April 27 and May 11 have been released in the first dataset from the ENE-COVID-19 project, the national seroprevalence study for SARS-CoV-2 infection in the Spanish population.
While the distribution is similar between men and women, there is a good deal of geographic variability. Infants, children, and the youth had lower levels while older individuals had static amounts. “Just over two million people would have had contact with the virus,” said Salvador Illa, the Minister of Health, while announcing the results at a press conference. Moreover, autonomous communities such as the Ceuta, Murcia, Melilla, Asturias, and the Canary Islands had prevalence rates of less than two percent, while Castilla-La Mancha and Madrid had more than 10 percent. These results confirm the suspicion that herd immunity is not within reach.
How was the study conducted?
Rapid immunochromatography tests were run to analyse IgG antibody levels. These tests are faster but less sensitive than immunoassays. While the latter were also used, their slow analysis means results have not yet been obtained. According to the preliminary report, “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.”
The study also took into account information on the backgrounds and risk factors of the participants. Of those who had had a positive PCR test, 87 percent showed presence of IgG antibodies. Of those who reported having had anosmia, a complete loss of smell, 43 percent came back positive for IgG antibodies. The presence of antibodies appeared to correlate with the rise in the number of symptoms. Whereas, among those who showed no symptoms, only 2.5 percent had produced IgG antibodies.
What does the test tell us?
The serological tests detect the presence of antibodies produced by the body in response to the virus, and not the virus itself. A positive test would suggest that the individual may have encountered the virus, but does not necessarily mean that the person is still a carrier. A PCR test that directly checks for the presence of the virus would be needed to confirm the infection.
The antibody test analyses the levels of IgG, as they reflect a more specific and powerful response to a virus or other antigen. Immune memory is also guided through IgG antibodies.
In addition to the low prevalence of antibodies against the new coronavirus, the duration of this immunity is another uncertainty. According to a study from the journal Emerging Infectious Diseases, immunity against SARS-CoV-1, a close genetic relative to SARS-CoV-2, lasts only two years.
The ENE-COVID-19 project will run two more testing rounds. The results may help see the evolution of the population’s immune response to SARS-CoV-2 over the course of time. The final database will also include results from the more sensitive immunoassays.