The Spanish COVID-19 prevalence study

Estudio de prevalencia coronavirus - preliminary results

Antibody testing (IgM + IgG) carried out between Apr 27th to May 5th, 60,983 participants with valid test results on mainland Spain and further 3,234 participants on the islands. Two tests were performed, one "quick" immunochromatographic assay and an antibody immunoassay. The authors report problems with the IgM testing, hence the preliminary report is mostly based on the IgG results.

https://www.mscbs.gob.es/ciudadanos/ene-covid/home.htm
https://www.mscbs.gob.es/ciudadanos/ene-covid/docs/ESTUDIO_ENE-COVID19_PRIMERA_RONDA_INFORME_PRELIMINAR.pdf
https://portalcne.isciii.es/enecovid19/indexev.html
https://portalcne.isciii.es/enecovid19/documentos/ene_covid19_sum_pre_rep.pdf

Total prevalence of SARS-Cov2 antibodies was estimated to be 5% of the population. In Spain, approximately 90% of COVID-19 infections went undetected by the health system (not a stellar performance, given that only 33.7% were found to be asymptomatic). The data shows a great geographical variation of prevalence from 1-2% for the least affected places up to almost 15% for the most affected places. No big differences by age or sex were detected - only young children had a markedly lower IgG prevalence. The reason for this is not known, might be less social contacts or biological reasons such different immune system response.

In the general population the most specific (but rare) symptom indicating a COVID-19 infection was anosmia: 3.07% of participants reported anosmia, of those 43.3% had antibodies. The other symptoms were more prevalent in the population but much less specific - having a combination of 5 or more other symptoms was correlated with 14.7% of positive cases. Apparently approximately 33.7% of cases were without any symptoms.

Analysis by Jesús Molina Cabrillana, epidemiologist of the Spanish Preventive Medicine Society (Sociedad Española de Medicina Preventiva) estimates the IFR mortality based on this study and other results to be 1-1.3%  - a preliminary estimate given that the excess mortality in Spain indicates COVID-19 might have had a much higher death toll than known.

https://elpais.com/sociedad/2020-05-13/solo-un-5-de-los-espanoles-se-han-contagiado-de-coronavirus-segun-los-primeros-datos-del-estudio-de-prevalencia.html
https://english.elpais.com/society/2020-05-14/antibody-study-shows-just-5-of-spaniards-have-contracted-the-coronavirus.html

https://www.dw.com/en/covid-19-death-rate-sinking-data-reveals-a-complex-reality/a-53365771

Strong points:
* large size of the sample, allowing meaningful statistical analysis of "positive" cases

Comment: even severely hit Spain is far away from anything like a herd immunity (if that exists). The health system in some parts of Spain was heavily overloaded so the numbers of hospitalizations are not meaningful and the number of deaths uncertain. Despite those problems this prevalence study offers interesting data regarding symptoms, prevalence of antibodies after PCR tests and mortality.

Stay tuned for the final results of this study.