It is the largest serological survey to date in Spain and probably in Europe. Between 27 April and 11 May (while the country was under lockdown), a representative group of over 61,000 people throughout all of Spain has been investigated.
The study shows that on average 5% of the population had developed antibodies against the virus. There were substantial differences between provinces, but even in hotspot areas such as Madrid seroprevalence was only 10 – 13%.
No differences were found between men and women. The youngest age group (0-19 years) showed a lower seroprevalence (around 3.5%). No significant differences were found between other age groups.
One-third of all infected people were asymptomatic. Only 20% of symptomatic patients who were seropositive had previously taken a PCR-test.
A large majority of the Spanish population is hence still seronegative towards SARS-CoV-2 infection. The study underlines the danger of no longer adhering to public health measures. By coincidence, Spain announced 2 new local lockdowns (La Mariña, Galicia, and Lleida/Lérida, Catalonia) in the 2 days preceding the publication of this study.