More than 51,400 people were tested and surveyed across Spain in the second half of November for the prevalence study, which suggests the infections by far exceed the number of confirmed cases in Spain, of just over 1.75 million.
"One in 10 people living in Spain would have been infected ..., half during the first wave and the other half during this second epidemic wave," said Raquel Yotti, director of Spain's Carlos III Health Institute, which co-led the study.
Prevalence in Madrid was the highest of all Spanish regions, with 18.6% of the population testing positive for COVID-19 antibodies.
Previous results of the study that were published in July after testing nearly 70,000 people in April-June, showed a prevalence rate of just over 5%.
Spain has been one of Europe's hardest-hit countries by the pandemic, both in terms of contagion and the economic impact. A total of 48,401 people have died from the coronavirus, with the toll climbing by 388 over the last 24 hours.
Data from the Health Ministry also showed 10,328 new coronavirus cases were reported on Tuesday, bringing the total since the onset of the pandemic to 1,762,212 infections.
The infection rate measured over the previous 14 days is up for a second day in a row at nearly 199 cases per 100,000 people.
The government decided a second state of emergency in October with new restrictions such as night-time curfews to stem resurgent infections, which helped to reduce new cases to less than 200 per 100,000 people this month.
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