The study, which tested nearly 70,000 people across Spain three times over the past three months
Results from the final stage of a nationwide antibody study show some 5.2% of Spain's population has been exposed to the new coronavirus, health officials said on Monday, confirming findings from earlier stages.
The study, which tested nearly 70,000 people across Spain three times over the past three months, found the virus' prevalence had not altered significantly since preliminary results were published in May.
It also suggested that immunity to the virus can be short-lived, with 14% of participants who tested positive for antibodies in the first stage subsequently testing negative in the last stage.
"Immunity can be incomplete, it can be transitory, it can last for just a short time and then disappear," said Dr Raquel Yotti, director of Spain's Carlos III Health Institute, which co-led the study.
The loss of immunity was most common among people who never developed symptoms.
Speaking at a news conference, she appealed to Spaniards to remain prudent, particularly those who had recovered from the virus and considered themselves immune.
"We can't relax, we must keep protecting ourselves and protecting others," she said.
Hit by one of the world's most severe outbreaks, Spain confined its population to their homes in mid-March, gradually lifting restrictions from May as the death rate fell.
International borders were opened at the beginning of July, providing a shot in the arm to the country's struggling tourism sector.
But in a sign that the risk is far from over, the regions of Galicia and Catalonia imposed local lockdowns over the weekend, isolating some 270,000 people after small-scale outbreaks were detected.
Catalonia - which was in 2019 the most visited Spanish region by foreign tourists - sought on Monday to reassure visitors, saying the lockdown in the Segria county only affected 2.5% of the region's population.