Some 14 percent of those who tested positive for antibodies in the first stage of the study, later tested 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 the Carlos III Health Institute in Madrid, which co-authored the study.
The loss of immunity was most common among people who never developed symptoms.
Results from the final stage of a nationwide antibody study showed 5.2 percent of the Spanish population has been exposed to the pathogen, health officials said.
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.