Herd immunity is achieved when enough of a population has become infected with a virus or bacteria -- or vaccinated against it -- to stop its circulation.

The European Center for Disease Control told CNN that Spain's research, on a nationwide representative sample of more than 61,000 participants, appears to be the largest study to date among a dozen serological studies on the coronavirus undertaken by European nations.

It adds to the findings of an antibody study involving 2,766 participants in Geneva, Switzerland, published in the Lancet on June 11.

There have been similar studies in China and the United States and "the key finding from these representative cohorts is that most of the population appears to have remained unexposed" to Covid-19, "even in areas with widespread virus circulation," said a Lancet commentary published along with Spain's findings.

"In light of these findings, any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable," said the Lancet's commentary authors, Isabella Eckerle, head of the Geneva Centre for Emerging Viral Diseases, and Benjamin Meyer, a virologist at the University of Geneva.

Doctors are uncertain whether having antibodies to the coronavirus means someone cannot be infected again.

It's not clear how long or how well antibodies protect people from the virus.

Spain's peer-reviewed study began in April while the nation remained on a strict lockdown, and was conducted by leading government research and epidemiological agencies.

"The relatively low seroprevalence observed in the context of an intense epidemic in Spain might serve as a reference to other countries.

At present, herd immunity is difficult to achieve without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems," the report reads.

The Spanish study's lead author, Marina Pollán, who is director of the National Center for Epidemiology, told CNN: "Some experts have computed that around 60% of seroprevalence might mean herd immunity.