Madrid CNN  — 

Spain’s large-scale study on the coronavirus indicates just 5% of its population has developed antibodies, strengthening evidence that a so-called herd immunity to Covid-19 is “unachievable,” the medical journal the Lancet reported on Monday.

The findings show that 95% of Spain’s population remains susceptible to the virus. 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. But we are very far from achieving that number.”

Spain has been one of the countries in Europe hit hardest by the coronavirus, with more than 28,000 deaths and 250,000 cases.

The Lancet published results of the first phase of Spain’s study, conducted from April 27 to May 11, which showed a nationwide antibody prevalence of 5%.

But the Madrid metropolitan area, the hardest-hit in the country by Covid-19, had more than 10% prevalence, and densely urban Barcelona had 7%, while many other coastal provinces had far lower rates.

Similarly, Geneva’s prevalence was 10.8% in the Swiss study conducted from April to early May, the Lancet reported.

“With a large majority of the population being infection naïve, virus circulation can quickly return to early pandemic dimensions in a second wave once measures are lifted,” the Lancet’s commentary authors Eckerle and Meyer wrote of the findings.

Spain’s second study phase results were released on June 4, showing a 5.2% national prevalence, just slightly higher than in the first phase. The results from the third and final phase were made public on Monday; they showed that national prevalence remained at 5.2%, Pollán said.