Despite being one of the worst-affected countries, only 5% of Spain’s population has developed antibodies to the novel coronavirus, according to a nationwide study of more than 60,000 people, which casted doubt on the feasibility of herd immunity as a way of tackling the pandemic.

Researchers from Harvard, MIT and several Spanish institutions analyzed findings from a widescale study on antibody prevalence in Spain, which is thought to be the largest of its kind on the coronavirus in Europe. Similar studies were done in China and the US, concluding that most of the population remained unexposed to the virus.

A total of 61,075 people participated in the study in Spain, done between April 27 and May 11. The participants had to answer a questionnaire on coronavirus symptoms and were given a point-of-care finger prick test, also having the option to donate blood to carry out more tests at the laboratory.

Only 5% of the participants presented with antibodies from point-of-care tests, while antibodies were detected in 4.6% of the blood samples, the study showed. At the same time, there was a large variability according to the location. Antibodies were found in 10% of the samples from Madrid but only 3% in those from coastal areas.

About a third of those who tested positive for COVID-19 antibodies had been asymptomatic while infected with the virus. Among those who reported having been unwell with symptoms prior to the study, 16.9% tested positive for antibodies. Meanwhile, 90% of those who had tested positive more than 14 days before the study had antibodies detected in their lab-tested blood samples.

“Despite the high impact of Covid-19 in Spain, prevalence estimates remain low and are clearly insufficient to provide herd immunity,” the authors said. “This cannot be achieved without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems. In this situation, social distance measures and efforts to identify and isolate new cases and their contacts are imperative for future epidemic control.”

Herd immunity happens when a large portion of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely. As a result, the whole community becomes protected. There are two paths to immunity, vaccines or natural infection.

Between 50% and 90% of a population must be immune to achieve herd immunity, depending on how contagious the disease is, according to experts at Johns Hopkins University. In the case of the novel coronavirus, they estimated that at least 70% of the population would have to be immune to reach herd immunity.

The health advisor to the White House Anthony Fauci said last month that if COVID-19 acted like other coronaviruses, there “likely isn’t going to be a long duration of immunity” from antibodies. Meanwhile, the WHO has said that it remains unclear whether those who have already caught the virus once will be immune to getting it again.

The study was published in The Lancet.

The situation in Spain

Spain has already reported more than 251,700 cases of coronavirus and 28,388 deaths, according to data compiled by Johns Hopkins University. The country has the third-highest number of deaths related to population in the world, with 607 deaths per million people, according to Our World in Data.

While it was one of the first countries to be hit in Europe, the situation started to stabilize with single figures in daily fatalities for most of the past three weeks. However, officials in the north-western region of Galicia have re-imposed restrictions on an area of 70,000 people following an outbreak. Officials linked local outbreaks to bars in the area.

The autonomous government of Catalonia re-imposed on Saturday controls on an area of 210,000 residents after a sharp rise in infections there. Catalan President Quim Torra said no-one would be allowed to enter or leave Segrià, a district west of Barcelona that includes the city of Lleida.