Covid-19 antibodies in Spain's population "are insufficient to provide herd immunity," a new study has claimed, despite the country being one of the worst-affected by the pandemic.
In a peer-reviewed paper published in the Lancet medical journal Monday, researchers from Harvard, MIT and several Spanish institutions analyzed findings from a widescale study on antibody prevalence in Spain.
More than 251,700 cases of coronavirus have been confirmed in Spain, while the virus has killed 28,388 people in the country to date, according to data compiled by Johns Hopkins University. With 607 deaths per million people, Spain has the third-highest number of deaths relative to population in the world, according to Our World in Data.
Households all over Spain were invited at random by the research team to take part in the study, which aimed to determine the proportion of the population that had developed antibodies for the coronavirus.
A total of 61,075 people agreed to participate in the study, which was carried out between April 27 and May 11. Participants answered a questionnaire on coronavirus symptoms, were given a point-of-care finger prick test, and had the option to donate blood for further laboratory testing (which 51,958 of the people in the study did).
Just 5% of participants presented with antibodies from point-of-care tests, while antibodies were detected in 4.6% of the blood samples.
According to the findings, there was "substantial geographical variability," with antibodies found in 10% of samples from Madrid but just 3% of those taken from coastal areas.
Around a third of those who tested positive for Covid-19 antibodies had been asymptomatic while infected with the virus, the study found.
Among those who reported having been unwell with symptoms of Covid-19 prior to the study, 16.9% tested positive for antibodies. Meanwhile, 90% of those who had tested positive for the coronavirus more than 14 days before taking part in 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 report's 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 is achieved when immunity is built among the general population through some exposure to a virus or infection. The strategy has been cited by health officials in Sweden, which controversially did not impose a lockdown.
However, many experts are skeptical about the effectiveness of such an approach, warning that immunity to the coronavirus is not guaranteed after infection or may only last a short while.
Speaking on CNBC's "Squawk Box Europe" on Monday, Danny Altmann, professor of immunology at Imperial College London, said it was not a "safe bet" to rely on immunity to Covid-19 as a strategy for coping with the pandemic, adding that herd immunity strategies were "probably never going to work."
Depending on how contagious an infection is, between 50% and 90% of a population must be immune to achieve herd immunity, according to experts at Johns Hopkins University, who estimate that at least 70% of the population would need to be immune to Covid-19 for herd immunity to be realized.
Top White House health advisor Dr. Anthony Fauci speculated last month that if Covid-19 behaved like other coronaviruses, there "likely isn't going to be a long duration of immunity" from antibodies. Meanwhile, the WHO has stated that it remains unclear whether those who have already caught the virus once will be immune to getting it again.
— CNBC's Berkeley Lovelace Jr. contributed to this article.