Study of Spain population’s COVID-19 antibody tests suggests herd immunity ‘unachievable,’ researchers say

A nationwide population-based study published by researchers in Spain in the medical journal “The Lancet” found that despite the high impact of COVID-19 on the country, antibody “prevalence estimates remain low and are clearly insufficient to provide herd immunity."

The study was conducted by the nation’s leading epidemiologists along with Spanish health authorities. A large-scale round of random testing of residents for antibodies to the new coronavirus was conducted, in which researcers found that a third of those infected do not develop symptoms.

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According to Harvard Medical School, herd immunity is when enough people become immune to a disease by either previous infection or a vaccine, making it increasingly difficult for the illness to spread. If enough people become immune, the entire community is protected.

But authors of the Spanish study say that “herd immunity is difficult to achieve without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems.”

“It is a wake-up call for public health: it is not possible to control (an outbreak) by just considering those who are symptomatic,” National Epidemiological Center Director Marina Pollán said.

“With this number of asymptomatic cases, we must follow the recommendations” for personal hygiene and social distancing, Pollán said.

Researchers in Madrid tested 61,075 participants from April 27 - May 11 and found that only 5% maintained antibodies developed from the virus. 

Additionally, only 14% of people who tested positive for coronavirus antibodies in the first round of the study’s testing eventually tested negative in future tests carried out only weeks later.

"These results emphasize the need for maintaining public health measures to avoid a new epidemic wave," the authors wrote. "Social distance measures and efforts to identify and isolate new cases and their contacts are imperative for future epidemic control."

"In light of these findings, any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable," they added. 

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Spain ended a nationwide lockdown in mid-June after restrictions on movement and public activity succeeded in reining in the country’s virus outbreak after it had pushed the health care system to the breaking point and killed thousands of people. The country is one of the hardest-hit by the pandemic in the world, with over 28,000 virus-related deaths.

The Health Ministry’s top virus expert, Fernando Simón, acknowledged that regional authorities are “correcting” their data and said he expects the national totals of deaths and infections to undergo revisions.

The troubling low prevalence of antibodies found in the Spanish populations comes as the U.S. Centers for Disease Control and Prevention updated its guidance in May on the use and reliability of antibody testing, saying that in a population where prevalence of COVID-19 is low, “less than half of those testing positive will truly have antibodies.”

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A report published on May 20 by the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP) points out that while antibody tests can provide useful information and evidence of current or previous infection, “antibody tests are generally not intended to be used as a diagnostic tool for confirming acute infection, except in unusual circumstances. Nor is it clear if having antibodies to the virus protects someone from being infected again in the future.”

Christopher Farnsworth, an instructor of pathology and immunology at Washington University School of Medicine, said antibody tests “may give some people a false sense of security.”

“Widespread antibody testing could do more harm than good if people do not understand the limitations of such testing,” he added.
As a result of the complications and potential inaccuracy, the CDC advised that antibody test results should not be used to make decisions “about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities,” or “returning persons to the workplace.”

The Associated Press contributed to this story.