A large epidemiological study published in the July 6, 2020, advance online issue of The Lancet found that most individuals who became infected with SARS-CoV-2 developed antibodies to the virus, confirming that infection usually results in at least a short-term immune response.

However, the results of antibody testing also showed that at least a third of SARS-CoV-2 infections were asymptomatic, and in some of those patients, the antibody response waned more quickly, potentially leaving them vulnerable to reinfection. 

And even in Spain, which was an early hotspot of the pandemic, only about 5% of individuals overall had antibodies to the virus – far below the estimated 70% that would be necessary to achieve what’s known as herd immunity.

The fact that many SARS-CoV-2 infections remain asymptomatic, and given the ability of SARS-CoV-2 to spread via asymptomatic transmission, has complicated attempts to understand its epidemiology and spread, which means that so far, basic questions of the pandemic remain unanswered

The ENE-COVID study was designed to capture how much of the Spanish population had been exposed nationally, and how exposure varied by region, as well as by demographic factors such as age and sex.

In their study, the authors repeatedly tested 61,000 individuals from 36,000 households across Spain for antibodies to SARS-CoV-2. They used two separate tests, a point-of-care test and, in individuals who agreed to donate blood, an additional laboratory assay. Participants were also asked about COVID-19 symptoms.

Overall, roughly 5% of the population had antibodies to the virus, Antibody prevalence was higher in hotspot areas, but was no higher than 15% even in heavily affected regions.

Symptoms of illness, diagnoses and antibody results were not, to say the least, tightly correlated. At least a third of those who were antibody-positive had had no symptoms, and only about 20% of individuals who self-reported symptoms compatible with COVID-19 had had a PCR test to check for active infection.

In a commentary that was published along with the study, researchers at the University of Geneva’s Center for Emerging Viral Diseases concluded, among other things, that the findings reinforce the necessity of a vaccine to achieve herd immunity.

“Any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable,” they wrote.

The authors of the study themselves made the same point. “Despite the high impact of COVID-19 in Spain” – which was the epicenter of the European outbreak after Lombardy, and has had the highest number of diagnosed cases of any country in the European Union – “prevalence estimates remain low and are clearly insufficient to provide herd immunity,” they wrote. Such herd immunity “cannot be achieved without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems.”