A Spanish study published in The Lancet journal has cast doubt over the feasibility of herd immunity — when enough people become infected with a virus to stop its spread — as a way of tackling the COVID-19 pandemic.
For herd immunity to work, at least 70% of a population needs to be immune to protect the uninfected people, experts say.
The study of over 60,000 people estimates that around just 5% of the Spanish population has developed antibodies, according to the researchers, including those from the National Centre for Epidemiology, Spain.
The aim of the population-based study was to estimate the seroprevalence — the level of a pathogen in a population, as measured in blood serum — of SARS-CoV-2 infection in Spain at national and regional level.
About 35,883 households were selected from municipal rolls using two-stage random sampling stratified by province and municipality size, the researchers said.
From April 27 to May 11, 61,075 participants answered a questionnaire on history of symptoms compatible with COVID-19 and risk factors, among others.
“Prevalences of IgG antibodies were adjusted using sampling weights and post-stratification to allow for differences in non-response rates based on age group, sex, and census-tract income,” the researchers wrote in the medical journal.
“Using results for both tests, we calculated a seroprevalence range maximising either specificity (positive for both tests) or sensitivity (positive for either test),” they said.
The researchers noted that seroprevalence was 5% by the point-of-care test and 4·6% by immunoassay, with no differences by sex and lower seroprevalence in children younger than 10 years.
There was substantial geographical variability, with higher prevalence around Madrid (over 10%) and lower in coastal areas (less than 3%), they said.
The findings suggest that majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hotspot areas, according to the researchers.
“Most PCR-confirmed cases have detectable antibodies, but a substantial proportion of people with symptoms compatible with COVID-19 did not have a PCR test and at least a third of infections determined by serology were asymptomatic,” the researchers wrote in the journal.
“These results emphasise the need for maintaining public health measures to avoid a new epidemic wave,” they said.