A Spanish study published in the Lancet Journal raised doubts about the feasibility of herd immunity – if enough people are infected with a virus to stop it from spreading – to combat the COVID-19 pandemic.
For herd immunity to work, at least 70 percent of the population must be immune to protect uninfected people, experts say.
The study of over 60,000 people estimates that only 5 percent of the Spanish population developed antibodies, the researchers said, including those from the National Center for Epidemiology in Spain.
The aim of the population-based study was to estimate the seroprevalence – the amount of a pathogen in a population measured in blood serum – of SARS-CoV-2 infection in Spain at national and regional level.
Approximately 35,883 households were selected from municipal lists based on a two-stage random sample, which is stratified by province and municipality size.
From April 27 to May 11, 61,075 participants answered a questionnaire on the history of symptoms that were compatible with COVID-19 and risk factors, among others.
"The prevalence of IgG antibodies was adjusted using sample weights and stratification to account for differences in non-response rates based on age group, gender, and income in the census tract," the researchers wrote in the medical journal.
"Using the results for both tests, we calculated a seroprevalence range that maximized either specificity (positive for both tests) or sensitivity (positive for both tests)," they said.
The researchers found that seroprevalence was 5 percent in the point of care test and 4.6 percent in the immunoassay, with no gender differences and lower seroprevalence in children under 10 years of age.
There was significant geographic variation, with a higher prevalence in Madrid (over 10 percent) and a lower prevalence in coastal areas (less than 3 percent).
The results suggest that the majority of the Spanish population, according to the researchers, is seronegative to SARS-CoV-2 infection even in hotspot areas.
"Most cases confirmed by PCR have detectable antibodies, but a significant number of people with COVID-19-compatible symptoms did not have a PCR test and at least a third of the serologically determined infections were asymptomatic," the researchers wrote in the journal.
"These results underscore the need to maintain public health policies to avoid a new epidemic wave," they said.
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