Seroprevalence of SARS-CoV-2 in Spain

Quick Takes

  • The seroprevalence of SARS-CoV-2 in Spain is 5% even in hotspot areas.
  • At least a third of people exposed to SARS-CoV-2 were asymptomatic.
  • A substantial proportion of people exposed to SARS-CoV-2 may not have detectable antibodies.

Study Questions:

What is the seroprevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Spain?

Methods:

ENE-COVID (Seroepidemiological Survey of SARS-CoV-2 Virus Infection in Spain) is a nationwide population-based cohort study assessing the seropositivity of SARS-CoV-2 in noninstitutionalized people, sampling 35,883 households selected across the 50 Spanish provinces and municipalities of various sizes. The first wave of the study, for which findings are published in this article, was conducted from April 27 to May 11, 2020. Two serologic tests were done: a point-of-care rapid test applied directly to finger-prick blood (Orient Gene Biotech COVID-19 IgG/IgM against spike protein Rapid Test Cassette; Zhejiang Orient Gene Biotech, Zhejiang, China), and an immunoassay (SARS-CoV-2 nucleoprotein IgG for use with ARCHITECT; Abbott Laboratories, Abbott Park, IL, USA) that requires analysis in a laboratory. The specificity of both tests was >95%, while sensitivity was 82% for the point-of-care test and 90% for the immunoassay. Seroprevalence was defined as the proportion of individuals with a positive result in the IgG band of the point-of-care test or a positive immunoassay result.

Results:

A total of 61,075 participants received the point-of-care test, and 51,958 underwent immunoassay testing for seroprevalence. The overall seroprevalence for the country was 5.0% (95% confidence interval, 4.7-5.4) by the point-of-care test and 4.6% (4.3-5.0) by immunoassay. Madrid and six other central provinces had higher seroprevalences (~10%) compared to coastal regions. Seroprevalence estimates were generally similar using both methods of testing. Seroprevalence steadily increased with age from 1.1% in infants until plateauing around 6% in people aged ≥45 years. Seroprevalence was greater in those with a confirmed coronavirus disease 2019 (COVID-19) contact. Participants working in essential sectors did not show higher seroprevalence values than the general population. Those reporting a history of symptoms consistent with COVID-19 had a prevalence of 16.9%. Only 20% of symptomatic patients who were seropositive reported undergoing testing for SARS-CoV-2, which was positive in about 75% of cases. The proportion of individuals with a positive test who were asymptomatic was 32.7%.

Conclusions:

The majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hotspot areas.

Perspective:

The findings of this first nationwide seroprevalence study are consistent with those from smaller ones suggesting that only 5% of the population in Spain—a European country heavily affected by COVID-19—has been exposed to SARS-CoV-2. If this is true, we are far from achieving herd immunity. Policy measures and social distancing are essential to mitigate uncontrolled spread and its consequences. However, while serologic surveys represent the best tool to determine the spread of infectious disease, whether a negative serology test truly indicates the absence of exposure to SARS-CoV-2 is unclear. Recent studies have reported a rapid loss of antibodies weeks after testing positive in certain patient populations, suggesting the seroprevalence might be an underestimate of the true exposure. Thus, the usefulness of serologic testing for SARS-CoV-2 is still undetermined. Understanding who is susceptible to severe forms of COVID-19 may be more clinically useful to identify at-risk patient groups and focus infection prevention efforts.

Clinical Topics: Prevention

Keywords: Antibodies, Coronavirus, COVID-19, Immunity, Herd, Immunoglobulins, Nucleoproteins, Primary Prevention, SARS Virus, Seroepidemiologic Studies, severe acute respiratory syndrome coronavirus 2


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