Seroprevalence of SARS-CoV-2 Infection in Portugal in May-July 2020: Results of the First National Serological Survey (ISNCOVID-19)

• Main Author: Kislaya, Irina
• Publication Date: 2021
• Other Authors: Gonçalves, Paulo, Barreto, Marta, De Sousa, Rita, Garcia, Ana Cristina, Matos, Rita, Guiomar, Raquel, Rodrigues, Ana Paula, ISNCOVID-19 group
• Format: Article
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10400.18/7679
• topic: COVID-19; Immunoglobulin G; Immunoglobulin M; SARS-CoV-2; Seroprevelence; Survey; COVID-19 Testing; Antibodies, Viral; Seroepidemiologic Studies; Anticorpos Antivirais; Estudos Seroepidemiológicos; Imunoglobulina G; Imunoglobulina M; Testagem COVID-19; Estados de Saúde e de Doença; Portugal

Introduction: The aim of this study was to estimate and describe the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) specific antibodies (immunoglobulin M and/or immunoglobulin G) in Portugal in May-July 2020. Material and methods: A cross-sectional seroepidemiological survey was developed after the peak of the first epidemic wave on a sample of 2301 Portuguese residents, aged 1 year or older. Survey sample was selected using a two-stage stratified non-probability sampling design (quota sampling). SARS-CoV-2 immunoglobulin M and immunoglobulin G antibodies were measured in serum samples by enzyme-linked immunosorbent assay. Seroprevalence estimates of immunoglobulin M and/or immunoglobulin G and 95% confidence intervals were stratified by sex, age group, health region and education. Results: Overall, seroprevalence was 2.9% (95% confidence interval: 2.0% - 4.2%). Higher prevalence rates were observed in male (4.1%, 95% confidence interval: 2.6% - 6.6%) and those with secondary education (6.4%, 95% confidence interval: 3.2% - 12.5%). Differences in seroprevalence by age group and region were not statistically significant. Discussion: The estimated seroprevalence of SARS-CoV-2 was higher than the cumulative incidence reported by the National Surveillance System but far from necessary to reach herd immunity. Conclusion: Our results support limited extent of infection by SARS-CoV-2 in the study population possibly due to early lockdown measures implemented in Portugal and support the need to continue monitoring of SARS-CoV-2 seroprevalence in order to increase our knowledge about the evolution of the epidemic and to estimate the proportion of the susceptible population over time.