SARS-CoV-2 seroprevalence in Portugal following the third epidemic wave: results of the second National Serological Survey (ISN2COVID-19)

• Published in: Infectious diseases (London, England) England), 2022-06, Vol.54 (6), p.418-424
• Main Authors: Kislaya, Irina, Gonçalves, Paulo, Gómez, Verónica, Gaio, Vânia, Roquette, Rita, Barreto, Marta, Sousa-Uva, Mafalda, Torres, Ana Rita, Santos, Joana, Matos, Rita, Manita, Carla, Almeida Santos, João, Soeiro, Sofia, de Sousa, Rita, Costa, Inês, Verdasca, Nuno, Guiomar, Raquel, Rodrigues, Ana Paula
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Aged; Antibodies, Viral; Child; COVID-19; COVID-19 - epidemiology; Cross-Sectional Studies; Humans; Immunoglobulin G; Middle Aged; Portugal; Portugal - epidemiology; SARS-CoV-2; Seroepidemiologic Studies; seroepidemiological survey; Seroprevalence; severe acute respiratory syndrome coronavirus 2; Young Adult
• ISSN: 2374-4235; 2374-4243
• DOI: 10.1080/23744235.2021.2025421

Integrated approaches to surveillance of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection are important for public health actions. The 2nd National Serological Survey (ISN2COVID-19) aimed to characterize the extent of SARS-CoV-2 infection and vaccine-induced response in the Portuguese population following the third epidemic wave and the launch of the vaccination campaign. A cross-sectional study was performed using data on 8463 Portuguese 1-79 years of age, collected in February and March, 2021. SARS-CoV-2 IgM and IgG (anti-nucleoprotein and anti-spike) antibodies were determined in serum samples using Abbott Architect chemiluminescent microparticle assays. Post-infection and vaccine-induced seroprevalence with 95% confidence intervals (95%CI) were estimated in the overall sample and stratified by population characteristics. The estimated seroprevalence was 15.5% (95%CI:14.6-16.5%), of which 13.5% (95%CI: 12.6-14.4%) was attributable to natural infection and 2.0% (95%CI:1.7-2.4%) to vaccination. The lowest seroprevelence was observed in persons aged 70-79 years (8.9% 95%CI:6.8-11.6), while seroprevalence in children (14.3%; 95%CI:11.5-17.6%) and adolescents (12.9%; 95%CI:10.5-15.7%) was similar to that of persons aged between 20 and 69 years. Of seropositive individuals, 22.6% (95%CI:19.7-25.9%) did not report any symptoms in 6 months prior to interview. Of persons with completed vaccination (2-doses), 98.6% (95%CI: 93.0-99.7%) had specific IgG (anti-S) antibodies. After the third epidemic wave, the post-infection SARS-CoV-2 seroprevalence was 1.7 times higher than the cumulative incidence based on PCR-testing, but was higher (2.7 times) in children may be due to the high proportion of asymptomatic and mild infections.