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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.

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