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

Abstract

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.

Controlling the pandemic during the SARS-CoV-2 vaccination rollout

“…The uncertainty in the model is based on 2000 parameter samples from the posterior distribution. The data (dots - percentage seroprevalence and error bars - 95% confidence intervals) in a and b are taken from the cross-sectional seroepidemiological survey (First National Serological Survey) conducted after the first pandemic wave 59 and supplied in the Mathematica notebook available in the GitHub repository, https://github.com/lynxgav/COVID19-vaccination 57 . The total seroprevalence refers to population older than 1 year 59 .…”

Controlling the pandemic during the SARS-CoV-2 vaccination rollout: a modeling study

“…Another strength is that, unlike this study [28] and the studies for the UK [26,27], the parameters of our model are statistically evaluated to match the course of the Portuguese pandemic as reflected by age-specific hospital admissions and age-specific seroprevalence [54]. In addition, our fitting procedure allows for estimation of temporal changes in age-dependent contact patterns as a response to prior control measures during this pandemic.…”

“…Portugal in May/July 2020 [54]. This cross-sectional seroepidemiological survey was conducted on a sample of n = 2, Portuguese residents, aged 1 year or older, after the first wave.…”

“…This cross-sectional seroepidemiological survey was conducted on a sample of n = 2, Portuguese residents, aged 1 year or older, after the first wave. The survey sample was selected using a two-stage stratified non-probability sampling design (quota sampling) [54]. SARS-CoV-2 IgM and IgG antibodies were measured in serum samples by enzyme-linked immunosorbent assay.…”

“…The seroprevalence data were stratified into the five age groups [1,10), [10,20), [20,40), [40,60) and 60+ [54]. Hence, for the hospitalization data and the transmission model, a finer age stratification is used than for the seroprevalence data.…”