Seroprevalence of severe acute respiratory syndrome coronavirus 2 in Slovenia: results of two rounds of a nationwide population study on a probability-based sample, challenges and lessons learned

Abstract

Objectives Seroprevalence surveys provide crucial information on cumulative severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exposure. This Slovenian nationwide population study is the first longitudinal 6-month serosurvey using probability-based samples across all age categories. Methods Each participant supplied two blood samples: 1316 samples in April 2020 (first round) and 1211 in October/November 2020 (second round). The first-round sera were tested using Euroimmun Anti-SARS-CoV-2 ELISA IgG (ELISA) and, because of uncertain estimates, were retested using Elecsys Anti-SARS-CoV-2 (Elecsys-N) and Elecsys Anti-SARS-CoV-2 S (Elecsys-S). The second-round sera were concomitantly tested using Elecsys-N/Elecsys-S. Results The populations of both rounds matched the overall population ( n = 3000), with minor settlement type and age differences. The first-round seroprevalence corrected for the ELISA manufacturer's specificity was 2.78% (95% highest density interval [HDI] 1.81%–3.80%), corrected using pooled ELISA specificity calculated from published data 0.93% (95% CI 0.00%–2.65%), and based on Elecsys-N/Elecsys-S results 0.87% (95% HDI 0.40%–1.38%). The second-round unadjusted lower limit of seroprevalence on 11 November 2020 was 4.06% (95% HDI 2.97%–5.16%) and on 3 October 2020, unadjusted upper limit was 4.29% (95% HDI 3.18%–5.47%). Conclusions SARS-CoV-2 seroprevalence in Slovenia increased four-fold from late April to October/November 2020, mainly due to a devastating second wave. Significant logistic/methodological challenges accompanied both rounds. The main lessons learned were a need for caution when relying on manufacturer-generated assay evaluation data, the importance of multiple manufacturer-independent assay performance assessments, the need for concomitant use of highly-specific serological assays targeting different SARS-CoV-2 proteins in serosurveys conducted in low-prevalence settings or during epidemic exponential growth and the usefulness of a Bayesian approach for overcoming complex methodological challenges.

Germany’s low SARS-CoV-2 seroprevalence confirms effective containment in 2020: Results of the nationwide RKI-SOEP study

“…Internationally, Germany can be classified among the countries with low seroprevalence in 2020, similar to Norway (0.9%, November to December 2020) 32 , Denmark (2.0%, August to October) 33 , and, earlier, Iceland (0.3%, April to June) 34 . Somewhat higher seroprevalences were found in nationwide population studies in Slovenia (4.3%, mid-October to mid-November 2020) 35 , Spain (5.2%, April to June) 36,37 , the Netherlands (4.5%, June to August) 38 , and England (8.9%, November) 39 and a much higher seroprevalence of 28% in October-November was estimated for the Czech Republic 40 . In Norway, the estimated ratio between seroprevalence and cumulative incidence was 1.1 32 , the study from Iceland showed a ratio of 1.8 34 , and the Danish study a ratio of 6 in August and 3 in October and December 41 .…”

SARS-CoV-2 Seroprevalence and Neutralizing Antibody Response after the First and Second COVID-19 Pandemic Wave in Croatia

“…Significantly higher seropositivity was observed after the second pandemic peak, varying from 11.4% in Poland (October–November 2020) [ 21 ], and 19.3% in Spain (Madrid, at the end of 2020) [ 22 ], to 21.1% in Switzerland (Geneva, November–December 2020) [ 23 ]. In Slovenia, the seroprevalence after the first (April 2020) and second (October–November 2020) waves was found to be 2.78% and 4.06%, respectively [ 24 ].…”

SARS-CoV-2 infections in 165 countries over time

“…To assess the accuracy of the above approach, the estimated cumulative infection fractions were compared to 22 previously published nationwide seroprevalence surveys across 14 countries (Supplemental Table S1) (Alharbi et al, 2021;Anand et al, 2020;Bogogiannidou et al, 2020;Espenhain et al, 2021;Hallal et al, 2020;Le Vu et al, 2021;Merkely et al, 2020;Murhekar et al, , 2021Nah et al, 2021;Poljak et al, 2021;Pollán et al, 2020;Reicher et al, 2021;Snoeck et al, 2020;Ward et al, 2020). Only surveys attempting to estimate nationwide seroprevalence in the general population (in particular, either using geographically or demographically stratified sampling or adjusting for sample demographics) were included.…”