Rapid increase of SARS-CoV-2 seroprevalence during the 2020 pandemic year in the population of the city of Tirana, Albania

Introduction While the identification of anti SARS-CoV-2 antibodies has been used to measure the hidden circulation of the COVID-19 in communities, there are few publications on the dynamics of SARS-CoV-2 seroprevalence during both waves of 2020. This study provides original data about the change in proportion of individuals showing immune response to COVID-19 between beginning of July and end of December 2020. Methods The study was conducted in two rounds, 27 June −3 July, and 21-28 December 2020, using two independently selected samples of individuals 20-70 years old. Study participants were randomly selected from lists of the inhabitants of the catchment communities of four primary health care centers in Tirana City. Serological testing was performed by an ELISA method which determines IgG class antibodies anti S1 protein of SARS-CoV-2 virus. The validity of the method was tested in a sample of blood donor’s sera of 2018. Results The proportion of individuals classified as seropositive during the first round, in early July was 7.5% (95% CI: 4.3% −10.7%). The proportion rose sharply in the second round, by late December 2020, reaching 48.2% (95% CI: 44.8% −51.7%). The same increasing pattern was observed in all studied categories. No statistical significance was found between men and women and between age categories. The prevalence of seropositive individuals was always significantly higher among those who reported symptoms and those who had done the molecular test. Conclusion The ratio of total infected cases over confirmed cases was estimated to be higher than 10 to 1 in Albania. The rapid increase in SARS-CoV-2 seroprevalence observed in Tirana City may have been facilitated by a number of factors, including the very low infection exposure during the period March -May 2020, and the consecutive high susceptibility in population. Despite the observed high seroprevalence, one month after the study, COVID-19 incidence continued to increase in Tirana.