Seroprevalence, Waning and Correlates of Anti-SARS-CoV-2 IgG Antibodies in Tyrol, Austria: Large-Scale Study of 35,193 Blood Donors Conducted between June 2020 and September 2021

Abstract

There is uncertainty about the seroprevalence of anti-SARS-CoV-2 antibodies in the general population of Austria and about the waning of antibodies over time. We conducted a seroepidemiological study between June 2020 and September 2021, enrolling blood donors aged 18–70 years across Tyrol, Austria (participation rate: 84.0%). We analyzed serum samples for antibodies against the spike or the nucleocapsid proteins of SARS-CoV-2. We performed a total of 47,363 samples taken from 35,193 individuals (median age, 43.1 years (IQR: 29.3–53.7); 45.3% women; 10.0% with prior SARS-CoV-2 infection). Seroprevalence increased from 3.4% (95% CI: 2.8–4.2%) in June 2020 to 82.7% (95% CI: 81.4–83.8%) in September 2021, largely due to vaccination. Anti-spike IgG seroprevalence was 99.6% (95% CI: 99.4–99.7%) among fully vaccinated individuals, 90.4% (95% CI: 88.8–91.7%) among unvaccinated individuals with prior infection and 11.5% (95% CI: 10.8–12.3%) among unvaccinated individuals without known prior infection. Anti-spike IgG levels were reduced by 44.0% (95% CI: 34.9–51.7%) at 5–6 months compared with 0–3 months after infection. In fully vaccinated individuals, they decreased by 31.7% (95% CI: 29.4–33.9%) per month. In conclusion, seroprevalence in Tyrol increased to 82.7% in September 2021, with the bulk of seropositivity stemming from vaccination. Antibody levels substantially and gradually declined after vaccination or infection.

Prevalence of Anti-SARS-CoV-2 Antibodies and Potential Determinants among the Belgian Adult Population: Baseline Results of a Prospective Cohort Study

“…It is striking that the seroprevalence among unvaccinated participants with one or more COVID-19 positive tests was only 68.7%. This observation is different than the one identified in a Austrian seroprevalence study among unvaccinated blood donors [ 10 ] and in a US study evaluating the prevalence of antibodies among unvaccinated US adults [ 24 ]. The lower seroprevalence identified in this study could be explained by the fact that the time since the infection was not considered in our analyses.…”

“…SalivaHIS results were compared with other national and international seroprevalence studies that were conducted during the same study period. From these studies it appears that the seroprevalence was 66.1% in the general population in Geneva (July 2021) [ 8 ], 89.4% in the general population in England (July 2021) [ 11 ], 75.9% among blood donors in Austria (July 2021) [ 10 ], and 98.0% (July 2021) among Belgian blood donors [ 12 ]. The seroprevalence of 78.1% (July 2021) observed in our study is, thus, relatively consistent with what was found in those other studies [ 8 , 10 , 11 , 12 ].…”

“…From these studies it appears that the seroprevalence was 66.1% in the general population in Geneva (July 2021) [ 8 ], 89.4% in the general population in England (July 2021) [ 11 ], 75.9% among blood donors in Austria (July 2021) [ 10 ], and 98.0% (July 2021) among Belgian blood donors [ 12 ]. The seroprevalence of 78.1% (July 2021) observed in our study is, thus, relatively consistent with what was found in those other studies [ 8 , 10 , 11 , 12 ]. Of course, these results should be interpreted with caution as the epidemiological situation and the vaccination campaign were different in each country.…”

“…Therefore, we can hypothesize a decrease in antibodies after SARS-CoV-2 infection over time. In the Austrian study, they identified a substantial decline in antibody levels during the first 6 months post-infection and a slower decline in the subsequent months [ 10 ]. However, a decrease in antibodies after infection is not always observed in other studies [ 24 , 25 ].…”

“…COVID-19 seroprevalence studies have been conducted in several countries; as, among others, in Switzerland [ 8 ], Portugal [ 9 ], Austria [ 10 ], and England [ 11 ]. In Belgium, studies have first been set up in specific population groups such as blood donors [ 12 ], hospital healthcare workers [ 13 ], primary healthcare workers [ 14 ], schoolchildren and school staff [ 15 ], and residents and staff in nursing homes [ 16 ].…”