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BackgroundBecause a large proportion of the Austrian population has been infected with SARS-CoV-2 during high incidence periods in winter 2021/2022, up-to-date estimates of seroprevalence of anti-SARS-CoV-2 antibodies are required to inform upcoming public health policies. MethodsWe quantified anti-Spike IgG antibody levels in 22,607 individuals that donated blood between October 2021 and April 2022 across Tyrol, Austria (participation rate: 96.0%). ResultsMedian age of participants was 45.3 years (IQR: 30.9-55.1); 41.9% were female. From October 2021 to April 2022, seropositivity increased from 84.9% (95% CI: 83.8-86.0%) to 95.8% (94.9-96.4%) and the geometric mean anti-Spike IgG levels among seropositive participants increased from 283 (95% CI: 271-296) to 1437 (1360-1518) BAU/mL. The percentages of participants in categories with undetectable levels, and detectable levels at <500, 500-<1000, 1000-<2000, 2000-<3000, and [≥]3,000 BAU/mL were 15%, 54%, 15%, 10%, 3%, and 3% in October 2021 vs. 4%, 18%, 17%, 18%, 11%, and 32% in April 2022. Of 2711 participants that had repeat measurements taken a median 4.2 months apart, 61.8% moved to a higher, 13.9% to a lower, and 24.4% remained in the same category. Among seropositive participants, antibody levels were 16.8-fold in vaccinated individuals compared to unvaccinated individuals (95% CI: 14.2-19.9; p-value < 0.001). ConclusionAnti-SARS-CoV-2 seroprevalence in terms of seropositivity and average antibody levels has increased markedly during the winter 2021/2022 SARS-CoV-2 waves in Tyrol, Austria.
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It is uncertain to which extent antibody and T-cell responses after vaccination against SARS-CoV-2 are associated with reduced risk of breakthrough infection and whether their measurement enhances risk prediction. We conducted a phase-4 open-label clinical trial in the pre-omicron era, enrolling 2,760 individuals aged [≥]16 years 35{+/-}8 days after having received the second dose of BNT162b2 (baseline 15-21 May 2021). Over a median 5.9-month of follow-up, we identified incident SARS-CoV-2 breakthrough infections using weekly antigen tests, a confirmatory PCR test, and/or serological evidence for incident infection. We quantified relative risks adjusted for age, sex, and prior SARS-CoV-2 infection for different immunological parameters and assessed improvements in risk discrimination. In contrast to the T-cell response, higher plasma levels of binding antibodies and antibodies in a surrogate neutralization assay were associated with reduced risk of breakthrough infection. Furthermore, assessment of anti-spike IgG levels enhanced prediction of breakthrough infection and may therefore be a suitable measurable correlate of protection in practice.
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BackgroundThere is uncertainty about the seroprevalence of anti-SARS-CoV-2 antibodies in the general population of Austria, and about the extent to which antibodies elicited by vaccination or infection wane over time. AimTo estimate seroprevalence, waning, and correlates of anti-SARS-CoV-2 IgG antibodies in the Federal State of Tyrol, Austria. MethodsWe 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 analysed serum samples for antibodies against spike or nucleocapsid proteins of SARS-CoV-2 with Abbott SARS-CoV-2 IgG assays. ResultsWe performed 47,363 serological tests among 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% (99.4-99.7%) among fully vaccinated individuals, 90.4% (88.8-91.7%) among unvaccinated with prior infection, and 11.5% (10.8-12.3%) among unvaccinated without known prior infection. Anti-spike IgG levels were reduced by 44.0% (34.9-51.7%) at 5-6 months compared to 0-3 months after infection. In fully vaccinated individuals, they decreased by 31.7% (29.4-33.9%) per month. In multivariable adjusted analyses, both seropositivity among unvaccinated and antibody levels among fully vaccinated individuals were higher at young age (<25 years), higher with a known prior infection, and lower in current smokers. ConclusionSeroprevalence 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.
