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. 2024 Sep 19;11(10):ofae547.
doi: 10.1093/ofid/ofae547. eCollection 2024 Oct.

Effectiveness of the First and Second Severe Acute Respiratory Syndrome Coronavirus 2 Vaccine Dose: A Nationwide Cohort Study From Austria on Hybrid Versus Natural Immunity

Alena Chalupka  1   2 , Uwe Riedmann  1 , Lukas Richter  2   3 , Ali Chakeri  2   4 , Ziad El-Khatib  2 , Martin Sprenger  5 , Verena Theiler-Schwetz  1 , Christian Trummer  1 , Peter Willeit  6   7   8 , Harald Schennach  9 , Bernhard Benka  2 , Dirk Werber  2 , Tracy Beth Høeg  10   11 , John P A Ioannidis  12 , Stefan Pilz  1
Affiliations

Effectiveness of the First and Second Severe Acute Respiratory Syndrome Coronavirus 2 Vaccine Dose: A Nationwide Cohort Study From Austria on Hybrid Versus Natural Immunity

Alena Chalupka et al. Open Forum Infect Dis. .

Abstract

Background: We aimed to evaluate the effectiveness of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccinations in previously SARS-CoV-2-infected adults in the general population of Austria during the Delta wave and with extended follow-up.

Methods: In a nationwide retrospective cohort study, we calculated age-, sex-, and nursing home residency-adjusted Cox proportional hazard ratios (HRs) of coronavirus disease 2019 (COVID-19) deaths, SARS-CoV-2 infections, and non-COVID-19 deaths from 1 October to 31 December 2021, and secondarily with extended follow-up to 30 June 2022. Relative vaccine effectiveness (rVE) is rVE = (1 - HR) × 100.

Results: Among 494 646 previously infected adults, 169 543 had received 2 vaccine doses, 133 567 had received 1 dose, and 190 275 were unvaccinated at baseline. We recorded 17 COVID-19 deaths (6 vaccinated, 11 unvaccinated) and 8209 SARS-CoV-2 infections. Absolute risk of COVID-19 deaths was 0.003%. rVE estimates for COVID-19 deaths and reinfections exceeded 75% until the end of 2021 but decreased substantially with extended follow-up. The risk of non-COVID-19 death was lower in those vaccinated versus unvaccinated.

Conclusions: First and second SARS-CoV-2 vaccine doses appear effective in the short-term, but with diminishing effectiveness over time. The extremely low COVID-19 mortality, regardless of vaccination, indicates strong protection of previous infection against COVID-19 death. Lower non-COVID-19 mortality in the vaccinated population might suggest a healthy vaccinee bias.

Keywords: COVID-19; SARS-CoV-2; mortality; observational study; vaccine.

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Conflict of interest statement

Potential conflicts of interest. All authors: No reported conflicts.

Figures

Figure 1.
Figure 1.
Participant selection chart. Abbreviations: COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.
See this image and copyright information in PMC

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