Vaccine-Induced or Hybrid Immunity and COVID-19-Associated Mortality During the Omicron Wave

Abstract

Background: It is not yet entirely clear to what extent vaccine-induced or hybrid immunity offers protection from death during the omicron wave of the COVID-19 pandemic in Germany.

Methods: In this retrospective study, we evaluated 470 159 cases aged ≥ 60 years in the German federal state of Bavaria who tested positive for SARS-CoV-2 between 1 January and 30 June 2022. Cox models were used to estimate adjusted hazard ratios (aHR) for dying within 60 days of the infection, depending on sex, age, time of infection, and different levels of immunity.

Results: Over the period of observation, 3836 COVID-19-associated deaths were registered (case fatality rate 0.82%). The risk of death was significantly lower in cases with a higher level of immunity than in unvaccinated cases (aHR for a full primary level of immunity if reached less than 6 months before the date of the infection: 0.30, 95% confidence interval [0.23; 0.39]; if reached more than 6 months before: aHR 0.46 [0.35; 0.60]). A boosted level of immunity lowered the risk of death even further (if reached less than 3 months before the infection: aHR 0.17 [0.15; 0.20]; if reached more than 3 months before: aHR 0.25 [0.21; 0.29]).

Conclusion: Among elderly persons in Bavaria, a higher immunity level was associated with a substantial degree of protection against death during the Omicron wave; however, the strength of protection may have diminished somewhat over time.

Figure 1

Flowchart of case selection according to the STROBE criteria

Figure 2

Adjusted hazard ratio for COVID-19-related death (with 95% confidence bands as gray areas), depending on age (left) and date of infection (right) (among cases in which the level of immunity was unknown). The values on the y-axis follow a logarithmic scale.

Figure 3

Adjusted hazard ratio for COVID-19-related death (with 95% confidence intervals) depending on the level of immunity (among cases in which the level of immunity was unknown) and with “unvaccinated” as reference category. The values on the y-axis follow a logarithmic scale.

eFigure 1

COVID-19-related 60-day survival probabilities (Kaplan–Meier curves) for cases ≥ 60 years with a positive RT-PCR test result, depending on the level of immunity. Cases with diverse gender are included. Please note the selective presentation of the y-axis (97% to 100%).

eFigure 2

Adjusted hazard ratio for COVID-19-related death (with 95% confidence bands as gray areas), depending on age (left) and date of infection (right) (including cases in which the level of immunity was unknown). The values on the y-axis follow a logarithmic scale. Instead of 60-day mortality, 30-day mortality was analyzed.

eFigure 3

Adjusted hazard ratio for COVID-19-related death (with 95% confidence intervals) depending on the level of immunity (including cases in which the level of immunity was unknown) and with “unvaccinated” as reference category. The values on the y-axis follow a logarithmic scale. Instead of 60-day mortality, 30-day mortality was analyzed.

eFigure 4

Adjusted hazard ratio for COVID-19-related death (with 95% confidence bands as gray areas), depending on age (left) and date of infection (right) (including cases in which the level of immunity was unknown). The values on the y-axis follow a logarithmic scale. Only fatalities in which COVID-19 was the direct cause of death were analyzed.

eFigure 5

Adjusted hazard ratio for COVID-19-related death (with 95% confidence intervals) depending on the level of immunity (including cases in which the level of immunity was unknown) and with “unvaccinated” as reference category. The values on the y-axis follow a logarithmic scale. Only fatalities in which COVID-19 was the direct cause of death were analyzed.