Abstract
SARS-CoV-2 Omicron quickly spread globally, also in regions with high vaccination coverage, emphasizing the importance of exploring the immunological requirements for protection against Omicron breakthrough infection. The test-negative matched case-control study (N = 964) characterized Omicron breakthrough infections in triple-vaccinated individuals from the ENFORCE cohort. Within 60 days before a PCR test spike-specific IgG levels were significantly lower in cases compared to controls (GMR [95% CI] for BA.2: 0.83 [0.73-0.95], p = 0.006). Multivariable logistic regression showed significant associations between high antibody levels and lower odds of infection (aOR [95% CI] for BA.2 spike-specific IgG: 0.65 [0.48-0.88], p = 0.006 and BA.2 ACE2-blocking antibodies: 0.46 [0.30-0.69], p = 0.0002). A sex-stratified analysis showed more pronounced associations for females than males. High levels of vaccine-induced antibodies provide partial protection against Omicron breakthrough infections. This is important knowledge to further characterize a threshold for protection against new variants and to estimate the necessity and timing of booster vaccination.