| Summary: | Background: Increased γ-glutamyl transpeptidase (GGT) levels can deplete plasma glutathione, which in turn impairs immune regulation; however, evidence on GGT levels and post-vaccine immunogenicity is lacking. Objective: To examine the association between GGT and SARS-CoV-2 spike IgG antibodies. Methods: Participants were 1479 medical staff (aged 21 to 75 years) who received a SARS-CoV-2 antibody test after their second vaccine and whose GGT levels were measured before the vaccine rollout. Elevated and highly elevated GGT levels were defined as 51–80 and ≥81 U/L, respectively. Multivariable linear regression was used to calculate the means of SARS-CoV-2 spike IgG. Results: In a basic model, both elevated and highly elevated GGT levels were associated with significantly lower antibody titers. The ratio of mean (95% CI) was 0.83 (0.72–0.97) and 0.69 (0.57–0.84) for elevated and highly elevated GGT levels, respectively. However, these associations were largely attenuated after additional adjustment for potential confounders. An inverse association between GGT levels and antibody titers was found in women [0.70 (0.51–0.97)], normal-weight adults [0.71 (0.51–0.98)], and non-drinkers [0.73 (0.46–1.14)] but not in men, overweight adults, and alcohol drinkers. Conclusions: Circulating GGT concentrations were associated with the humoral immune response after COVID-19 vaccination, but this relationship could be ascribed to confounders such as sex, BMI, and alcohol drinking rather than GGT per se.
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