Serum vitamin D and the risk of SARS-CoV-2 and seasonal influenza infection during the twindemic period.

BACKGROUND & AIMS: The association between circulating vitamin D levels and the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or seasonal influenza infection in vaccine recipients remains unclear. We prospectively examined these associations among healthcare workers.

METHODS

This study included 1434 staff members at a tertiary hospital who completed a questionnaire and provided blood samples in December 2023. Participants were followed for SARS-CoV-2 and seasonal influenza infections until March 2024, based on data from an in-house registry and responses to a follow-up questionnaire. Baseline serum 25-hydroxyvitamin D levels were categorized as sufficient (≥20 ng/mL), insufficient (12-19 ng/mL), or deficient (<12 ng/mL). Cox proportional hazard models were used to estimate the hazard ratio for SARS-CoV-2 or seasonal influenza infection risk across vitamin D categories, adjusting for potential confounders.

RESULTS

Among the participants (median age: 40 years; 73 % female), 1372 (96 %) had received at least three doses of a COVID-19 vaccine, and 1196 (83 %) had received a seasonal influenza vaccine. Baseline vitamin D status was not associated with SARS-CoV-2 infection risk; the adjusted hazard ratios (95 % confidence intervals) for those with vitamin D insufficiency and deficiency, compared to those with vitamin D sufficiency, were 0.69 (0.43, 1.10) and 0.95 (0.49, 1.83), respectively. No association was observed between vitamin D status and the risk of seasonal influenza infection.

CONCLUSIONS

Our findings suggest that sufficient vitamin D levels may not offer additional protection against SARS-CoV-2 and seasonal influenza infection after vaccination.