Influence of Military Preventive Policy for ReCruit Training on COVID-19 seroconversion: the IMPACT-COVID-19 study

Introduction

Recruitment and training is vital to maintaining the size, deployability and effectiveness of armed forces, but was threatened early in the COVID-19 pandemic. Reports suggested asymptomatic seroconversion driving SARS-CoV-2 transmission in young adults. Potential association between lower vitamin D status and increased infection risk was also highlighted. We aimed to prospectively determine seroconversion and test the hypothesis that this would vary with vitamin D supplementation in representative populations.
Methods

Two cohorts were recruited from Yorkshire, Northern England. Infantry recruits received daily oral vitamin D (1000 IU for four weeks, followed by 400 IU for the remaining 22 weeks of training) in institutional countermeasures to facilitate ongoing training/co-habitation. Controls were recruited from an un-supplemented University population, subject to social distancing and household restrictions. Venous blood samples (baseline and Week 16) were assayed for vitamin D and anti-SARS-CoV-2 spike glycoprotein antibodies, with additional serology (weeks 4, 9, 12) by dried blood spot. Impact of supplementation was analysed on an intention-to-treat basis in volunteers completing testing at all timepoints and remaining unvaccinated against SARS-CoV-2. Variation in seroconversion with vitamin D change was explored across, and modelled within, each population.
Results

In the military (n=333) and University (n=222) cohorts, seroconversion rates were 44.4% vs 25.7% (P=0.003). At week 16, military recruits showed higher vitamin D (60.5 ± 19.5 mmol.L-1 vs. 53.5 ± 22.4 mmol.L-1, p < 0.001), despite <50% supplementation adherence. A statistically significant (p=0.005) effect of negative change in vitamin D (%) on seroconversion in recruits (OR of 0.991 and 95% CI of 0.984-0.997) was not evidenced in the University cohort.
Conclusion

Among unvaccinated populations, SARS-CoV-2 infection of infantry recruits was not reduced by institutional countermeasures, versus civilians subject to national restrictions. Vitamin D supplementation improved serum levels, but implementation did not have a clinically meaningful impact on seroconversion during military training.