Influence of military preventive policy for recruit training on COVID-19 seroconversion: the IMPACT-COVID-19 study

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.

Two cohorts were recruited from Yorkshire, Northern England. Infantry recruits received daily oral vitamin D (1000 IU for 4 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. The impact of supplementation was analysed on an intention-to-treat basis in volunteers completing testing at all time points and remaining unvaccinated against SARS-CoV-2. Variation in seroconversion with vitamin D change was explored across, and modelled within, each population.

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 vs 53.5±22.4 mmol/L, 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 to 0.997) was not evidenced in the University cohort.

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 the implementation did not have a clinically meaningful impact on seroconversion during military training.