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

Stacey, MJ, Ferentinos, P, Koivula, F, Parsons, IT, Gifford, RM, Snape, D, Nicholson-Little, A, Faustini, S, Walsh, NP, Lamb, LE, O'Shea, MK, Richter, AG, Greeves, JP, O'Hara, J and Woods, D (2025) Influence of military preventive policy for recruit training on COVID-19 seroconversion: The IMPACT-COVID-19 study. BMJ Military Health. military-2024. ISSN 2633-3767

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Abstract

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

Item Type:	Article
Additional Information:	This article has been accepted for publication in BMJ Mil Health,2025 following peer review, and the Version of Record can be accessed online at http://doi.org/10.1136/military-2024-002940
Uncontrolled Keywords:	COVID-19; EPIDEMIOLOGY; Health policy; IMMUNOLOGY; Other metabolic, e.g. iron, porphyria; 32 Biomedical and Clinical Sciences; 3202 Clinical Sciences; Clinical Research; Dietary Supplements; Complementary and Integrative Health; Prevention; Infectious Diseases; Coronaviruses; Nutrition; Emerging Infectious Diseases
Subjects:	T Technology > TX Home economics > TX341 Nutrition. Foods and food supply Q Science > QR Microbiology > QR180 Immunology U Military Science > U Military Science (General) H Social Sciences > HV Social pathology. Social and public welfare. Criminology > HV697 Protection, assistance and relief
Divisions:	Sport and Exercise Sciences
Publisher:	BMJ
Date of acceptance:	17 February 2025
Date of first compliant Open Access:	20 May 2025
Date Deposited:	20 May 2025 14:44
Last Modified:	20 May 2025 15:00
DOI or ID number:	10.1136/military-2024-002940
URI:	https://researchonline.ljmu.ac.uk/id/eprint/26344

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