2385. Factors associated with delay of completion of a two-dose COVID-19 vaccine primary series among servicemembers in the Military Health System

BACKGROUND: Efficacy of a two-dose COVID-19 vaccine series at preventing infection is ∼50-80% after the first dose and ∼95% following the second, if completed on the recommended schedule. Although >90% of active-duty servicemembers (SM) completed a primary series, understanding timing of doses as recommended among Military Health System (MHS) beneficiaries is vital to prevent COVID-19 outbreaks. [Figure: see text] METHODS: Active-duty and National Guard/Reserve on active-duty SM aged ≥17 years who were beneficiaries of the MHS during 1/1/2020-6/30/2022 and completed a two-dose primary series were assessed for timing of the vaccines (stratified as early, grace period, recommended, and late) and associated demographic factors. SARS-CoV-2 infection (laboratory confirmed or ICD-10 diagnosis) prior to initiating the primary series, between the two doses, or after completion of the primary series was also assessed. Descriptive statistics (chi-square) were used to identify associations with timing of the second primary dose. RESULTS: Among 1,426,436 SM, 97.9% were aged 17-49, 81.4% were male, 69.8% were white, and 35.6% served in the Army (Table 1). Overall, 92.7% received their second primary dose in the recommended or grace periods, and 7.3% were late. Timing of the first and second vaccine doses showed peaks in the months following the vaccine EUA, as well as after the DoD vaccine mandate. SM who had a COVID diagnosis between their two primary doses were more likely to receive a late second dose, although the diagnosis was likely not the cause for the late second dose, as were those who received their first dose during the pre-mandate period. SM receiving a late second dose were more likely to be younger, in the Air Force or Army, and reside in HHS regions 4, 5, 6, 7, and 8. [Figure: see text] CONCLUSION: Most of the population received their second primary dose on the recommended schedule or earlier. However, SM in the Air Force and Army were more likely to receive their second dose later than recommended, as were those who lived in the South, West, and Midwest and those who had COVID-19 infection between their primary doses. Future research will investigate adverse disease outcomes associated with a delayed second dose in the primary series, as well as potential interactions between factors predicting delayed second dose uptake. DISCLOSURES: Simon Pollett, MBBS, AstraZeneca: The IDCRP and the Henry M. Jackson Foundation (HJF) were funded to conduct an unrelated phase III COVID-19 monoclonal antibody immunoprophylaxis trial Timothy Burgess, MD, MPH, AstraZeneca: The IDCRP and the Henry M. Jackson Foundation (HJF) were funded to conduct an unrelated phase III COVID-19 monoclonal antibody immunoprophylaxis trial