2387. Assessment of COVID-19 primary series vaccination and factors associated with early and late uptake among Servicemembers in the Military Health System
Authors: Stewart Laveta , Craig-Kuhn Megan Clare , Sercy Erica , Stern Caryn , Graham Brock , Michel Amber , Parmelee Edward , Shackelford Stacy , Pollett Simon , Burgess Timothy , Tribble David R , Tribble David R 
Published: November 2023

Abstract

Background
Identifying predictors of COVID-19 vaccination receipt in servicemembers (SM) is critical for protecting US military health. We report active-duty (AD) SM vaccination across 20 months of eligibility and assess factors associated with adoption of the primary series vaccine pre/post the Department of Defense (DoD) mandate, 8/24/21.

Methods
Data on AD, National Guard/Reserve on AD, ages &gt;17, Military Health System (MHS) beneficiaries (1/1/20 - 6/30/22) were extracted from the MHS Data Repository. Descriptive statistics were assessed using Chi-square. Univariate and multivariate modeling was conducted to examine the relationship of demographics and COVID-19 diagnosis with initiation of the COVID-19 primary series vaccine pre mandate. Statistical significance was defined as p&lt; 0.05.

Results
Of 1,889,750 SMs, most were ages 17-49, male, lived in the US, white, not Hispanic, and served in the Army. Overall, 90% received &gt;1 primary series dose; with 85% completing the primary series, 5% had partial primary series, and 10% had no record of vaccination at the end of observed time.
Of SMs with &gt;1 primary series dose, 78% received the 1st dose pre mandate (11% had COVID-19 prior to 1st dose) and 22% post mandate (19% had COVID-19 prior to 1st dose; p&lt; 0.001) (Table 1 and Fig. 1).
SM ages &gt;50 had higher odds of vaccine initiation pre mandate (P&lt; 0.001), compared to ages 17-49 (Table 2). States in the Midwest and North central US had lower odds of pre mandate initiation compared to states in the southeast, aOR=0.85 and 0.87 respectively.
Black/African American SM had lower odds (aOR=0.71) of pre mandate initiation, and Asians or Pacific Islanders had higher odds (aOR=1.78) (P&lt; 0.001) compared to white SM (Table 2). Coast Guard and Navy had higher odds of pre mandate initiation; Air Force and Marine Corps had lower odds (P&lt; 0.001) compared to Army. SM who had COVID infection prior to 1st primary dose had lower odds (P&lt; 0.001) of initiating pre mandate.Table 1:Demographic factors of the active-duty and Guard/Reserve on active-duty population who received ≥1 primary series vaccine dose according to primary series initiation timing (pre-mandate versus post-mandate)
1 Domestic regions of residence are defined by Health and Human Services (HHS) regions and all others are considered outside the US.
Figure 1: COVID-19 vaccine primary series initiation pre/post DoD mandate, by month among Servicemembers in the Military Health System Table 2. Univariate and adjusted associations with receiving a first primary dose prior to the COVID-19 vaccine military mandate (odds ratios&gt;1 indicate higher odds of receiving the vaccine before the mandate)
1 The model was adjusted for all factors shown in the table: age, gender, region of residence, race, Hispanic ethnicity, branch of service, COVID-19 infection prior to initiation primary series, and active-duty status. 2 Domestic regions of residence are defined by Health and Human Services (HHS) regions and all others are considered outside the US.

Conclusion
Although the majority of SM initiated the COVID-19 primary series prior to the DoD mandate, age, race, history of COVID-19 diagnosis, and region were associated with uptake post-mandate. Understanding the factors that affect vaccine uptake among SM is important to guide vaccine policy to enhance medical readiness and optimal vaccine effectiveness.

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