Introduction: The unique characteristics of the U.S. Armed Forces, with frequent deployments exposing personnel to diverse infectious threats, result in an exceptionally highly vaccinated population. While various vaccines are administered concurrently, the impact of multiple vaccinations on health outcomes remains uncertain. This study investigates the association between concurrent vaccinations and medical encounters in service members, considering demographic variables and specific medical symptoms and the types of vaccines administered. Data and methods: Utilizing a population-based prospective design, we tracked a cohort of active-duty service members from 2004 to 2014, with complete vaccine records. Concurrent vaccinations were defined as receiving ?1 vaccine within a 7-day period. Logistic and Poisson regression analyses were employed, controlling for gender, age, race, service, military grade, and deployment status. The top 12 medical encounters and 12 commonly used vaccines were studied. Results: Demographic analysis revealed a well-matched distribution between single and multiple vaccine groups. Both regressions showed that for concurrent vaccines numbering less than 4, adverse effects were minimal, but risks increased with 5 or more vaccines. However, the effects on medical encounters varied across individual symptoms and types of vaccines, with 5 or more vaccines showed higher risks for medical encounters. Conclusion: Concurrent vaccine injection appears safe, particularly with 4 or fewer vaccines. Risks increase with a higher number of vaccines. The study contributes valuable insights into the safety of concurrent vaccinations, emphasizing the need for cautious administration in specific scenarios.