COVID-19 and Influenza Incidence among Active Component United States Military Service Members at Installations in US–Mexico Border States (2020–2023)

The objective of this study was to compare coronavirus disease 2019 (COVID-19) and influenza among US active-component service members (ACSMs) stationed at military installations near the US-Mexico border (Arizona, California, New Mexico, and Texas). A retrospective cohort study was conducted to assess diagnoses of influenza (2020-2024) and COVID-19 (2020-2023). Incidence rate ratios (IRRs) comparing influenza and COVID-19 at border and non-border locations were calculated for laboratory-confirmed and/or symptom-based encounter diagnoses. Forty-five percent of ACSMs included in the influenza (N = 650,505) and COVID-19 (N = 660,333) analyses were near the border. There was no statistically significant increase of either COVID-19 or influenza incidence among ACSMs stationed near the border compared with those located elsewhere in the same states. A statistically significantly reduced adjusted IRR (aIRR) of laboratory-confirmed influenza (aIRR: 0.93; 95% CI: 0.89-0.98) and COVID-19 (aIRR: 0.97; 95%: CI: 0.96-0.99) was observed among ACSMs stationed near the border compared with those located elsewhere in the same states for all states combined for the entire study period. Similarly, significantly lower aIRRs were observed for all diagnoses (laboratory-confirmed or symptoms-based encounter) of influenza (aIRR: 0.81; 95% CI:0.80-0.82) among ACSMs near the border compared with ACSMs not near the border for all states combined for the entire duration of the study. However, incidence rates of COVID-19 and influenza varied across some specific states and time periods. By assessing the potential health threats of cross-border transmission, public health officials can more effectively deploy preventive and response measures among ACSMs living and working near the US-Mexico border.