The U.S. Army, Navy and Air Force have reinstated mandatory influenza vaccinations for new recruits, reversing a policy change implemented earlier this year by Defense Secretary Pete Hegseth after military health officials linked the absence of flu immunization requirements to outbreaks at several training facilities.

Amid a worsening flu outbreak at Lackland Air Force Base in San Antonio, Texas, the U.S. Air Force, Army and Navy are once again requiring new recruits to get vaccinated against the influenza virus, a...