All branches of the U.S. military began once again requiring their recruits to get flu vaccines earlier this month, an exception to Defense Secretary Pete Hegseth's decision to lift the military's vaccine mandate, a Pentagon official confirmed to CBS News on Wednesday. 

The news comes as Lackland Air Force Base in Texas — home to the Air Force's Basic Military Training program — grapples with a flu outbreak that has infected 275 people in recent weeks, a congressional staffer with knowledge of the matter told CBS News. The process of reinstating the mandate for recruits began before the Lackland outbreak was publicly acknowledged.

The unit at Lackland has implemented mitigation measures, is monitoring trainees who may have been exposed and is treating symptomatic trainees with antiviral medications such as Tamiflu, an Air Force spokesperson told CBS News last week.

Hegseth announced in late April that he was making the annual flu vaccine voluntary for service members "effective immediately." In a video posted to social media, Hegseth said requiring people to get vaccinated was "overly broad and not rational." 

By early May, all military departments had formally requested exemptions that allow them to keep requiring flu vaccinations for certain service members, and those exemptions were granted in early June, according to the congressional staffer. The exemptions typically apply to vulnerable populations like people who live in communal environments, healthcare workers and other categories, the staffer said.

Chief Pentagon spokesman Sean Parnell said in a statement last week that exceptions to the voluntary flu vaccine policy had been issued following a "comprehensive review."

"The decisions were based upon thorough risk assessments and are designed to maximize operational readiness, lethality, and force generation, while safeguarding at-risk populations," Parnell said. "The Department remains committed to the health and readiness of our warfighters and civilian personnel."

Asked for comment Wednesday, the Pentagon referred CBS News to Parnell's earlier statement.

The flu vaccine was first mandated for troops in 1945, leading to millions of vaccinations, according to a 2022 analysis of vaccine mandates in the military. The requirement was lifted in 1949 but reinstated in the 1950s, and flu vaccines remained mandatory until Hegseth's order.

The Pentagon has long required personnel to get vaccinated against a range of diseases from hepatitis B to measles, mumps and rubella. Military vaccination programs date back to Gen. George Washington's leadership of the Continental Army during the Revolutionary War, a 2021 Congressional Research Service report notes.

The Biden administration also required service members to receive COVID-19 vaccines, leading thousands of people who declined to get vaccinated to voluntarily or involuntarily leave the military. That mandate was lifted in 2023, and last year, President Trump authorized service members who refused the COVID-19 vaccine to be reinstated.