The Pentagon has reinstated the requirement for flu vaccinations among recruits across all military boot camps following a significant influenza outbreak at the U.S. Air Force’s Lackland Air Force Base in San Antonio, Texas. The outbreak, ongoing for roughly three weeks, has sickened nearly 300 trainees, according to Democratic Representative Joaquin Castro, whose district includes the base.

The reversal comes nearly two months after Defense Secretary Pete Hegseth lifted the mandatory flu shot policy for military personnel at the end of April, citing reasons of medical autonomy and religious freedom. At that time, services were given 15 days to request exceptions to continue requiring vaccinations. A Pentagon official, speaking anonymously due to the sensitivity of the matter, asserted that the decision to reimpose the mandate was unrelated to the current outbreak and noted that exception requests were being finalized earlier in June, prior to the surge in cases at Lackland.

During the period when the vaccine was optional, only about 40% of new recruits at Lackland chose to receive the flu shot, according to a source familiar with the situation. The base typically processes around 700 new trainees weekly, who live and train under conditions conducive to the rapid spread of infectious diseases. These include close quarters sleeping arrangements, communal showers, and extensive close-contact drills, compounded by high physical and psychological stress and limited sleep.

Experts note that while flu outbreaks are most common during the traditional late fall and winter "flu season," the virus can circulate year-round, particularly in environments where large groups are confined indoors. Dr. Arnold Monto, a flu specialist and emeritus professor at the University of Michigan, described the Lackland outbreak as not unusually concerning for a military setting during the off-season but emphasized vaccination as a key preventive measure in such group environments.

The Pentagon confirmed that exceptions to the vaccine mandate had been granted not only for the Air Force but also for the Army, Navy, National Security Agency, and Defense Health Agency, though specific details were not released. Additionally, the Army and Navy had sought permission to require flu shots for broader groups, including troops deploying overseas, healthcare workers, and child care personnel.

Advocacy groups have welcomed the renewed emphasis on vaccinations. Michele Slafkosky, executive director of Families Fighting Flu, highlighted the prior military policy that mandated flu vaccines for recruits as a long-standing effort to protect troop and public health. She criticized the suspension of the requirement, linking it to the recent outbreak at Lackland, and expressed optimism that the updated guidance would help prevent similar incidents in the future.