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By Alice Gibbs
Senior Reporter
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An influenza outbreak at a Texas Air Force training base is drawing attention to a military vaccination policy that stood for more than seven decades before being overturned this spring and partially reinstated just weeks later.
Hundreds of trainees at Joint Base San Antonio-Lackland have been infected in an outbreak that emerged shortly after Defense Secretary Pete Hegseth ended the military's longstanding flu vaccine requirement, making annual influenza vaccinations voluntary for service members. Military officials later approved exceptions allowing the Air Force and other services to again require flu shots for trainees and other groups.
The outbreak has also coincided with the death of 25-year-old trainee Keon McDaniel, who suffered a medical emergency during basic training and died days later. While Rep. Joaquin Castro said this week that the Air Force had informed his office that McDaniel died from influenza during the outbreak, the Air Force has not publicly released a cause of death and has said a medical review remains ongoing.
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While the Pentagon has not said the policy change caused the outbreak, the rapid decision to allow military services to restore vaccination requirements has raised questions about why the mandate existed for decades and what role vaccination plays in preventing outbreaks in military training environments.
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Timeline: From Pandemic Lesson to Policy Reversal
The roots of the military's influenza vaccination program trace back to the 1918 flu pandemic, when an estimated 20 to 40 percent of Army and Navy personnel became ill and more than 26,000 U.S. soldiers died.
The military introduced mandatory flu vaccination in 1945. Although the requirement was suspended in 1949 amid concerns about vaccine effectiveness against changing influenza strains, it was restored in the early 1950s and remained in place for more than 70 years.
That changed on April 21, when Hegseth announced that annual influenza vaccination would become voluntary for active-duty personnel, reservists and Department of Defense civilian employees.
"The notion that a flu vaccine must be mandatory for every service member, everywhere, in every circumstance, at all times, is just overly broad and not rational," Hegseth said at the time. "If you believe that the flu vaccine is in your best interest, then you are free to take it; you should. But we will not force you."
Within weeks, influenza began spreading through Air Force Basic Military Training at Joint Base San Antonio-Lackland. By mid-June, military officials confirmed a localized outbreak among trainees.
ABC News reported that vaccination rates among trainees had fallen from nearly 100 percent to about 40 percent after the policy change. The Air Force has not publicly confirmed that figure.
On June 24, the Pentagon confirmed it had granted exceptions to Hegseth's policy for the Air Force, Army, Navy, Defense Health Agency and National Security Agency, allowing those organizations to restore influenza vaccination requirements in certain circumstances.
Newsweek reached out to the U.S. Air Force via email for comment but they did not immediately respond to requests for comment before publication.
What Officials Have Said
The Air Force has described the situation as a "localized influenza outbreak" affecting trainees at Basic Military Training.
"Medical professionals and Public Health officials have implemented mitigation measures to isolate and treat symptomatic trainees to reduce further exposure and continue to monitor the situation," an Air Force spokesperson said in late June. Officials said symptomatic trainees were being treated with antiviral medications, including Tamiflu, while exposed trainees were being monitored.
Chief Pentagon spokesperson Sean Parnell told Newsweek that that following a review of requests exceptions to Hegseth's new policy were granted for the Army, Navy, Air Force, National Security Agency and Defense Health Agency.
He added that the decisions were "based upon thorough risk assessments" and were designed to "maximize operational readiness, lethality, and force generation, while safeguarding at-risk populations."
Parnell said the department remained "committed to the health and readiness of our warfighters and civilian personnel."
Meanwhile, Rep. Joaquin Castro, whose district includes Lackland, has linked the death of trainee McDaniel to the outbreak in a statement calling on the Pentagon to restore its vaccine mandate.
“The Air Force confirmed that trainee Keon McDaniel died from the flu during the outbreak at Lackland Air Force Base in San Antonio,” said Castro in a statement on June 30. “This is a tragedy that could have been prevented were it not for the reckless actions of Secretary Hegseth. I will continue to push for the Pentagon to fully restore its vaccine mandate and protect lives. Our military must be guided by science, not politics.”
In a June 17 statement, the Air Force said: "The cause of the medical emergency is currently under investigation, and a comprehensive medical review is being conducted to determine the facts."
Why Military Bases Are Vulnerable to Flu Outbreaks
The outbreak has unfolded in an environment that infectious disease experts say is particularly susceptible to the rapid spread of respiratory viruses.
Simon Clarke, an associate professor in cellular microbiology at the University of Reading who previously served in the military, said the nature of basic training can accelerate the spread of infections.
"Military training environments involve people living in close confines and being with one another pretty much 24-7," Clarke told Newsweek. "So that increases the likelihood of transmission of infection."
Clarke said vaccination can be particularly important in settings where large groups of people spend extended periods together because it helps interrupt the chain of transmission.
Similarly, Dr. William Schaffner, a professor of infectious diseases at Vanderbilt Heath, told Newsweek: "This is an ideal environment for the spread of a respiratory infection."
Asked whether lower vaccination rates could have contributed to the outbreak, Clarke cautioned that it is impossible to draw firm conclusions without more information about the virus involved. However, he said reduced vaccine uptake could be a significant factor.
"There is always the chance that it is a strain of flu that is not covered by the vaccine, or you've got a bad batch of vaccines—but it is most likely to be the fact that they just haven't bothered," he said.
When it comes to reinstating vaccinations for the military, Schaffner was clear: "Reinstituting the obligation to be vaccinated is a good idea. And again, I speak from my own personal experience as a health care provider where in my institution, everyone who works in the institution is obliged to be vaccinated. The vaccine prevents admission to the hospital, admission to intensive care unit, and admission to cemeteries," he said.
Contact Newsweek editors on this story: Rebecca Flood and Emma Lee-Sang.
Update 07/02/26 08:40 a.m. ET: This story was updated with comment from the Pentagon and Dr. William Schaffner.
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