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, according to ABC News. The move comes two months after Defense Secretary Pete Hegseth rescinded the U.S. military’s mandate that they do so.
As of June 23, 2026, at least 222 recruits on the base have fallen ill and four have reportedly been hospitalized.
In his April 21 announcement making the flu vaccine optional, Hegseth cited medical autonomy and religious freedom, describing the vaccination requirement as “overly broad and not rational,” telling troops that “your body, your faith and your convictions are not negotiable.”
The flu shot requirement that Hegseth ended had been in place since 1945, with one brief pause in 1949. It was part of a tradition of military vaccine mandates nearly as old as the United States itself.
As an epidemiologist who studies vaccine-preventable diseases, I found the end of the flu mandate striking less for its immediate impact than for what it signals. For most of American history, military commanders took for granted that infectious disease could cost them a war, which is why vaccination was considered a matter of military readiness rather than personal choice.
The Lackland outbreak is evidence that the underlying epidemiology has not changed – only the political climate surrounding public health.
A tradition that started with George Washington
The first American military vaccine mandate predates the Constitution. In the winter of 1777, Gen. George Washington ordered the mass inoculation of the Continental Army against smallpox.
His decision wasn’t ideological – it was strategic. The year before, a smallpox outbreak had torn through American troops outside Quebec, contributing to the collapse of the northern campaign. John Adams famously wrote to his wife, Abigail, that smallpox was killing 10 soldiers for every one felled in battle.
Inoculation in 1777 was itself risky. The procedure, called variolation, involved deliberately infecting a soldier with a small amount of smallpox virus to build immunity. Washington gambled that losing some to inoculation was better than losing a war to the virus. Historians have credited the decision with saving the Continental Army.
That pattern held for centuries: When an infectious disease threatened to take more soldiers off the line than enemy fire did, the military required protection.
U.S. troops received smallpox vaccinations from the War of 1812 through World War II. During World War I, the Army added typhoid vaccination. During World War II, it expanded vaccine requirements to also include tetanus, cholera, diphtheria, plague, yellow fever and, in 1945, influenza.
1945: New war, new vaccine
The flu vaccine mandate grew out of military experiences during the influenza pandemic of 1918. That spring, a novel influenza strain spread through crowded Army training camps and traveled to Europe with American troops. About 45,000 American soldiers died of influenza during World War I – nearly as many as the roughly 53,000 killed in combat.
The 1918 pandemic made clear that a respiratory virus could cripple an army. In 1941, as the country prepared to enter another world war, the U.S. Army organized an influenza commission that partnered with the University of Michigan to develop the first influenza vaccine. Clinical trials in military recruits showed that the vaccine reduced the incidence of influenza illness by 85%, and in 1945 the military mandated the vaccine. Roughly 7 million service members were vaccinated that year.
The mandate was briefly paused in 1949 after scientists realized the vaccine needed regular updates due to the virus changing. Once formulations could be adjusted seasonally, the mandate returned in the early 1950s and has stayed in place continuously – until Hegseth’s change of policy.
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COVID-19 changed vaccine politics
For decades, vaccine mandates were an unremarkable fact of military life, but COVID-19 changed that.
In August 2021, all service members were ordered to be vaccinated against COVID-19. More than 98% of active duty troops complied, but the mandate became a flash point. More than 8,000 service members were involuntarily discharged for refusing the shot.
In 2023, Congress passed a law requiring the Pentagon to rescind the military COVID-19 vaccine mandate. This reversal reframed the politics of military vaccine requirements. In January 2025, President Donald Trump ordered the reinstatement, with back pay, of troops discharged over COVID-19 vaccine refusal.
In announcing the end of the flu mandate, Hegseth relied heavily on “medical freedom” language that emerged from the COVID-19 vaccine debate, rather than on any new evidence about influenza or the effectiveness of the flu vaccine.
The medical freedom movement opposes government involvement in what its supporters see as personal health decisions – including public health recommendations such as vaccine mandates, masking and social distancing.
Does the vaccination rationale still hold?
Critics of the military flu vaccine mandate argued that flu is a milder threat than it was in 1918, that service members are healthier than the general population, and that personal choice should outweigh public health logic for a seasonal virus.
The epidemiology – and now the outbreak at Lackland – tell a different story.
Although flu seasons can vary in disease severity, the virus mutates so unpredictably that pandemic flu seasons – like those in 1918, 1957, 1968 and 2009 – are a recurring possibility. Flu still hospitalizes and kills tens of thousands of Americans annually. The Centers for Disease Control and Prevention estimates the influenza vaccine prevented roughly 180,000 hospitalizations and 12,000 deaths during the 2024-2025 season.
The military operates in precisely the conditions that favor the spread of respiratory viruses: recruit training centers, barracks, ships and submarines where people live in close quarters. When vaccination rates among Lackland recruits dropped from nearly 100% to 40% after the mandate was lifted, according to ABC News, the virus did exactly what respiratory viruses do in close quarters – it quickly spread.
The logic that drove Washington in 1777 and the Army surgeon general in 1945 to require vaccination hasn’t really changed. A sick soldier can’t deploy, can’t train and can spread illness through an entire unit. At Lackland, more than 220 recruits learned that firsthand.
This an updated version of an article originally published on April 28, 2026.
