Mandatory Vaccines in the US Military - a Complicated History

April 2026: the Pentagon declared that the influenza vaccine would no longer be mandatory, changing decades of military policy. Less than two months later, an outbreak of influenza at Lackland Air Force Base led to the death of an airman and an emergency reinstatement of the influenza vaccine for all basic training environments.

This current contention around vaccine mandates in the military is a continuation of the Covid-19 vaccine mandate in 2021, however, it is not a new debate by any means.

The US military has a long and complicated history with immunizations. During the American Revolution, smallpox was a substantial threat to the ragtag forces of the newly independent colonies. Although a smallpox vaccine had not yet been developed, the much riskier smallpox inoculation had been around for a long time. Inoculated soldiers would be sick for weeks; they were contagious, and there was still some risk of death. After initially banning smallpox inoculation due to the drain on military resources, General George Washington was forced to reconsider due to the increasing number of deaths from the infection. In the winter of 1777, he mandated smallpox inoculation for all troops, and thus began the long, controversial relationship.

Illness has long been one of the greatest threats to any military force. Louse-borne typhus, in part, ended Napoleon’s excursion into Russia. During the Spanish-American War, combat deaths numbered less than 400, while nearly 3,000 troops died from disease, including malaria, yellow fever, and dysentery. Vaccines are a quick, effective way to increase group immunity.

The history of the influenza vaccine itself is intertwined with that of the US military. During WWI, American troops served as a key vector in the spread of the infection, resulting in a global pandemic that cost millions of deaths worldwide, and more American deaths than the war itself. Influenza was not identified as a virus until 1933, and with tensions building towards a second world war, the pressure was on to develop a vaccine. The US Army not only commissioned this research but provided troops for the clinical trials. By 1945, an influenza vaccine mandate was in place for US military forces.

The military's vaccine initiative has certainly not been without its flaws. In 1901, Walter Reed confirmed that mosquitoes were the carriers of yellow fever, and while aggressive mosquito control saved thousands of lives, a yellow fever vaccine was developed in 1937, on the cusp of WW2. This vaccine was derived from cell cultures using human serum. Despite warnings from Brazilian scientists about the risks of such a vaccine, the US military implemented mass administration of the vaccine. At least 50,000 American troops (potentially as many as 300,000) were infected with hepatitis B as a result.

The influenza vaccine debate is far from over and may not be as simple as it seems. Vaccine efficacy varies widely from year to year; therefore, a threshold for herd immunity has not been established. In the past, the military has strived for a 90-100% vaccination rate. The current influenza outbreak occurred after only 40% of trainees reportedly opted for the influenza vaccine. However, the Pentagon is adamant that the timing is coincidental, and the lifted mandate was not in effect long enough to factor into the current situation.

References

Associated Press. (2026). Flu vaccine no longer mandated for U.S. troops, Hegseth says. https://apnews.com/article/hegseth-pentagon-flu-vaccine-mandate-us-military-ce6069bf42de217092f9ca3154764593

Associated Press. (2026). Pentagon restores mandatory flu shots for all recruits as boot camp outbreak sickens nearly 300. https://apnews.com/article/pentagon-flu-shot-requirement-boot-camp-outbreak-4255f063ef99ea2d00cb24fec8793c32

Centers for Disease Control and Prevention. (n.d.). History of 1918 influenza pandemic. https://archive.cdc.gov/www_cdc_gov/flu/pandemic-resources/1918-commemoration/1918-pandemic-history.htm

Crosby, M. C. (2007). The American plague: The untold story of yellow fever, the epidemic that shaped our history. Berkley Books.

Löwy, I. (2021). The 1942 massive contamination of yellow fever vaccine: A public health consequence of scientific arrogance. American Journal of Public Health, 111(9), 1654–1660. https://doi.org/10.2105/AJPH.2021.306313

Marr, J. S., & Cathey, J. T. (2017). The yellow fever vaccine misadventure of 1942. Journal of Public Health Management and Practice, 23(6), 651–657. https://doi.org/10.1097/PHH.0000000000000565

World Health Organization. (n.d.). History of influenza vaccination. https://www.who.int/news-room/spotlight/history-of-vaccination/history-of-influenza-vaccination

Yen, C. F., Chan, J., Coonan, K. M., & Eberly, L. A. (2022). A historical analysis of vaccine mandates in the United States military and its application to the COVID-19 vaccine mandate. Military Medicine, 187(11–12), e1354–e1360. https://doi.org/10.1093/milmed/usac181