The flu shot has been around for about 80 years. The first influenza vaccine was developed in the 1940s by researchers at the University of Michigan, initially for use by the U.S. military. It became available to the general public shortly after, and the vaccine has been reformulated and improved every year since.
Why the 1918 Pandemic Sparked the Search
The push to create a flu vaccine started decades before one actually worked. The 1918 influenza pandemic killed tens of millions of people worldwide, and scientists scrambled to develop something protective. The problem was that no one had yet identified the actual cause of influenza. Researchers in 1918 believed a bacterium called Pfeiffer’s bacillus was responsible, so the vaccines developed during that pandemic targeted the wrong organism entirely.
In New York City, the health department’s laboratory director developed a heat-killed bacterial vaccine and promoted it as protection against influenza. At Tufts Medical School in Boston, another researcher created a similar vaccine from locally isolated bacterial strains and marketed it as both a preventive and a treatment. In Pittsburgh, a team isolated their strains, prepared a vaccine, did minimal safety testing, and handed it to the Red Cross for human use, all within a single week. None of these vaccines worked against influenza because the virus itself wouldn’t be identified until the 1930s.
The First Real Flu Vaccine
Once scientists could actually isolate and grow the influenza virus, a real vaccine became possible. In the 1940s, Thomas Francis Jr. and Jonas Salk (who would later become famous for the polio vaccine) led the effort at the University of Michigan, with funding and support from the U.S. Army. The military had a direct stake in this work: influenza had devastated troop ranks during World War I, and they wanted to prevent a repeat.
Francis and Salk developed an inactivated vaccine, meaning it contained killed virus that couldn’t cause infection but could train the immune system to recognize influenza. They grew the virus in fertilized chicken eggs, a production method that is still used to manufacture most flu vaccines today. This original vaccine was monovalent, covering only a single strain of influenza A.
The First Large-Scale Trials
For the 1943-44 flu season, Francis and Salk organized a major field trial involving 12,500 subjects, all participants in the Army Specialized Training Program at eight universities and five medical schools. The results were striking: only 2 percent of vaccinated subjects got sick. Researchers also observed a “herd effect,” where flu rates dropped even among unvaccinated people living in the same dormitories as vaccinated individuals. This was strong early evidence that widespread vaccination could protect entire communities, not just the people who received the shot.
From Military Use to Public Health Staple
After its success in military trials, the flu vaccine moved into civilian use within a few years. But influenza is a moving target. The virus mutates constantly, which means a vaccine that works one year may not match the strains circulating the next. This reality shaped the entire infrastructure around the flu shot as we know it today.
Starting with the 1968 influenza pandemic, the World Health Organization began holding annual consultations with experts to recommend which virus strains should be included in the following season’s vaccine. That system of global surveillance and yearly reformulation continues today, which is why you need a new flu shot every fall rather than a one-time vaccination.
How the Vaccine Has Changed Over Time
The flu shot has gone through several major upgrades since its debut. The original 1940s version was monovalent, protecting against just one strain of influenza A. The 1968 pandemic, caused by a new strain the existing vaccine didn’t cover, led to the development of trivalent vaccines that protected against three strains (two influenza A subtypes and one influenza B). In 2012, the FDA approved the first quadrivalent flu vaccine, which added a second influenza B strain for broader coverage. Quadrivalent vaccines are now standard.
Production methods have also evolved. While egg-based manufacturing has remained the dominant approach for over 80 years, newer options have emerged. Cell-based and recombinant vaccines, which don’t rely on chicken eggs at all, are now available. These newer methods can be scaled up faster during a pandemic and avoid potential issues for people with egg allergies.
When Annual Vaccination Became Universal
For decades, flu shots were recommended primarily for high-risk groups: older adults, young children, pregnant women, and people with chronic health conditions. That changed in 2010, when the CDC’s Advisory Committee on Immunization Practices issued a “universal” recommendation that everyone 6 months and older get a flu vaccine annually. The reasoning was straightforward: influenza is unpredictable, and even healthy people can develop serious complications or spread the virus to vulnerable individuals.
That recommendation has held steady for over a decade. Today’s flu shot is the product of more than 80 years of refinement, from a single-strain military vaccine grown in chicken eggs to a quadrivalent shot reformulated each year based on global surveillance of circulating viruses.