Here’s What We Know: How Vaccines Are Made

The very first vaccine was created at the tail end of the 18th century by Dr. Edward Jenner, an English physician and scientist, to protect against smallpox. He injected his patients with the cowpox virus, a related but far milder infection. And, lo and behold, it worked. 

Jenner’s vaccine was refined later, but that “first” was one of the most stunning discoveries in the history of medicine. Smallpox was declared eradicated globally in 1980, with the last case reported in Somalia in 1977. 

In what follows, Dr. Sean Cullen— The Friedman Family Foundation Clinical Scholar in Newborn Medicine and Assistant Professor of Pediatrics and Weill Cornell Medicine—explains the ins and outs of vaccines, including: 

What are vaccines, and how do they work? 

“Vaccines are designed to teach the immune system how to fight off a particular infection,” Dr. Cullen says. “They use antigens, which are specific parts of disease-causing bacteria or viruses, to help your immune system recognize infections and develop a defense against them. That defense comes in the form of antibodies—specialized proteins that can prevent a disease from occurring altogether or mitigate its most severe symptoms.” 

A great vaccine success story 

Dr. Cullen is too young to have encountered polio in his lifetime, but he names the polio vaccine as one of the best illustrations of what a vaccine can accomplish. 

Polio is a highly contagious viral infection of the nervous system. Before the polio vaccine was developed and administered to children during the 1950s, the disease often resulted in partial or total paralysis, especially in children. It was even called infantile paralysis, although adults could sometimes contract it. That happened to Franklin Delano Roosevelt during his 30s. The disease left the future president unable to walk, and he was confined to a wheelchair for the rest of his life. 

“When I was in medical school, one of our lectures featured a film showing images of rows and rows of children in ‘iron lung’ machines. Polio sometimes caused paralysis of the muscles that helped children breathe. Regrettably, many of these children were confined to iron lungs for life and often died early as a result.” 

But starting in the 1950s, children in this country were routinely vaccinated against polio, and the disease was officially eradicated from the U.S. in 1979. 

How vaccines are tested 

Dr. Cullen goes on to explain how vaccines are developed, tested and re-tested to ensure their safety and efficacy. 

Through a stepwise series of research studies and clinical trials, here’s how the process works: 

“Even after a vaccine is approved for use,” he says, “there are even larger trials designed to ensure that there are no long-term side effects, including extremely rare side effects that only appear through testing a very large number of people. That added layer of monitoring is called VAERS: the Vaccine Adverse Event Reporting System.” 

With VAERS, anyone can report any side effect they’ve experienced from any vaccine—even vaccines that have been around for decades. These data are collected and monitored by the various government agencies assigned to the task. 

Vaccine hesitancy 

Dr. Cullen points to the very success of vaccine development as a major driver of hesitancy and skepticism.  

There are upwards of 20 vaccine-preventable diseases that are part of the standard childhood immunization schedule recommended by the Centers for Disease Control and Prevention (CDC). “That means there are at least 20 diseases that children aren’t getting at the rates they did in earlier periods. Or aren’t getting at all, like polio. 

“Parents today may ask themselves why they should have their children vaccinated against diseases they’ve never heard of and have never seen,” he continues. 

Some wellness influencers on social media have also undermined confidence in vaccines. That has amplified vaccine hesitancy, leading to lower-than-optimal vaccination rates in some parts of the country. 

Another reason for vaccine hesitancy? Parents’ fears of a link between vaccines—especially the MMR vaccine—and autism. But no such link has been established. 

“The idea of a link between vaccines and autism has been one of the most examined questions in modern pediatric research,” Dr. Cullen says. “I can say conclusively that there is no evidence for a causal link between routine childhood vaccines and autism spectrum disorders. 

“The initial paper that came out and reported such findings was formally retracted in 2010,” he continues. “For interested parents, I’d like to recommend a book by Dr. Peter Hotez, titled Vaccines Did Not Cause Rachel’s Autism: My Journey as a Vaccine Scientist, Pediatrician, and Autism Dad.” (Dr. Hotez, affiliated with Baylor College of Medicine, has an adult daughter with autism.) 

Measles outbreaks 

In the U.S., measles wasn’t exactly eradicated, but with the development of a highly effective measles vaccine in the late 1960s—the same one that is combined with vaccines for mumps and rubella today—this formerly prevalent childhood disease was declared eliminated in 2000. Elimination is different from eradication. When a disease is “eliminated,” it means there’s an absence of its continuous spread over a period greater than 12 months. 

However, according to the Centers for Disease Control and Prevention (CDC), measles has been making a comeback, with numerous outbreaks across multiple states reported in 2025 thus far.

The MMR vaccine 

The MMR vaccine contains live, very weakened forms of the three viruses, Dr. Cullen explains. Therefore, they can’t replicate like the actual measles, mumps and rubella viruses. But there’s enough to train your immune system to recognize what the real viruses look like, and to develop those all-important antibodies mentioned above. What’s more, the MMR vaccine confers lifelong immunity to all three diseases. 

“Measles is highly contagious,” Dr. Cullen says. “It can spread through a community like wildfire”—especially when vaccination rates are suboptimal. 

Protecting vulnerable children