How Do Vaccines Work?

Vaccine Questions in the Age of COVID-19

Vaccines have changed the course of human history. Over the last 200 years, they have reduced, eliminated, and, in the case of smallpox, even eradicated diseases that had plagued people for centuries.

So when a fast-spreading and dangerous disease—COVID-19—emerged, scientists worked at unprecedented speed to develop this line of defense against the new threat, too. COVID-19 vaccines drew on decades of research and new technologies, but most of these weren’t familiar to the public.

Even though billions of people have been vaccinated worldwide, others still have questions—some new to the COVID-19 pandemic, and some as old as vaccines themselves.

Illustration of a woman being vaccinated by a doctor while seated in a train compartment, while three other women and a child look on. Vaccinating American-bound passengers on the Grand Trunk Railway of Canada during the Montreal smallpox epidemic of 1885.
Courtesy of J. Marvin/Osler Library of the History of Medicine, McGill University
Woman seated in the passenger seat of a car receives a vaccination from a healthcare worker, and the driver of the car takes a cellphone photo. Drive-through vaccinations for COVID-19, April 2021.
Courtesy of M. Corvello/Wikimedia Commons

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