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How vaccines work
April 26, 2021

Neal Browning receives a shot in the first-stage safety study clinical trial of a potential vaccine for COVID-19, the disease caused by the new coronavirus, Monday, March 16, 2020, at the Kaiser Permanente Washington Health Research Institute in Seattle. Browning is the second patient to receive the shot in the study. (AP Photo/Ted S. Warren)


As people worldwide await approval of one or multiple COVID-19 vaccines in the U.S., it’s important to understand how vaccines work.

Several vaccine candidates against the novel coronavirus have shown promise in animal studies. Now, clinical trials are underway to find out if these same vaccines are safe and effective in humans.

Vaccines 101

“Vaccines work by training your body’s immune system,” says Sandhya Vasan, director of the Henry M. Jackson Foundation component of the U.S. Military HIV Program and Emerging Infectious Disease Branch at the Walter Reed Army Institute of Research, which is conducting several COVID-19 vaccine trials.

“They expose your body’s immune system to something that looks like the pathogen that you are trying to prevent,” she says. “Your body forms an immune response and specific antibodies targeted to that particular pathogen. So when you’re exposed to the actual pathogen later, your body’s memory response kicks in and is already primed to fight off that pathogen.”

Scientists make different types of vaccines for different diseases. For some diseases — such as measles, chickenpox and yellow fever — they create vaccines using a live but weakened (or attenuated) form of the pathogen, which can be a virus, bacteria or other infectious agent. Other vaccines, such as the injections to prevent influenza, are developed using a killed (inactivated) pathogen. Over the last few decades, polio vaccines have switched from using an attenuated approach to an inactivated approach.

Two other common approaches involve using either fragments of the germ or a toxin produced by the germ to elicit the immune response.

Coronavirus vaccines

For the novel coronavirus vaccine, scientists are exploring new approaches. “There’s been an amazing development of technologies that allow us to make vaccines in different ways that can be more safe, easy and cost-effective to mass-produce,” says Vasan. Here are three examples:

  • One of the coronavirus vaccines in phase III clinical trials uses genetic material called messenger RNA to instruct cells to produce a coronavirus protein within the human body, which can then generate antibodies against the actual virus.
  • A second approach in phase III clinical trials involves inserting a gene from the coronavirus into a nonreplicating virus, which then creates the coronavirus protein that can generate an immune response.
  • A third approach involves creating the coronavirus protein in the lab and incorporating that into the vaccine, which is then administered to generate an immune response.

“It’s a good idea to not put all your eggs in one basket, [but rather to] try a lot of different things,” says Richard M. Novak, who is leading a phase III clinical trial of a coronavirus vaccine at the University of Illinois at Chicago.

Testing the safety and efficacy of potential vaccines is critically important to the public’s trust, and “trying to make the process as transparent as possible is a key part of the effort,” Vasan says.

Freelance writer Linda Wang wrote this article.