Vaccine Science: How are vaccines made?

From the Vaccine Education Center at the Children’s Hospital of Philadelphia

The strengths and limitations of each approach are described here.

Weaken the Virus

“Viruses are weakened, so they reproduce very poorly once inside the body. Vaccines for measles, mumps, rubella, rotavirus, polio (oral version; not used in the U.S.), chickenpox (varicella), and influenza (intranasal version) vaccines are made this way. These are all vaccines that protect against viruses. Viruses cannot reproduce on their own. They need cells from other organisms to make more virus particles. Viruses reproduce thousands of times during a natural infection, meaning an infection that occurs after exposure to the virus in the community.”

“Live, weakened vaccine viruses usually reproduce fewer than 20 times. This is enough times to cause immunity, but not enough to cause disease. The immune response generates "memory B cells" that protect against infection in the future. Find out more about these and other cells of the immune system.”

“The advantage of live, weakened vaccines is that one or two doses usually lead to life-long immunity. The limitation of this approach is that these kinds of vaccines usually cannot be given to people with weakened immune systems (like people with cancer or AIDS). Find out more about what happens when the immune system doesn’t work properly.”

“Watch this video to see how viruses are weakened to make vaccines.”

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