How Vaccines Work

By: Alia Hoyt  | 

Vaccine Basics

Vials of the measles, mumps and rubella vaccine
Vials of the measles, mumps and rubella vaccine are displayed on a counter at a Walgreens Pharmacy on Jan. 26, 2015 in Mill Valley, California. Justin Sullivan/Getty Images

Jenner was operating on the now widely accepted principle that once a person catches a certain disease, he or she is immune to it for the rest of their life. For example, once you've had the chickenpox, it's extremely unlikely that you'll ever catch it again. This is because your body, when exposed again, will recognize the disease and fight it off. The beauty of vaccines is that they help the body develop disease-fighting abilities without making you sick. Vaccines accomplish this amazing feat by tricking the body into believing it already has the full-blown disease. Here are the steps in this process, known as the "immune response":

  1. The immune system identifies these foreign substances (viruses and bacteria), also known as antigens.
  2. Once antigens are identified, the immune system develops proteins that circulate in the blood. These proteins are called antibodies. They fight the infection by killing the antigens. Antibodies are made by white blood cells called lymphocytes, also known as B cells. The main purpose of B cells is to create antibodies to fight infection.
  3. The body stockpiles these antibodies so they are available to fight off the disease if exposed later on. Unfortunately, antibodies are disease-specific, so previously acquired chickenpox antibodies will be useless if faced with other diseases.

It's very important to note that when the actual disease infects a person, the antigens multiply thousands and thousands of times until a raging infection is underway. The vaccine provides just enough of these antigens for the body to recognize them and complete the immune response process, therefore protecting them from exposure to the disease in the future.


The procedure for developing a vaccine takes many years and even more money, often hundreds of millions of dollars. Therefore, according to John Bradley, M.D., member of the committee on infectious diseases of the American Academy of Pediatrics (AAP), developing vaccines are prioritized in this order:

  • Vaccines that fight diseases that cause the most deaths and damage, like meningitis
  • Vaccines that prevent severe diseases like measles and influenza
  • Vaccines, like the one for rotavirus, that prevent significant suffering

Additionally, vaccines are studied and produced by companies, so the return on investment must be significant in order to justify the large expense. Vaccines are currently in development to prevent malaria. The malaria vaccine has been slighted in the past because the financial return was not worth the investment the industry had to make, according to Dr. Bradley. It's also extremely difficult to develop a malaria vaccine because of the complex nature of the parasite. As of April 2020 there is no commercially available malaria vaccine, although it continues to be studied in hopes that eventually this will change [source: World Health Organization].

Another reason that vaccines can be tricky to produce is that some viruses mutate so quickly that traditional vaccines are ineffective. A prime example is HIV (human immunodeficiency virus). Despite these hurdles, there is currently a tremendous movement to develop a vaccine to fight HIV and AIDS. The COVID-19 pandemic has resulted in millions of people being infected with the virus and hundreds of thousands of deaths worldwide. The race is on to develop a vaccine, with currently more than 100 projects going on in 2020. So far no one knows when one will be available. It's estimated that even with compressing vaccine trials, a vaccine may not be ready before 2021, which shows how difficult it is to bring a safe vaccine to the public.