Because of the COVID-19 pandemic, most of us have heard more about vaccines in the last year than we previously had in our lifetimes. You might be reading about efficacy, herd immunity, and vaccine safety, which are all great things to understand. But it’s important to start at the beginning. Vaccines prevent illness and disease from germs — but how do they actually work?
The simplified version is: Vaccines teach your body’s immune system how to respond when you are exposed to a germ or pathogen. Pathogens are organisms that can make you ill, including bacteria, viruses, and fungi. The immune system has many moving parts and uses a complex process to deal with pathogens. This process can take some time, especially when you are first exposed to a germ, and you can get sick before it is completed. Vaccines are designed to speed up that process and prevent you from getting sick when you’re exposed to a pathogen.
Traditional vaccines work by introducing a weak or inactivated form of a pathogen into the body. The immune system recognizes the pathogen, gets rid of it, and remembers how to do this again later when you are exposed to it.
Newer vaccine technology works differently. It teaches your immune system how to respond without actually exposing it to the germ. The end result is the same: Your immune system knows how to quickly respond when it encounters the real thing.
Regardless of how the vaccine introduces the antigen, it activates the immune system, which is composed of:
- B cells: These produce antibodies, which prevent the germ from entering other cells or mark it for destruction.
- T cells: Their role isn’t understood as well, but they likely clear out marked and infected cells.
- Memory cells: These help the body “remember” the antigen so it can respond appropriately the next time the antigen is encountered.
Many vaccines give you lifelong immunity, meaning you only need to get them once. However, some vaccines require additional “booster” shots, because immunity may wane over time or because more than one dose is needed to get full protection. In the case of influenza, a virus that continuously mutates, an annual shot is needed to protect against different strains.
Here’s a guide to how vaccines are made and how they work.
How are vaccines made?
How a vaccine is developed depends on many things, including how the germ acts in the body, who needs protection from the germ, and how the immune system is expected to react. These factors then help scientists decide which type of vaccine is best for the type of infection they want to protect against.
The most important part of the vaccine is the active ingredient, called the antigen. This is what stimulates the immune system to respond once it’s in your body. There are several kinds of vaccines, which include antigens in different ways:
- Inactivated: These vaccines use a killed form of the germ. They often require boosters to maintain a strong immune response.
- Live attenuated: These vaccines use a live, weakened germ. People with weakened immune systems may not be able to receive this type.
- mRNA: These vaccines don’t include a germ; they have instructions for the body to make proteins that look like a germ. These proteins then stimulate the immune response.
- Viral vector: These vaccines use a different virus to teach the immune system to respond to the virus that you need protection from.
- Recombinant, subunit, and conjugated: These vaccines use a piece of a germ, which is sometimes linked to a different protein.
- Toxoid: These vaccines use a weakened form of the toxin created by a germ.
Once the antigen is identified, a few more steps take place:
- The antigen is grown, for example, a virus is grown in cell culture.
- The antigen is isolated from the growth material and purified.
- In many cases, strengthening and stabilization are necessary. This usually involves adding ingredients such as adjuvants and stabilizers (see below).
- Once all components have been added, the vaccine can be packaged and distributed.
You can see step-by-step how the Pfizer COVID-19 vaccines are made here.
Vaccine ingredients
In addition to the antigen, vaccines often contain other ingredients that help them perform better or keep them safe. Here are some examples:
- Preservatives prevent contamination of vials meant to be used more than once.
- Stabilizers help prolong a vaccine’s shelf life.
- Adjuvants are ingredients that boost the immune response.
It’s important to know that each ingredient in every vaccine is tested several times to be sure it’s safe and effective.
Clinical trials
After a vaccine is developed, and researchers know that it triggers an immune response, it is then tested in humans. This is done in three phases:
- In phase I testing, the vaccine is given to a small number of healthy adult volunteers.
- Phase II testing involves hundreds of volunteers that have similar characteristics (such as age) to those of the target vaccine population.
- Phase III testing compares the effects of the vaccine in thousands of people (usually at several research sites) with those who did not receive the vaccine (called controls).
Through these phases, researchers are able to understand which dose is best and how people should respond to the vaccine. If at any point there are serious concerns about safety, the testing stops. The benefits of the vaccine must outweigh any risks before it is approved.
After a vaccine is approved by the FDA, testing continues. This is called phase IV testing, in which researchers continue to collect information on how people respond to vaccines in the real world. This helps identify possible side effects that are so rare they don’t show up in clinical trials.
What is herd immunity?
To understand why vaccines are so important, it’s helpful to know about herd immunity. When a large percentage of a community has immunity against an infectious illness, it’s harder for that illness to spread between people. For the small percentage of people who can’t get vaccinated (for example, those with weakened immune systems), this lower rate of spread helps keep them safe.
Can the body fight diseases naturally without a vaccine?
Yes. The immune system consists of different cell types that work together to fight off infection and disease. However, when it first encounters a pathogen, it takes some time to mount a defense. Meanwhile, it’s possible to become very ill from the germ, and you can even develop complications in some cases. For that reason, vaccines are a safer way to protect yourself from a serious illness.
Why should you follow the CDC’s immunization schedule?
After a vaccine is approved for use by the FDA, the next step is figuring out when and how it should be given. A team of experts called the Advisory Committee on Immunization Practices (ACIP) meets three times per year (more often during a pandemic) to review all available vaccine-related information, such as clinical trial data and vaccine safety updates.
This panel consists of experts in:
- Public health
- Adult medicine
- Pediatric medicine (treatment of children)
- Infectious diseases
- The immune system
After reviewing the data, the panel makes recommendations to the Centers for Disease Control and Protection (CDC) for vaccine schedules for children and adults, or it updates its existing recommendations. The final schedules tell us how many doses are needed for each vaccine and when they should be given.
The vaccine schedules for infants and children are approved by the:
- American Academy of Pediatrics
- American Academy of Family Physicians
- American College of Obstetricians and Gynecologists
The adult vaccine schedule is approved by the:
- American Academy of Family Physicians
- American College of Obstetricians and Gynecologists
- American College of Physicians
- American College of Nurse-Midwives
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