The Science Behind Vaccines
Your immune system has a natural way of defending you from the viruses and bacteria that cause illness. As these germs multiply rapidly and infect your body, an army of white blood cells fights back, producing antibodies that attack the invaders.
The first time you are infected by a specific virus or bacteria, your immune system often cannot destroy the germ quickly enough to prevent you from becoming sick. After you’ve recovered, certain immune cells hold onto the memory of the infection. This allows them to mobilize quickly if they encounter the same germ again, preventing illness in the future. Immunity, your body’s resistance to certain types of infection, is the outcome of this process.
Vaccines — which were first developed in 1796 for smallpox and continue to evolve with new research and technology — build on your immune system’s natural function to protect you from serious diseases. Just like white blood cells and the other components of your immune system, vaccines help you develop immunity, but without the initial illness and risk of health complications. Here’s a look at how vaccines work.
Understanding how vaccines work
To understand how vaccines work, it’s helpful to know how your immune system naturally fights an infection. B-lymphocyte white blood cells produce blood proteins called antibodies, which attack antigens — the part of the invading germ that triggers an immune response. T-lymphocyte white blood cells attack already infected cells in the body. After the infection has ended, a few T-lymphocytes, or memory cells, remain to ensure a fast immune response should the same germ reappear.
Vaccines imitate infections, triggering the process of building immunity to a specific disease without causing illness. Your immune system responds to these imitation infections the same way it would to an actual infection. Detecting antigens from the vaccine, your immune system produces antibodies and T-lymphocyte cells. The imitation infection leaves your body with T-lymphocyte memory cells and B-lymphocytes, which will know how to quickly fight that antigen in the future. This prevents you from becoming sick if you are exposed to the virus or bacteria that cause the disease.
It typically takes several weeks for your body to produce T-lymphocyte and B-lymphocyte cells in response to a vaccine, which is why in most cases you will not immediately develop immunity. This means that you could become infected with a disease and develop symptoms shortly after vaccination, so it’s important to take necessary precautions until you become fully immune to the virus or bacteria. The healthcare professional administering your vaccine will inform you of the anticipated timeline for immunity.
Sometimes the imitation infection created by a vaccine causes mild symptoms such as a fever. These side effects are normal and indicate that your body is building immunity.
Types of vaccines
While all vaccines arrive at the end result of immunity, there are several paths for getting there. When deciding which type of vaccine to create for a disease, researchers consider a variety of factors, including the immune system’s response to the germ that causes the disease, the population likely to receive the vaccine, and the regions of the world where it will be used. These are the seven types of vaccines commonly used in the U.S.
Live, attenuated vaccines
Live vaccines use a weakened version of the living virus or bacteria that causes a disease. They are most similar to the natural infection and produce long-lasting immunity. The weakened form of the germ does not lead to serious disease, like that caused by the wild form of the germ, for most individuals with strong immune systems. However, live vaccines aren’t safe for those who are immunocompromised, including children undergoing cancer treatment. Examples of live vaccines include the measles, mumps, and rubella (MMR) vaccine and the chickenpox vaccine.
Inactivated vaccines are produced by inactivating, or killing, the germ that causes a disease. Examples include the polio, hepatitis A, and flu vaccines. They don’t provide as strong of immunity as live vaccines, so additional doses, also referred to as booster shots, are often needed for ongoing protection.
Toxoid vaccines target a toxin, a harmful substance produced by a germ, rather than the entire germ itself. They contain a weakened version of the toxin to be targeted called a toxoid, which teaches your immune system how to fight off the natural toxin. Similar to the germs used in live vaccines, toxoids are weakened to a point where they do not cause illness. These vaccines are used to prevent diseases caused by toxin-producing bacteria such as diphtheria and tetanus.
Subunit vaccines use portions of a virus or bacteria called subunits rather than the entire germ. Vaccines for diphtheria, tetanus, and pertussis, or whooping cough, are combined into the DTaP vaccine. The pertussis component is a subunit vaccine.
Conjugate vaccines fight a specific type of bacteria whose antigens are more difficult for a child’s immune system to detect due to their outer coating. The vaccines link the coating to stronger antigens that the immune system can more easily respond to. Like subunit vaccines, these vaccines are more targeted and only use part of the germ. Examples of conjugate vaccines include the haemophilus influenzae type B (Hib) vaccine.
Viral vector vaccines
Viral vector vaccines produce immunity to a virus by using a modified version of a different virus to deliver instructions to your immune cells. The modified version of the other virus is referred to as a vector virus. Recent Ebola vaccines have used this technology, and the Johnson & Johnson COVID-19 vaccine is also a viral vector vaccine.
Messenger RNA (mRNA) vaccines
mRNA vaccines help your body produce a protein that triggers an immune response and the development of antibodies. This technology was used to create the Pfizer-BioNTech and Moderna COVID-19 vaccines.
Vaccine safety standards
All vaccines in the United States require approval by the Food and Drug Administration (FDA) and are held to rigorous safety and effectiveness standards. The vaccine approval process starts with lab tests, which may take years in some cases. The FDA analyzes the results to determine if the vaccine should be tested with people. If the vaccine moves forward, the next step is clinical trials where the vaccine is tested on volunteers.
Clinical trials typically begin with a smaller group of 20 to 100 volunteers before including thousands more. The trials gather information about vaccine safety, the typical immune response, and dosing. If any safety concerns emerge during this stage, the FDA must address them before it can authorize the vaccine.
Vaccine manufacturers are required to continuously monitor vaccine safety after approval and test every batch produced to ensure effectiveness, quality, and purity. These test results are reviewed by the FDA, which also conducts an inspection of the factory where the vaccine was produced. If you experience severe side effects after vaccination, contact your healthcare provider so that they can report potential concerns to the Centers for Disease Control and Prevention (CDC)’s vaccine monitoring database.
COVID-19 mRNA vaccines were developed and approved more quickly than a typical vaccine given the urgent nature of the pandemic. This expedited timeline was made possible by significant investment from governments and companies in the vaccine development process and decades of research on mRNA vaccine technology. These vaccines were as thoroughly vetted for quality and safety as any other vaccine.
Stay current with vaccines
Many vaccines are received during childhood, like the polio, chickenpox, and MMR vaccines, but there are others you may need as an adult. The CDC recommends the zoster vaccine, which prevents shingles, for adults ages 50 and older and the pneumonia vaccine for those over 65. Other vaccines may be recommended based on additional factors including job or school requirements, travel, and health conditions. Check in with your doctor about which vaccines you need.
The CDC also recommends an annual flu vaccine for everyone 6 months and older. New versions of flu vaccines are developed annually in anticipation of circulating flu virus strains, so health experts recommend getting one every year to prepare for cold and flu season. It’s especially important to be proactive about flu vaccination if you are over the age of 65 or have a chronic condition such as COPD or diabetes.
For detailed vaccine recommendations by age group, reference the CDC’s recommended immunization schedules for children and adolescents and adults.
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This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or another qualified health provider with any questions you may have regarding a medical condition.