Vaccines (immunizations) - overview
Vaccinations; Immunizations; Immunize; Vaccine shots; Prevention - vaccine
Vaccines are used to boost your immune system and prevent the risk of infection and the severity of infections, including those that cause serious, life-threatening diseases.
Ouch! Vaccines help to give the body immunity from infections. Different vaccines work in different ways. Some vaccines inject fragments of a virus or bacteria called antigens into the body. Once in the blood, these antigens circulate among the blood cells, which include red blood cells and white blood cells. White blood cells, such as B and T cells, help fend off foreign invaders. When antigens invade tissue, they attract macrophages. These are scavenger cells that engulf the antigens. The macrophages then signal to T-cells that the antigens are invading. Killer T cells gather and attack the antigens. Then suppressor T cells stop the attack. After the vaccination, B-cells make defensive antibodies against the antigen. These antibodies help the cells remember this particular antigen, so that they can fight it off if the body is infected again.
HOW VACCINES WORK
Vaccines "teach" your body how to defend itself when germs, such as viruses or bacteria, invade it:
- Vaccines expose you to a very small, very safe amount of viruses or bacteria that have been weakened or killed.
- Your immune system then learns to recognize and attack the infection if you are exposed to it later in life.
- As a result, you will not become ill, or you may have a milder infection. This is a natural way to deal with infectious diseases.
Five types of vaccines are currently available:
- Live virus vaccines use the weakened (attenuated) form of the virus. The measles, mumps, and rubella (MMR) vaccine and the varicella (chickenpox) vaccine are examples.
- Killed (inactivated) vaccines are made from a protein or other small pieces taken from a virus or bacteria. The whooping cough (pertussis) vaccine is an example.
- Toxoid vaccines contain a toxin or chemical made by the bacteria or virus. They make you immune to the harmful effects of the infection, instead of to the infection itself. Examples are the diphtheria and tetanus vaccines.
- Biosynthetic vaccines contain manmade substances that are very similar to pieces of the virus or bacteria. The Hepatitis B vaccine is an example.
- mRNA vaccines contain the genetic code of the virus the body can use to create antibodies to fight the virus. Some SARS-CoV2 vaccines use this technology.
WHY WE NEED VACCINES
For a few weeks after birth, babies have some protection from germs that cause diseases. This protection is passed from their mother through the placenta before birth. After a short period, this natural protection goes away.
Vaccines help protect against many diseases that used to be much more common. Examples include tetanus, diphtheria, mumps, measles, pertussis (whooping cough), meningitis, and polio. Many of these infections can cause serious or life-threatening illnesses and may lead to life-long health problems. Because of vaccines, many of these illnesses are now rare.
SAFETY OF VACCINES
Some people worry that vaccines are not safe and may be harmful, especially for children. They may ask their health care provider to wait or even choose not to have a vaccine. But the benefits of vaccines far outweigh their risks.
The American Academy of Pediatrics, the Centers for Disease Control and Prevention (CDC), and the National Academy of Medicine have all concluded that the benefits of vaccines outweigh their risks.
Vaccines, such as the measles, mumps, rubella, chickenpox, and nasal spray flu vaccines contain live, but weakened viruses:
- Unless a person's immune system is weakened, it is unlikely that a vaccine will give the person the infection. People with weakened immune systems should not receive these live vaccines.
- These live vaccines may be dangerous to the fetus of a pregnant woman. To avoid harm to the baby, pregnant women should not receive any of these vaccines. Your provider can tell you the right time to get these vaccines.
Thimerosal is a preservative that was found in most vaccines in the past. But now:
- There are infant and child flu vaccines that have no thimerosal.
- NO other vaccines commonly used for children or adults contain thimerosal.
- Research done over many years has NOT shown any link between thimerosal and autism or other medical problems.
Allergic reactions are rare and are usually to some part (component) of the vaccine.
The recommended vaccination (immunization) schedule is updated at least every 12 months by the US Centers for Disease Control and Prevention (CDC). Talk to your provider about specific immunizations for you or your child. Current recommendations are available at the CDC website:
The CDC website (
Bring your immunization record with you when you travel to other countries. Some countries require this record.
- Chickenpox vaccine
- DTaP vaccine (Diphtheria, Tetanus, Pertussis)
- Hepatitis A vaccine
- Hepatitis B vaccine
- Hib vaccine
- HPV vaccine
- Influenza vaccine
- Meningococcal vaccine
- MMR vaccine
- Pneumococcal conjugate vaccine
- Pneumococcal polysaccharide vaccine
- Polio vaccine
- Rotavirus vaccine
- Shingles vaccine
- Tdap vaccine
- Tetanus vaccine
- COVID-19 vaccine
Bernstein HH, Kilinsky A, Orenstein WA. Immunization practices. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 197.
Centers for Disease Control and Prevention website. Thimerosal FAQs.
Freedman MS, Hunter P, Ault K, Kroger A. Advisory committee on immunization practices recommended immunization schedule for adults aged 19 years or older - United States, 2020. MMWR Morb Mortal Wkly Rep. 2020;69(5):133-135. PMID: 32027627
Kroger AT, Pickering LK, Mawle A, Hinman AR, Orenstein WA. Immunization. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 316.
Robinson CL, Bernstein H, Poehling K, Romero JR, Szilagyi P. Advisory committee on immunization practices recommended immunization schedule for children and adolescents aged 18 years or younger - United States, 2020. MMWR Morb Mortal Wkly Rep. 2020;69(5):130-132. PMID: 32027628
Strikas RA, Orenstein WA. Immunization. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 15.
Last reviewed on: 7/3/2022
Reviewed by: Neil K. Kaneshiro, MD, MHA, Clinical Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.