Vaccines contain the same germs that cause disease. (For example, measles vaccine contains measles virus, and Hib vaccine contains Hib bacteria.) But they have been either killed or weakened to the point that they don't make you sick. Some vaccines contain only a part of the disease germ.

A vaccine stimulates your immune system to produce antibodies, exactly like it would if you were exposed to the disease. After getting vaccinated, you develop immunity to that disease, without having to get the disease first.

This is what makes vaccines such powerful medicine. Unlike most medicines, which treat or cure diseases, vaccines prevent them.
Before a vaccine is ever given to people, FDA oversees extensive lab testing of the vaccine that can take several years to make sure it is safe and effective. After the lab, testing in people begins, and it can take several more years before the clinical studies are complete and the vaccine is licensed.

Once a vaccine is licensed, FDA, CDC, National Institutes of Health (NIH), and other federal agencies routinely monitor its use and investigate any potential safety concerns.
Like any medicine, vaccines can cause side effects such as a low-grade fever, or pain and redness at injection site. Mild reactions go away within a few days on their own. Severe, long lasting side effects are usually very rare.

The administration of vaccines is called vaccination. 
Vaccination is the most effective method of preventing infectious diseases; widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world. The effectiveness of vaccination has been widely studied and verified; for example, vaccines that have proven effective include the influenza vaccine, the HPV vaccine, and the chicken pox vaccine. The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different preventable infections.


All ingredients of vaccines play necessary roles either in making the vaccine, triggering the body to develop immunity, or in ensuring that the final product is safe and effective. Some of these include:

  • Adjuvants help boost the body's response to vaccine. (Also found in antacids, buffered aspirin, antiperspirants, etc.)
  • Stabilizers help keep vaccine effective after manufactured (Also found in foods such as Jell-O® and resides in the body naturally.)
  • Formaldehyde is used prevent contamination by bacteria during the vaccine manufacturing process. Resides in body naturally (more in body than vaccines). (Also, found in environment, preservatives, and household products.)
  • Thimerosal is also used during the manufacturing process but is no longer an ingredient in any vaccine except multi-dose vials of the flu vaccine. Single dose vials of the flu vaccine are available as an alternative. No reputable scientific studies have found an association between thimerosal in vac­cines and autism.



https://en.wikipedia.org/wiki/Vaccine
https://www.cdc.gov/vaccines/vpd/vpd-vac-basics.html