How vaccines actually work: A simple guide

By Mayo Clinic Staff

Your body is built to fight germs that can make you sick. Most of the time, your immune system can stop bacteria and viruses before they cause serious illness.

But sometimes a new germ appears that your body hasn't seen before. When that happens, your immune system may not react quickly enough to protect you.

Vaccines help fill that gap. They safely teach your immune system to recognize certain germs and respond faster. This training makes you less likely to get very sick.

Ever wonder how these shots actually work? It comes down to the different types of vaccines and the ways they train your immune system.

It's all about the germ

Most vaccines use a small, safe part of a germ to help your body recognize and fight the disease if you're ever exposed to it. The main difference between vaccines is how they use that germ to train your immune system.

Live vaccines

Live vaccines use a weakened form of a germ to help your body build protection without making you sick. Live vaccines protect against:

• Chickenpox, also called varicella.
• Measles, mumps and rubella (MMR).
• Rotavirus.
• Smallpox.
• Yellow fever.

Live vaccines are usually given to people who have healthy immune systems. They aren't recommended for people who have weakened immune systems, like those getting chemotherapy.

Inactivated vaccines

Inactivated vaccines use a germ that has been killed, so it can't make you sick. This type of vaccine can protect against:

• Flu, also called influenza.
• Hepatitis A.
• Polio.
• Rabies.

Inactivated vaccines may not provide protection as strong or as long as that of live vaccines. They often require more than one dose to work their best.

Vaccines based on an organism component

These vaccines use only parts of the germ, like a protein or sugar. Your immune system learns to recognize it without being exposed to the whole germ. These vaccines protect against:

• COVID-19.
• Hepatitis B.
• Human papillomavirus (HPV).
• Meningococcal disease.
• Pneumococcal disease.
• Shingles.
• Whooping cough, also called pertussis.

These vaccines can generally be tolerated by most people, including those with weakened immune systems or serious health conditions.

Messenger RNA (mRNA) vaccines

An mRNA vaccine doesn't use the germ itself. Instead, it tells your cells to make a harmless piece of the germ based on an mRNA blueprint, which causes your immune system to respond. The cells can only make the piece of the germ for a short while, while the mRNA is still present. The mRNA does not become part of your cells, and your body does not continue to make the protein long term.

Currently, mRNA vaccines are used to protect against COVID-19. Scientists are developing other vaccines of this type, including new kinds of flu shots.

Toxoid vaccines

Toxoid vaccines use a harmless form of a poison made by bacteria. They train your immune system to recognize and block the real poison if you're exposed later. Toxoid vaccines protect against:

• Diphtheria.
• Tetanus.

This type of vaccine often requires follow-up booster shots to help your body keep its protection strong.

What you can do

Staying current on your vaccines is an easy habit that goes a long way in protecting your health. If you're not sure which vaccines you need, talk with your healthcare team. They can review your health history, explain your risks and recommend the vaccines that are safest and most effective for you.