Vaccine is a safe way to protect against diseases. Most vaccine reactions are temporary, like a sore arm or fever. It is rare for vaccines to cause serious side effects but they are carefully monitored and reported. It is much more common for people to be seriously injured or even die from disease-causing pathogens (like polio, measles and mumps) than to experience vaccine-related side effects.

Scientists learned how to make vaccines by studying the proteins that pathogens use to hide from and attack your immune system. They then created weakened versions of these proteins, which stimulate your immune system without making you sick. Modern vaccines are also being used to supercharge your immune system to fight certain cancers, which have evolved to resist standard treatments.

Most vaccines contain a dead or inactivated germ that doesn’t cause disease, a weakened version of a protein made by the germ to trigger your immune system, or a combination. Vaccine ingredients are tested to ensure that they are safe. Some vaccines contain formaldehyde, mercury or aluminium to help them work better, but these are in very small amounts and don’t pose any health risk.

When you receive a vaccine, the antigen enters your body and is displayed by special cells called antigen-presenting cells. These cells act as a kind of “wanted poster” for the pathogen, alerting your immune system to destroy it. Your memory cells are then ready to attack any future invaders, and antibodies are produced that kill or inactivate the pathogen.