It is pretty hard for some people to understand what exactly a vaccine is.
Dating back to the late 18th century, vaccination was first discovered by Edward Jenner, a doctor, noticed a correlation between being infected with cowpox and not being infected with smallpox. He decided to try experimenting by infecting children with small amounts of cowpox, then noted that they were immune to smallpox afterwards (times were different back then.) A century later Louis Pasteur created a second vaccine, one that helped with rabies, and the rest is history. Scientists continued expanding on the vaccines they made until we have what we do today, which helps to save many lives through preventative action.
Vaccines work by injecting the body with a fake disease that’s mimicking the real one, but doesn’t carry real threat. However, the body (and specifically the immune system) responds by creating cells to defend against that specific disease, and even when it deals with the fake disease, it has all the tools and knowledge it needs to beat the real disease if it tries to invade. The vaccine is like a dress rehearsal, so that when the time comes everyone is prepared.
However, no matter how much scientific evidence there is surrounding vaccination, there are still naysayers. Originally started by Andrew Wakefield, a medical researcher who falsified data to claim vaccines cause autism, people today put their children at huge risks off of the basis of data that was known to be falsified. Even prominent politicians subscribe to the lies, which is a worrying trend. Just remember that, like cigarettes causing lung cancer, all scientific evidence points towards vaccines being safe for children.