The correct answer is: vaccinate.
To vaccinate is to give someone an injection of a vaccine, which is a substance that helps the body’s immune system fight off a particular disease. Vaccines are one of the most effective ways to prevent disease, and they have saved millions of lives around the world.
The other options are not as effective as vaccines at preventing disease. To insure is to protect someone from financial loss in the event of something happening, such as illness. To brace is to prepare for something difficult or unpleasant. To ensure is to make certain that something happens. To innoculate is to give someone a vaccine.
Vaccines are safe and effective, and they are the best way to protect yourself and your loved ones from disease. If you are not sure whether you are up to date on your vaccinations, talk to your doctor.