How Well Do Vaccines Work? Vaccines work really well. No medicine is perfect, of course, but most childhood vaccines produce immunity about 90–100% of the time. What about the argument made by some people that vaccines don’t work that well and that diseases would be going away on their own because of better hygiene or sanitation, even if there were no vaccines? That simply isn’t true. Certainly better hygiene and sanitation can help prevent the spread of disease, but the germs that cause disease will still be around, and as long as they are they will continue to make people sick. All vaccines must be licensed (approved) by the Food and Drug Administration (FDA) before being used in the United States, and a vaccine must go through extensive testing to show that it works and that it is safe before the FDA will approve it. Among these tests are clinical trials, which compare groups of people who get a vaccine with groups of people who get a control. A vaccine is approved only if FDA makes the determination that it is safe and effective for its intended use.