Transmitting a mild infection to prevent a more serious one: since its first rational use in the late 18th century, vaccination has undoubtedly been one of the greatest national and international achievements in public health. Thanks to this preventive measure, which has saved millions of lives, it has been possible to halt the transmission—and even eradicate—several contagious infections. But today, many French people are still not adequately protected against certain risks. That is why it is essential to identify failures and their causes. This is where the InVS comes in: evaluating and adapting the vaccination schedule, strengthening surveillance, and assessing the impact of a new vaccine—all of these challenges put its epidemiological expertise to the test. (R.A.)