First, scientists will test the vaccine virtually, using computer models. Next, they’re tested on animals, and it’s only when they’ve passed those tests that they make it to human testing – albeit on a very small scale. Once they’ve been proven effective and not harmful on a small number of people, they will be approved for clinical trials, which will include a larger number of people, and children.
It’s only after they’ve been tested like this, including as many as 10,000 human volunteers, that vaccines are approved for manufacture, but even then the monitoring is not over. As the vaccines are distributed, any side effects and complications are monitored, and if, as in the case of a specific Rotavirus vaccine, there is a problem, the vaccine will be recalled from the market.
It’s worth speaking to your doctor if you are worried, but for the most part, vaccines are safe, and provide valuable protection against disease.