Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: A vaccine can prevent a serious disease. When a new vaccine is developed, it is given to a small group of people. Why would it be important to test each vaccine on a small group of people before everyone can have one?
to determine how the vaccine will affect people
To ensure safety and effectiveness before widespread use.