Many students make the mistake of using the same type of search strategies that they use in Google. They cram their keywords into one search box when Advanced search boxes are available.
Click on Advanced Search to be able to combine multiple terms and limit by field.
bioethics
AND
vaccine trials
These types of searches are a good starting point, so you can view the results and locate more search terms in the SUBJECTS, ABSTRACTS, and TITLE fields.
OR
(vaccine trials) AND (Bioethics or morality or ethical aspects)
(bioethics or morality or ethical aspects) AND (vaccines or vaccinations or immunizations) AND (clinical trials or human experimentation)
There is no one right way to construct searches. You will probably have to try many different search combinations to cover all the aspects of your topics and to find relevant results.
(bioethic* or morality or ethic*) AND (vaccin* or immunization*) AND (clinical trial* or human experiment*)
Truncation Examples:
bioethic* will retrieve bioethics, bioethical
vaccin* will retrieve vaccine, vaccines, vaccination, vaccinations, vaccinate, vaccinating, vaccinated, vaccinization
trial* will retrieve trials, trial
Example:
(bioethic* or morality or ethic* or "research ethic*") AND (vaccin* or immunization*) AND ("clinical trial*" or "human experiment*")