Locate possible subjects and keywords you can use to refine and narrow your search.
For example, if you're looking for information about the possible ethical issues involved with developing a vaccine for the Coronavirus, you can use the search the terms, vaccines and bioethics. You'll pull up this record:
While you might not end up using this particular article, notice that there are subjects that might be useful in refining your search strategy. There are the SUBJECTS: Vaccines--Product development and Vaccines--Ethical aspects. You could also include the term "clinical trials" in your search.
Abstracts that summarize the articles are another important place to look for ideas to refine your thoughts on the assignment as well as a way to find some keywords for further searching.
This summary describes how the desire to shorten the vaccine timeline for clinical trials may lead to an evaluation of the risk-benefits for the study participants. This summary may give you some ideas for an essay on the ethical aspects of vaccine trials. There are some keywords in the abstract that might prove useful: challenge trial, safety, efficacy, risk-benefit, public good.
Titles may contain keywords that help you construct effective searches or ideas that you might want to pursue for your essay.
(Title: FDA ramps up effort to convince public that vaccine decisions will be based on science, not politics)
The title above brings up the idea that there could be controversy about whether or not vaccine development will be based on science or influenced by political pressures. Articles like these might help in discussions about public reluctance to take a vaccine.