A recent letter to the editor questions whether the COVID-19 vaccines may cause recipients of the vaccine to develop more severe COVID-19 disease than if they had not been vaccinated. A letter in the Oct. 28, 2020 “International Journal of Clinical Practice” criticizes the informed consent protocols given to volunteers during tests of the vaccines, saying that "disclosure of the specific risk of worsened COVID-19 disease from vaccination calls for a specific, separate, informed consent form and demonstration of patient comprehension in order to meet medical ethics standards." They felt that the informed consent protocol for people to agree to being tested in research projects was not adequate.
Earlier in the article, however, the researchers say, "Current data on COVID-19 vaccines is limited, but does not so far reveal evidence of ADE of disease." ADE is short for Antibody-dependent Enhancement (the possibility of vaccine making subsequent infections worse than if one hadn't received the vaccine).
On Feb. 10, 2021, Dr. Paul Offit from the vaccine education center of the Children's Hospital of Philadelphia states that, from five specific instances and research of COVID viruses in humans and animals, that it's "very unlikely to happen" that there is Antibody-dependent Enhancement from vaccines for COVID-19. You can find this on You Tube by searching for "Do COVID-19 vaccines cause Antibody-dependent Enhancement?"
The issue, therefore, seems to be that researchers should have provided more specific warnings to volunteers in vaccine trials about the vaccines potentially causing worse disease, but that subsequent research showed that the vaccines are safe.