Prior to the release of data from the Moderna and Pfizer Phase III clinical trials (as well as the large accumulation of data since the vaccines were authorized under the EUA), there was widespread glurge from those wishing to sow fear and uncertainty trying to raise the fear of ADE (Antibody Dependent Enhancement)
ADE is a real but rare phenomenon wherein during the early phases of a vaccine trial, it's possible to create antibodies that bind to the virus in question in such a way that increases the severity of the disease in a subsequent infection.... exactly the opposite of what we want a vaccine to do.
This phenomenon is rare, but tragic and real. It happened a couple of times a half-century ago in an RSV vaccine trial in the US, and in an early measles vaccine trial (in the UK, I think). There was a vaccine trial in 2016 that was terminated due to ADE (Dengue-virus, Philippines), and I think there has been one other in the 21st century, though the details escape me right now.
ADE is easily detectable during clinical trials, and in the rare event it happens, it's (usually) detected in phases 1 or 2. As soon as evidence accumulates that the vaccinated cohort is falling seriously ill from the virus at a higher rate than the unvaccinated cohort, the trial is immediately halted.
Anyone reading these words on my page knows by now that the real world efficacy of the SARS-CoV-2 vaccines in the US (Pfizer, Moderna, and Janssen) shows the exact opposite of ADE: the vaccinated cohort is overwhelmingly protected against hospitalization and death compared to the unvaccinated cohort. In short, not only is there no evidence of ADE with the "big-three" SARS-CoV-2 vaccines, there is overwhelming evidence against ADE.
The people pushing fears of ADE (sometimes called VAED or "Vaccine Associated Enhanced Disease") are simply trying to sow uncertainty and doubt about the SARS-CoV-2 vaccines. You can tell this is true because the people who try to dishonestly push this hypothesis never bother to show vaccine efficacy data stratified by age and vaccination status.
If you run across one of these walnuts, you can safely ignore their whaarglebaargle.