Not a physician, but we learned this week that this is why we don't have an RSV vaccine. The vaccine (which was not mRNA), was producing worse cases of RSV in patients who had already had it because it was producing antibody dependent enhancement (ADE). This also happened with the dengue vaccine, which is mRNA. The professor emphasized that Sars-Cov-2 was a good candidate for this to happen with a vaccine, specifically stating that the fact that COVID-19 has been overall worse in older adults than in young people indicated that there might be antibodies present in older people from a different but similar coronavirus in the past (even decades ago) that younger people were never exposed to.
I'm not sure how much validity there is to his statement, and I don't yet have the medical knowledge to go digging for answers, but I thought it was interesting in light of the coming vaccines. I did find this article though:
Mechanisms of antibody-dependent enhancement of disease and mitigation strategies for SARS-CoV-2 vaccines and therapies are discussed.
www.nature.com