According to Dr. Larry Kwak, Dr. Steven T. Rosen and Dr. Idit Shachar, these are things we could be on the look out:

"mRNA also activates danger sensors in the primal immune system, which in turn indirectly promotes the release of pro-inflammation factors, specifically interferons, which have been associated with autoimmunity. This issue is underscored by a clinical trial of an mRNA lung cancer vaccine in 2019, in which blood tests revealed elevated indicators for autoimmunity concerns in 20% of patients. "

Immune responses directly against RNA molecules themselves cause autoimmune diseases, such as systemic lupus. In 2014 in the early days of the technology, an mRNA COVID-19 vaccine inventor published on this potential long-term concern of mRNA vaccines.

Published animal safety studies showed traces of COVID-19 spike protein in the brain, heart, and other vital organs, and the European Medicines Agency’s assessment report acknowledged that low levels of mRNA itself were detected in most tissues. The potential consequences of vaccines crossing the natural blood-brain barrier in children’s developing brains are of the utmost concern to all future humanity.

One last theoretical concern is the Jannsen vaccine, which uses a virus stripped of its own genes to deliver payload DNA. Such virus vectors can cause cancer when they randomly insert their payload near a gene that causes cancer. This is more than a hypothetical concern, as this year, a clinical trial of gene therapy for sickle cell disease (admittedly, using a different virus) was suspended when two patients who received the therapy more than five years ago developed blood cancers.

https://www.washingtontimes.com/news/2021/oct/28/applying-brakes-on-warp-speed-covid-19-vaccination/