The FDA has ordered that the use of the Jannsen virus-vector COVID vaccine for those aged 18 and older in all cases ave for when other vaccines (the mRNA vaccines by Pfizer and Moderna) are not an option for various reasons. I have not researched this aspect of the order but I believe that would rarely be the case as I am not aware of any particular disorders when a virus-vector vaccine would be OK but an mRNA one would not. The ostensible reason for the near-ban is supposedly rare blood clots that the Janssen vaccine causes. The "rare" part of it is likely just word play aimed at making it sound benign - in reality, it is likely anything but.
Remember how a few months ago you were a crazy conspiracy theorist if you dared question the axiomatic and sacred notion that COVID vaccines are safe and effective? Well, by now, at least as regards one of the three such vaccines in use in the US, it is all but officially acknowledged that it is neither. Being one of those crazy conspiracy theory, I expect a full and public apology from all those in the government, media, etc. who so loudly proclaimed the "safe and effective" mantra. But I am not holding my breath as prolonged oxygen deprivation tends to be unhealthy.
References
FDA sharply limits use of Johnson & Johnson shot due to rare blood clots
Laurie McGinley and Carolyn Y. Johnson, The Washington Post, 6 May 2022
Janssen COVID-19 Vaccine
FDA, archived 17 February 2000
US COVID vaccines: their power fleeting, their usefulness limited
@borepstein , 7 November 2021