The COVID vaccine is required if you are going through a process to receive your green card, be it a adjustment of status or a spouse visa. What that means is that you have to receive that very first vaccination which is either two doses or one dose, depending on which vaccine. That is considered completion of that requirement for the COVID vaccine.
There’s been a ton of boosters that have happened after that but it’s just that initial vaccine whether one or two doses that is required to be completed. You have to show proof of that vaccination to the immigration doctor before they can complete your medical exam or they will give you that vaccine while you’re in the process of completing your medical exam.
But when the couple gets to that process with the US embassy or consulate to complete the vaccine requirement to complete the medical exam, there is an exception for fiancé visa applicants to complete some of their vaccines once they arrive in the US and before they actually file their green card application. But with the COVID vaccine, it’s a little unique and so that is one vaccine that you would have to receive before you’re able to actually receive that fiancé visa to come to the United States.