You should follow the advice of your healthcare provider. If you have already received one type of vaccine but your provider would like you to receive another, please bring a prescription or some other form of documentation from your doctor to the location where you get your vaccine. That will make sure that you are able to receive that vaccine.
Some immunocompromised people may be eligible for an additional dose of COVID-19 vaccine because of their medical condition. The additional dose should be administered at least 28 days after the second mRNA dose or the first Johnson & Johnson dose. No doctor’s order necessary, but you may be asked to give verbal confirmation of eligibility.
CDC recommendations for moderately or severely immunocompromised people can be found here.