Any vaccine used in the past, correct me if I'm wrong (🙄) required one shot.
Some childhood vaccinations require one shot to help the body build a defense. Other vaccinations have to be repeated so the body can maintain a defense (cholera, typhoid, yellow fever, rabies, etc.). My job/career requires me to carry the infamous yellow International Certificate of Vaccination issued by the WHO. I have received all these vaccines multiple times over the years. I have to admit the rabies vaccine (2 shots) was the worst. That one really hurt. Covid is just another vaccine on the yellow card to me.