The COVID-19 vaccines made by Pfizer and Moderna both require two doses to be administered, several weeks apart, for them to be fully effective. Initial clinical trials by the manufacturers suggested both Pfizer and Moderna were about 50% effective after the first shot. Other independent studies, however, have found even higher rates of effectiveness, possibly up to 80% after the first dose.
What happens if you only get one shot of the Pfizer or Moderna vaccine? See story here. https://abc7chicago.com/health/what-if-you-only-get-1-dose-of-pfizer-or-moderna-vaccine/10458064/