According to the Food and Drug Administration, in order to approve a vaccine the vaccine must be at least 50% effective. In other words, a yet to be approved COVID-19 trial vaccine must prevent half the people who receive the vaccine from becoming infected with the virus.
A recent survey from National Opinion Research Center at the University of Chicago found 49% of 1,056 American adults surveyed in mid-May 2020 plan on getting the COVID-19 vaccine as soon as it becomes available. The other 51% either are not sure if they will or they will not get the vaccine immediately.