The paper linked shows a reduced rate of COVID in the pool of people that received a vaccine candidate compared to the pool that received a placebo:

  A total of 43,548 participants underwent randomization, of whom 43,448 received injections:
    21,720 with BNT162b2 and 
    21,728 with placebo.
  There were 8 cases of Covid-19 with onset at least 7 days after the second dose among participants assigned to receive BNT162b2 and 162 cases among those assigned to placebo;

  BNT162b2 was 95% effective in preventing Covid-19

Again, no vaccine ever really 100% prevents infection - what they do is reduce the number and severity of cases. Vaccines are literally created to increase the human bodies ability to fight off infection - efficacy varies by age and many other factors.