There are currently nine vaccines in phase 3 trials, the last stage before regulatory review and approval, according to WHO. Although very variable, historically, market entry probability for phase 3 vaccine candidates is up to 70%. The ambition is to have one approved before year-end.
A vaccine approved before year-end would be the fastest produced vaccine on record, and the only way to develop a vaccine this fast is by combining phases. The current record is five years (vaccine against Ebola). Normally it takes more than 10 years to develop a vaccine. The fact that the development is rushed has consequences, as we do not know much about long-term effectiveness and side-effects.
Authorities are taking a different approach than usual by supporting several candidates simultaneously. Approval depends on safety and efficacy. Health authorities have hinted that an efficacy of 50% is sufficient for approval, as long as it is safe (efficacies of flu vaccines are also around 40-60%) - the perfect should not be the enemy of the good. Widespread vaccination is not expected until mid-2021, according to WHO.
Many candidates are gene-based and none of that kind has been approved for human use so far. While theoretically better, the effectiveness of gene-based vaccines developed so far has been low in humans. The advantage is that they are easier and cheaper to mass-produce.