There is significant evidence that it prevents you getting the disease. That's the 95% efficacy everyone is talking about. Whether or not it fully prevents spreading has not been tested yet, but early signs sound positive.
The BioNTech/Pfizer press release mentions a reduction in confirmed cases (8 vs. 162), not in severity. There is no good reason why such an effective vaccine should not reduce the risk of spreading the disease. However, even a large phase 3 trial doesn't have enough statistics to answer this question.
