They don’t stop spreading.
Next question.
Well, they do actually:
https://www.cnbc.com/2021/03/11/pfizer-covid-vaccine-blocks-94percent-of-asymptomatic-infections-and-97percent-of-symptomatic-cases-in-israeli-study.html
94% of asymptomatic infections blocked, 97% of symptomatic infections blocked for Pfizer. They absolutely do stop the spread by blocking infection of the vaccinated person. For reference, the smallpox vaccine is 95% effective and the pertussis (Whooping Cough) vaccine is about 80% effective. So the COVID-19 vaccines are among the most effective vaccines ever developed.