What was happening is that we didn't know if they do or do not prevent infection or the spreading of the virus. The phase 3 studies were designed to see if the

What was happening is that we didn't know if they do or do not prevent infection or the spreading of the virus. The phase 3 studies were designed to see if the vaccine prevented disease. Measuring infectiousness is more difficult, since you can't isolate healthy unvaccinated people and see if they get infected. Since we know the viral load is an important factor for COVID infectiousness, you can reasonably use the viral load as a proxy for measuring infectiousness, and the viral load DOES drop significantly for people who have had the vaccine. *Edit Now we know: it does reduce transmission.