I don't know - I'm also not the person that was arguing that. I have pretty high confidence that if you have the vaccine you won't transmit the disease - I sus

I don't know - I'm also not the person that was arguing that. I have pretty high confidence that if you have the vaccine you won't transmit the disease - I suspect the scientists are just being overly cautious because they don't have the data yet (though it is now starting to prove this out). They don't have the data in part, because the trials did not test asymptomatic people. This means it's possible that some of the vaccinated people in the trial caught the virus, but at a reduced severity that was asymptomatic (but still transmissible). Pretend that the vaccine reduced severity, but most people still caught it and were just asymptomatic. In that world, the vaccine wouldn't be enough (and the trials would not have revealed this). Since we know that some people who get the vaccine do catch the virus, but just have a mild case - it seems like a plausible outcome that others catch it without symptoms, but can still transmit it. That said, it's looking like that's not the world we're in - and the vaccines do just stop you from contracting the disease in addition to reducing severity in the rare case you catch it anyway. mRNA vaccines are new and given the asymptomatic transmission of the virus is why we have a pandemic, it's a reasonable thing to be cautious about (which is why I was surprised the trials only tested symptomatic patients).