The first 2 links I posted were referencing quotes from people who will have seen the data from initial vaccine trials indicating the vaccines will most likely reduce the severity of illness but not prevent transmission.
The 30-50% was just something I knew I had read before so I googled it an came up with the lancet article.
Now I have looked back it was this news piece I came across 30-50% being discussed
Covid vaccine rush could make pandemic worse, say scientists
Fair enough we don't know the exact figure yet, we could even be lucky to get 30%, or it might be 0% or 100%
Can you provide a quote from the lancet article that says this? What I'm reading is
" the impact of these COVID-19 vaccines on infection and thus transmission is not being assessed. Even if vaccines were able to confer protection from disease, they might not reduce transmission similarly. "
and
"Challenge studies in vaccinated primates showed reductions in pathology, symptoms, and viral load in the lower respiratory tract, but failed to elicit sterilising immunity in the upper airways. Sterilising immunity in the upper airways has been claimed for one vaccine, but peer-reviewed publication of these data are awaited."
and
"These observations suggest that we cannot assume COVID-19 vaccines, even if shown to be effective in reducing severity of disease, will reduce virus transmission to a comparable degree. The notion that COVID-19-vaccine-induced population immunity will allow a return to pre-COVID-19 “normalcy” might be based on illusory assumptions. "
You also asked how this can be negative. Here is an example
I get vaccinated and I'm protected from Covid19 but can still transmit it
My dad gets vaccinated but is one of the 50% who is not protected
I catch and transmit the coronavirus to my dad
