You’re confusing recipients with spreaders. If your goal is to curb spread by eliminating the spreaders, and half of those you vaccinated can still get infected, and transmit it 3x as much as they would’ve done without being vaccinated (less symptoms and less tests so won’t know to isolate, there are now more spreaders than beforehand.
Obviously we don’t know it’s the case, but we know there’s a possibility it is the case.
There's a lot of assuming going on here all around. Some points:
1) It's 40% of the vaccinated who can still get infected.
2) It's very likely that the infection will be less severe, and the viral shed will be much lower, as well.
3) The theory that they may be 3x more likely to transmit it is based on the assumption that people take precautions when they have symptoms (even though most spread happens before symptoms) and that they are less likely to get tested after an exposure. This is only relevant is you also assume that vaccinated people will take zero precautions, which is a stretch. It also doesn't factor in that X% of people they come in contact with will also be vaccinated, lowering the risk of transmission even further.
There was a Washington Post article (
https://www.washingtonpost.com/graphics/2020/world/corona-simulator/) from the beginning of the pandemic that attempted to show exponential spread and explain the ever-so-popular "Flatten The Curve" policy. There were some graphics used that come to mind whenever I try to reconcile the current vaccination approach with how I think it should be done. Without revisiting the policy or anything the article says (because I haven't read it in 10 months), the image that is burned into my mind is all the balls bouncing around, changing from grey (not infected, no antibodies) to red (infected, spreading) to pink (antibodies, immunity). The fewer grey balls we have bouncing around, the lower the impact the red balls will have. And the slower we can make the spread in the time leading into the third wave, the more time we buy to vaccinate even more people and eliminate more grey balls from the equation.