No point shipping a vaccine early if it doesn't actually work. That could do more damage, people thinking it's safe to be around others maskless, when in fact the world population instead received an ineffective vaccine.
I think you're misunderstanding the problem space here. We don't expect a coronavirus vaccine to prevent close to 100% of infections, so it's not a binary question of working vs. not working. The question is, how strong of an effect at what level of confidence is required to conclude that it's working well enough?
The effect needs to be strong, because in ordinary times, many social circles cross paths and a much higher level of human contact occurs. The average person won't read into the nuance of 'the vaccine is X% effective, depending on risk factors Y and Z'. They'll rush to their ordinary behavior. While there isn't a vaccine, it's easier to obligate folks into social distancing. That strategy won't exist anymore once we have a vaccine, regardless of its efficacy.
I agree, but that's what the challenge is. We have to solve for the values of X, Y, and Z where we're comfortable with people's inevitable rush back to their ordinary behavior, and it's extraordinarily hard to figure out what those values should be.
I don't know the biological reasons behind it (I don't even know if the reasons are known, or if it's just based on early results), but Dr. Fauci's said that scientists are hoping for 75% effectiveness and 50-60% effectiveness would be acceptable. (https://www.cnbc.com/2020/08/07/coronavirus-vaccine-dr-fauci...)
We already know that some of the vaccine candidates work (ie. immune response). I’m not arguing we should ship untested vaccines, but challenging how many months we need to wait after testing.
We know that they trigger an immune response, but that alone doesn't necessarily mean much.
For example, an early SARS vaccine candidate triggered an immune response in tests, but that immune response in practice actually made SARS infections worse.[1]
Yeah, certainly we ought to see how animals and people with the vaccine react when exposed to the virus, but have we not already done that in the months these vaccine candidates have been tested? The current trials did things like give the vaccine to thousands of Brazilian health-care workers, which should have uncovered any issues like that (in the article you linked, the issue was uncovered by testing in a small sample of animals).
What I’m arguing is that we don’t need complete phase 3 trials to establish efficacy - we can establish that based on early data from those (and earlier) trials, while accepting larger error bars on the risk of side effects.