But he's ignoring the issue of having the actual infection as the prime, with one dose of the vaccine as the boost.
No, he's not ignoring that issue. He mentions the prime-boost concept, then asks re: those who've recovered from covid, "Good question is do I really need two doses, or
is one dose enough? And the answer to that is that I don't know. I don't know the answer to that because that's not been studied. And it's plausible and makes sense that possibly that would be an approach - maybe a way of saving some doses or whatever. But we just don't know..."
The people asking whether those with antibodies should take the vaccine aren't concerned that "they would have less immune response, or get more sick, or have more side effects"
Boruch Hashem! You were saved from worrying about those possibilities! But the experts working on the vaccine were, in fact, worried about them, based on their experience with other vaccines and based on some observations of Covid. But what he says is that although we're not dismissing those possibilities out of hand, the available evidence right now suggests that we don't have to be concerned about them.
...on the contrary, they are questioning to what extent the vaccine is necessary, and if indeed it would give a boost (in a safer way than re-exposure to the actual virus, that would most likely also give a boost) wouldn't a single dose be enough?
And he answers "...we just don't know." He thinks it's a reasonable question to ask, but since the study hasn't been done, we have no evidence on which to base a definite answer. And "What we do know, is that two vaccine doses work..."
And if "we don't know" why not test it, there are millions of people that recovered and have antibodies? And if we're concerned about the inverted U curve he described as far as the immune system vs the severity of the disease, then why not test it primarily with those that had mild or moderate symptoms yet developed antibodies?
Last summer, there were millions of people who had recovered from Covid, and billions of people who had not gotten it. The priority was testing a vaccine to protect the billions, and it took about 5 months to do that - recruiting subjects, giving the vaccine, and watching till a significant number developed Covid. Waiting for a significant number of post-Covid subjects to develop Covid would take that much longer, because they have some degree of immunity, so let's say at least a year or two to get results.
We can't wait for that study to be done, so public health experts need to make a decision in the face of this uncertainty. But this is always the case. Doctors always have to treat the patient in front of them today, despite lacking complete certainty about the best treatment.