[pessimist]

This vaccine is an inactive host virus whose payload has been replaced by the coronavirus "spike". It has always been a worry that the body will generate immunity to the host virus and thus make the vaccine less effective. That is a likely explanation for the first-full-dose regimen being less effective.

[aquadrop]

Shouldn't it warrant a research if even smaller dose has even higher efficiency?

[pessimist]

Yes. But clinical trials are expensive ($100m plus) and time consuming.

[_Gyan_]

If both regimens underwent Phase 2 testing, there should be data on comparative immunogenicity.