I note that the Chinese military is rolling out a vaccine.
I believe that their vaccine suffers from a problem. It uses a human adeno virus, adenovirus-5, (Ad5), as a vector, modified to deliver genetic material from the SARS-CoV-2 (corona), virus, (which causes Covid19).
This delivered viral genetic material then hijacks your cells to make corona virus proteins that you generate an immune response to. Making you have immunity to corona virus.
The problem is that about half the people already have an immune response to the Ad5 adenovirus being used so their immune system destroys it before it can do all the gubbins to let you develop immunity to the SARS-CoV-2.
You can get round this by using chimpanzee adeno virus as your vector, but they have not done so.
To draw a slightly dodgy analogy, imagine you have dogs and want to give each of them a pill. The pill is like the genetic material from the corona virus.
Your dogs won't just swallow it, so you hide it in a bit of bacon. The bacon is like your Ad5 vector. BUT half your dogs have tried bacon before and it makes them vomit.
So only half of them get the payload as intended because the other half reject the vector.
It is discussed here, about half way down, starting at the line,
"CanSino/AMMS: their Ad5-nCov..."
https://blogs.sciencemag.org/pipeline/archives/2020/06/29/coronavirus-vaccine-update-june-29