The Gamelaya vaccine is similar to the AZ vaccine. The Russians used two vectors for the primer and booster rAd26 and rAd5 wihich are common human adenoviruses. So some people might have got infected by these viruses and have developed immunity against it. Then the body will fight against the vector itself and the vaccine will not have the desired effect. So they chose two vectors such that many will not have immunity against BOTH and at least one of them will work.
OTOH the Oxford team went ahead with a Chimp adenovirus. Only a person who might work in a zoo might have immunity against a Chimp virus.
That's the only difference between the vaccine methodology. None of the vaccines will give a positive result for PCR.
PS: Note that the Australian developed vaccine was abandoned simply because it gave a false positive for HIV. Not because of it's efficacy. Then the whole regime of HIV testing has to be altered.
