Remember that these carrier viruses are _not_ common in humans. Astra Zeneca even uses one that only infects chimpanzees naturally, and iiuc, the other adenoviruses used by other vaccines mostly infect non-human primates.
You have to use something uncommon, or otherwise the vaccine wouldn't work reliably due to people already having antibodies to it. Sputnik V actually uses two different ones, to make it less likely for the 2nd dose to be defeated by antibodies developed in the 1st dose.
Specifically, the particular strains used. Some strains of adenoviruses are common in humans. Others are not.
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