These vaccines use a harmless virus as a vehicle, but the body can develop defences against it too.

Vaccines from AstraZeneca, Russia’s Gamaleya Institute and Johnson & Johnson fight the coronavirus with another virus, leaving scientists concerned the shots may lose potency if annual inoculations become necessary to fight new variants.
So-called viral vector shots – also used by several Chinese Covid-19 vaccine developers – use harmless modified viruses as vehicles, or vectors, to carry genetic information that helps the body build immunity against future infections. However, there is a risk that the body also develops immunity to the vector itself, recognising it as an intruder and trying to destroy it.
Most vector-vaccine developers have opted to use an adenovirus, a harmless class of common-cold viruses…