I obviously can't say for sure, but given the topic was the development of artificial resistance to vaccines, I would've thought the alternative they had in mind was natural resistance - i.e. getting infected naturally.
Just in my non-expert head - a live vaccine is using the virus as it already exists to confer immunity - not trying to code against a ton of hypothetical variants which could select for more radical variants
Ah, that isn't how a live vaccine works though. They get attenuated (aka damaged/killed/broken) or similar but not the same viruses (aka cowpox used to be a live vaccine for smallpox), then introduced. The body reacts to the not-really-functional virus, but it isn't the same as if it was an actual infection.
It still triggers the immune system, but mutations in any of the major receptors or the like will cause it to be less effective too.
