having said all that, I did consider another angle - that the vax makes people immune to snake venom. Maybe it kills some people, and we know it doesn't prevent "the flu" but maybe it makes you resistant to venom.

later they could go wild with spraying or contaminating food parcels with high doses of venom. The vaxxed would be immune but the unvaxed (dissidents generally) would die on the spot as they had not become immune.

Taking a poison in low doses actually makes you resistant to poison (some poisons)