I think that Covid virus was just a hybrid of many lethal ones @Alticus
The authorities claimed that if you got other vaccines it wouldn't protect you against Covid but now studies are starting to appear which indicated that people who cross-vaxxed either had a better chance of surviving it or avoiding it altogether. ie - the MMR vaccine was shown to offer limited protection against COVID & some recipients who got vaxxed were better protected than the ones who got the Pfizer shot.