IIRC my biology lessons in high school, viruses mutate in order to escape from vaccines. Just like mosquitoes mutate and adapted to DDT, we created new chemicals and then mosquitoes adapted to them, under evolutionary pressure.
So vaccinated populations are creating the pressure on the virus to mutate. And un-vaccinated population may get effected (irrespective of which version of virus they encounter at the time)..
Conclusion: This study provides the first in vivo evidence that zinc sulfate in combination with hydroxychloroquine may play a role in therapeutic management for COVID-19.