Leaky vaccines. That really should send a shudder down everyone's spine with the new Covid "vaccine". So pretty much, keep a virus alive in the vaccinated host. The host does not fall ill but sheds the virus load at an exponential rate. Virus continues to evolve, grow stronger in the vaccinated hosts because it never died out. Infect and kill all unvaccinated hosts quickly until only the vaccinated survive and all new births require vaccination.