There are a few reasons why COVID-19 cases may continue to rise even as vaccination rates increase. For one, no vaccine is 100% effective, and highly contagious strains like the Delta variant have led to breakthrough infections among a small percentage of vaccinated people. As the total number of vaccinated people increases, the pool of unvaccinated people gets smaller—that means proportionally, more cases will be among the vaccinated. Breakthrough cases are still a small percentage of total cases.
The COVID-19 vaccines continue to be highly effective in reducing risk of severe disease, hospitalization, and death. When COVID-19 cases rise, breakthrough infections among vaccinated individuals are more likely to be mild cases, while unvaccinated people are more likely to become severely ill or require hospitalization. CDC data show that in August 2021, the risk of dying from COVID-19 in the U.S. was more than 11 times greater for unvaccinated people than for fully vaccinated people.
Added December 9, 2021