evidence: Although all of the vaccines approved and authorized for use in the United States are effective at preventing symptomatic disease, so-called "breakthrough" cases of COVID-19 after vaccination are to be expected — perhaps even more so now because of the more contagious delta variant of the virus. [...] In addition to the point about all vaccines being less than 100% effective, the September 2017 CHOP post noted that more unvaccinated people in a population leads to more virus transmission. [...] Johns Hopkins University epidemiologist David Dowdy made a similar point.