Why Vaccinated People Are Getting ‘Breakthrough’ Infections

"The uncertainty about Delta results in part from how it differs from previous versions of the coronavirus. Although its mode of transmission is the same — it is inhaled, usually in indoor spaces — Delta is thought to be about twice as contagious as the original virus.
Significantly, early evidence also suggests that people infected with the Delta variant may carry roughly a thousandfold more virus than those infected with the original virus. While that does not seem to mean that they get sicker, it does probably mean that they are more contagious and for longer.
Dose also matters: A vaccinated person exposed to a low dose of the coronavirus may never become infected, or not noticeably so. A vaccinated person exposed to extremely high viral loads of the Delta variant is more likely to find his or her immune defenses overwhelmed."