Scott Hensley of the Perelman School of Medicine said breakthrough COVID-19 infections tend to be mild because the vaccines give the immune system a head start. “The virus is stopped in its tracks within a few days,” he said.
https://www.washingtonpost.com/politics/rare-breakthrough-covid-cases-are-causing-alarm-confusion/2021/07/21/371a2e1c-ea5f-11eb-a2ba-3be31d349258_story.html The Washington Post