And in Sandwich, a schoolteacher developed COVID-19 after receiving one shot, according to town officials. The Cape Cod Healthcare employees had mild symptoms or were asymptomatic, said Dr. Kevin Mulroy, senior vice president and chief quality officer of Cape Cod Healthcare. “None of them have been seriously ill,” he said during an interview last week. Health officials said vaccine “breakthroughs” are to be expected, since none of the COVID-19 vaccines are 100% effective. “That likelihood exists. We’ve known it all along,” said Vaira Harik, interim director of the Barnstable County Department of Human Services. While the number of post-vaccination COVID-19 infections that have been detected so far are exceedingly small, their existence points to the need to maintain safety protocols and community mitigation efforts, Harik said during a Thursday morning briefing by the Cape Cod COVID-19 Response Task Force.