For some, breakthrough infections have felt like mild allergies, coming with symptoms including a cough, sniffles and a scratchy throat. Others have had more severe cases, where they are bedridden with body aches, fevers and chills. And still others have had some of the telltale signs of Covid such as loss of taste and smell, “Covid rash” and brain fog.

“We were calling it floaty-head syndrome,” said Molly O’Brien-Foelsch, 47, a marketing executive in Pennsylvania who tested positive for Covid after a trip to the British Virgin Islands with her husband last month. “It felt like there was a huge marshmallow on my head.”

Scientists believe that breakthrough infections rarely result in severe illness, but there have been cases of prolonged hospitalizations. Elaina Cary-Fehr’s father Isaac, a 64-year-old Uber driver in Austin, was transferred to a long-term care facility after being hospitalized with Covid pneumonia in June and later receiving a tracheotomy tube. He was released from the facility this week.