But researchers are also looking at whether those breakthrough cases could lead to long COVID-19, which is when people experience persistent, returning, or new symptoms a month or more after infection. The condition can develop after severe initial infections or even those who initially had mild or no symptoms.
Some estimates indicate about 30% of unvaccinated COVID-19 patients develop long-term symptoms, including shortness of breath, fatigue, difficulty concentrating, insomnia, and brain fog. Similar symptoms can develop after other viral infections, too.
A small study from Israel published recently found apparent long COVID-19 in several health workers with breakthrough infections. They developed mild symptoms, including cough, fatigue, and weakness, that persisted for at least six weeks.