What's concerning about rare cases of Johnson and Johnson COVID-19 vaccine blood clots Right now, researchers are still trying to get to the bottom of the blood clot mystery with possible connections to the Johnson & Johnson COVID-19 vaccine. and last updated 2021-04-28 10:14:22-04 Right now, researchers are still trying to get to the bottom of the blood clot mystery with possible connections to the Johnson & Johnson COVID-19 vaccine. But ABC15 Health Insider Dr. Janice Johnston of Redirect Health says even if there is a connection, it's still very uncommon. "The biggest thing to keep in mind is that this is very rare," she explains. "There have been six cases in about seven million doses. To try to put that into perspective. This is a very rare type of thing... but it is significant and potentially very serious."