EU agency says blood clot 'very rare' side effect of Johnson & Johnson vaccine Europe's Johnson & Johnson campaign was delayed after US health regulators said the shot should be paused over blood clot fears. Share Via Email | A+A A- In this April 8, 2021 file photo, the Johnson & Johnson COVID-19 vaccine is seen at a pop up vaccination site in the Staten Island borough of New York. (Photo | AP) By AFP THE HAGUE: Europe's medicines regulator said Tuesday that blood clots should be listed as a "very rare" side effect of Johnson & Johnson's coronavirus vaccine, but that the benefits of the shot still outweighed the risks.