What is this potential side effect of the J&J vaccine for COVID-19? About one in a million: Six cases out of the 6.8 million doses of the J&J vaccine administered in the U.S. This will make it possible to determine if the clotting problem is a vaccine side effect or not. What are the CDC and FDA specifically recommending for the J&J vaccine? On April 13, 2021, Johnson & Johnson announced it is delaying the rollout of its vaccine in Europe in response to the U.S. review.