WASHINGTON The U.S. on Tuesday recommended a âpauseâ in using the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of rare but potentially dangerous blood clots, setting off a chain reaction worldwide and dealing a setback to the global vaccination campaign. The Centers for Disease Control and Prevention and the Food and Drug Administration announced that they were investigating unusual clots in six women between the ages of 18 and 48. One person died. The acting FDA commissioner expected the pause to last only a matter of days. But the decision triggered swift action in Europe and elsewhere as the drugmaker and regulators moved to halt the use of the J&J vaccine, at least for now. Hundreds of thousands of doses were due to arrive in European countries this week. The European Union has been plagued by supply shortages, logistical problems and concerns over blood clots in a small number of people who received the AstraZeneca vaccine.