The agency investigated clusters of anxiety-related events, with a total 64 incidents out of 8,624 doses administered, reported to the CDC by five mass vaccination sites across five different states.

Researchers said that these anxiety-related cases “can occur after any vaccination” if a person has a physical reaction within 15 minutes of inoculation due to their worries about getting the shot. 

The incidents were reported between April 7 and 9, about five weeks after the Food and Drug Administration (FDA) issued its emergency authorization approval for the Johnson & Johnson vaccine.

These anxiety-related reactions are not related to the rare cases of blood clots that led the CDC and FDA to recommend pausing administration of the Johnson & Johnson vaccine across the country. The anxiety-related cases occurred before the pause, which ended last week. 

In response to the anxiety-related cases, which included 17 cases of fainting, or syncope, four of the five mass vaccination sites temporarily shut down during an investigation. 

In total, the CDC determined that the reporting rate of fainting after a Johnson & Johnson vaccine to the Vaccine Adverse Events Reporting System was 8.2 per 100,000 doses. In comparison, the rate for the 2019-2020 flu shot was 0.05 per 100,000 doses, about 164 times less common.

Recipients who underwent anxiety-related events most commonly reported light-headedness or dizziness, sweating, fainting, nausea or vomiting, and hypotension. Thirteen of the patients were taken to the emergency department for additional care.

Researchers noted that because Johnson & Johnson is the only single-dose shot available in the U.S., it may be selected more by those who have needle aversion and therefore more likely to have anxiety-related incidents post-vaccination.

They called on vaccine providers to be “aware” on anxiety-related incidents after the shot and observe all recipients for any reactions for at least 15 minutes after the vaccine was administered.

The Associated Press identified the five states where the incidents occurred as California, Colorado, Georgia, Iowa and North Carolina.

The anxiety-related events were different from the at least 17 recipients of the Johnson & Johnson vaccine who developed rare blood clotting in unusual places and low levels of platelets. Last week, the FDA included a warning for these clotting cases, mostly in women aged 18 to 49, as it resumed the Johnson & Johnson vaccinations.

Johnson & Johnson vaccine recipients have also reported less severe side effects, such as sore arm, fatigue and headache, as have those who received the Pfizer-BioNTech and Moderna vaccines.