Anxiety is likely responsible for dozens of reports of dizziness and nausea from people who got J&J's shot at mass-vaccination sites, CDC says

• The CDC said Friday that anxiety was likely responsible for a cluster of adverse J&J reactions earlier this month.
• About 64 people at mass sites in five states reported dizziness and nausea after getting the shot.
• Experts say the side effects were probably just a physical manifestation of anxiety.

Three weeks after mass vaccination sites in five different states reported clusters of adverse reactions following the administration of the Johnson & Johnson shot, the CDC said Friday, it was likely anxiety - and not J&J - that caused dizziness, fainting, and nausea among dozens of recipients.

The post-shot reactions investigated in the CDC's Friday report are different from the rare blood clot adverse event that led to a nationwide pause in the administration of the J&J vaccine earlier this month.

The anxiety-related clusters, which took place between April 7-9 in California, Colorado, Georgia, Iowa, and North Carolina, resulted in four of the five sites temporarily suspending vaccinations. But experts said none of the approximately 64 reported reactions were considered "serious."

In fact, the likely culprit behind the reactions was a physical manifestation of vaccine anxiety, scientists found after investigating. The most common side effects among those affected were light-headedness or dizziness and nausea or vomiting, while a few also noted chest pains or a racing heart - all possible symptoms of anxiety.

"We knew we were going to see this," Dr. Noni MacDonald, a Canadian researcher who has studied such incidents told NBC News.

She told the outlet that studies indicate 10% to 15% of adults are afraid of injections. The CDC found that about 25% of those who reported side effects in these cases had previously had similar reactions to past vaccinations.

Twenty percent of the patients in question were taken to an emergency department for further medical evaluation, and most of the events resolved within 15 minutes of the patient receiving supportive care, experts found.

Health officials at the time said they had no reason to believe there was anything wrong with the shots, themselves.

CDC experts theorized that people with shot anxiety may have chosen J&J's shot because it appealed to them as the only vaccine among the three authorized in the US that requires only one injection.

Administration of the J&J shot was paused earlier this month, following reports of recipients who developed a rare blood clot and abnormally low levels of the platelets that form clots. But the US resumed use of the vaccine last week after the Food and Drug Administration added a warning to the vaccine about the risk of unusual blood clots in women under 50.