The CDC studied reports of the dozens of physical reactions from vaccine recipients from April 7 to 9, from clinics in California, Colorado, Georgia, Iowa and North Carolina. The reactions prompted some vaccine centers to temporarily shut down while officials determined the source of the problem.

Many of the 64 people who were affected reported either fainting or experiencing dizziness, while others became nauseous or vomited, and a few had racing hearts, chest pain or other symptoms. No one was seriously ill, and the CDC determined that the reactions were brought on by anxiety caused by receiving injections, the Associated Press reported.

“We knew we were going to see this” Dr. Noni MacDonald, a Canadian researcher who has studied similar incidents, told AP.

For more reporting from the Associated Press, see below.

Of the three COVID-19 vaccines authorized in the U.S., only J&J’s requires just one dose. That probably makes it more appealing to people who are nervous about shots and might leave them “more highly predisposed to anxiety-related events,” the CDC report said.

Some of the sites advertised they were giving J&J shots, said Dr. Tom Shimabukuro, who leads the CDC’s COVID-19 vaccine safety monitoring work and is one of the study’s authors.

The CDC found that about one-quarter of the people reporting side effects had similar things happen following past vaccinations.

The post-shot reactions differ from a very rare kind of side effect that led to a pause in administration of the J&J vaccine. At least 17 vaccine recipients have developed an uncommon kind of blood clot that developed in unusual places, such as veins that drain blood from the brain, along with abnormally low levels of the platelets that form clots.

Other types of side effects from the coronavirus vaccines are not unusual. Another CDC report released Friday looked at side effects reported by more than 300,000 J&J vaccine recipients. More than half said they experienced a sore arm, fatigue or headache. A third reported fever or chills, and about one-fifth said they were nauseous.

But the clusters at the five clinics are believed to be stress-related.

MacDonald, a professor of pediatrics at Dalhousie University in Nova Scotia, Canada, said studies have indicated that 10% to 15% of adults are afraid of injections.

Many people who experience stress-related symptoms are younger, and past clusters from other shots have involved students. Some hyperventilate, some experience nausea, some reported headaches. And some had what at first appear to be more severe, neurological symptoms, she said.

One cluster that MacDonald reviewed involved 14 U.S. military reservists who developed symptoms after getting flu shots in 2009. The first was a 23-year-old man who one day reported progressive weakness in his arms and legs but fully recovered.

“Everybody thinks this is (only) young teenage girls” who experience this, MacDonald said. “Well, it isn’t.”

It can start with one person fainting that can set off a chain reaction of symptoms in anxious people who see or hear about that first person. These days, people also react to things they read or watch in Facebook posts or on other sites.

Some doctors have referred to the phenomenon as a form of mass hysteria, but MacDonald rejected the term.

“These people are not crazy,” but rather are experiencing real physical responses to psychological stress, she said.