While some people have an onset of COVID-19 vaccine side effects that begin within a day or so, fainting is reported shortly after receiving the shot in smaller numbers. For example, an analysis released by the CDC identified 64 “anxiety-related events” post-vaccination out of 8,624 doses between April 7 and April 9, of which 17 fainted or “syncope.” All recipients received the Johnson & Johnson vaccine.

The CDC says it receives reports of fainting “after almost all vaccines” that are usually triggered by pain or anxiety (post-vaccine fainting is most commonly reported in adolescents). Twenty percent of patients with anxiety-related events in the CDC analysis had a history of fainting associated with receiving injections or an aversion to needles. Dizziness or lightheadedness were the most commonly reported symptoms out of the 64 events analyzed in the CDC report, and a few experts POPSUGAR spoke with fainting, dizziness or lightheadedness are examples of a nervous response called vasovagal reaction.

Amina Abdeldaim, MD, MPH, medical director of Picnic and a clinical assistant professor at the NYU Grossman School of Medicine, explained that the vagus nerve that runs through the thorax (in the center of the body) affects the heart. “There’s a reflex that slows down the heartbeat, and that’s what causes your lightheadedness and your dizziness,” she said. There is also a sudden drop in blood pressure, which can lower blood flow to the brain.

Sofija Volertas, MD, an assistant professor in the UNC’s Department of Rheumatology, Allergy and Immunology, noted that this response does not always have to be associated with anxiety. “[People] may still have this kind of reaction even if they do not have significant emotional distress. A nurse who received the Pfizer COVID-19 vaccine back in December and fainted during a post-shot interview revealed that she has an “overactive vagal response” to You can read about other causes of syncope (fainting) specifically here.

Dr. Volertas also clarified that vasovagal reactions and immediate severe allergic reactions or anaphylaxis are different. While both can occur shortly after a vaccine, vasovagal reactions tend to occur within seconds to a few minutes after injection, while anaphylaxis, she said, can only occur when the allergen is absorbed into your system and recognized by your immune system ( this is why people are asked to wait 15 minutes after vaccination before leaving the vaccination site, or 30 minutes if they have previously had anaphylaxis or, as the CDC notes, any vaccine or injection-inducing immediate allergic reaction).

“The other thing with vasovagal is usually that your heart rate gets really slow, you get really clammy skin, and you get really pale,” said Dr. Volertas. “With allergic reactions, you really get redness, red skin and hives, and your heart rate gets really fast.” A non-severe immediate allergic reaction may occur up to four hours after vaccination, while vasovagal reactions do not occur.

When someone faints from a vaccine or shows other symptoms of a vasovagal reaction, it is typically short-lived – for the 64 cases reported by the CDC, most were resolved within 15 minutes. That’s what Purvi Parikh, MD, an immunologist and allergist with the Allergy & Asthma Network, classified as “a very benign, short, limited” response. “People recover themselves, even within minutes, and they do not have dangerous changes in their vital signs and what not,” she explained.

Bottom line? If you experience fainting when you receive the COVID-19 vaccine, this should not be a cause for concern, but you can always contact the base with your doctor. In addition, it is not considered a vasovagal reaction if you experience fainting or other side effects after leaving the vaccination site. In that case, it may be wise to consult a doctor, and the same applies to all symptoms that last for a long period of time.

POPSUGAR aims to provide you with the most accurate and up-to-date information on coronavirus, but details and recommendations regarding this pandemic may have changed since its publication. For the latest information on COVID-19, please see resources from WHO, CDC and local public health departments.