Syncope is the medical term for fainting. It’s a potential side effect of any vaccine, including those used to prevent COVID-19. Fainting is when you pass out due to a lack of oxygen to your brain. It’s most commonly a vaccine side effect among younger adults and teenagers, according to the Finnish Institute for Health and Welfare, but it can affect anybody.

In most cases, the stress and anxiety of getting a vaccine cause fainting, not the vaccine itself. In very rare cases, a severe allergic reaction to one of the ingredients can cause a drop in blood pressure that leads to loss of consciousness.

Read on to learn more about why some people faint after receiving a COVID-19 vaccine and what factors cause people to faint.

According to the Centers for Disease Control and Prevention (CDC), fainting is a reported side effect of almost every vaccine. It’s most common after vaccines against:

In most cases, fainting is caused by the stress and anxiety of receiving a vaccine. These emotions can trigger a condition called vasovagal syncope. Vasovagal syncope is the most common cause of fainting in general.

Nerves send messages from your brain to your heart and blood vessels to control your heart rate and blood pressure. Vasovagal syncope occurs when these nerves don’t send an appropriate signal, causing a drop in blood pressure and inadequate blood flow to your brain.

Strong emotions, like the fear of receiving a vaccine, and other factors like dehydration or pain can trigger vasovagal syncope.

In a May 2021 report published by the CDC, the reported frequency of fainting from Janssen COVID-19 vaccines and flu shots was 8.2 and 0.05 per 100,000 people, respectively, between 2019 and 2021.

Of the people who fainted, 62 percent were ages 11 to 18. And 25 percent were ages 19 to 49.

Nearly a quarter of people who experienced fainting or other anxiety-related side effects after receiving the Janssen vaccine reported a history of similar anxiety-related events from other vaccines.

Allergic reaction

In fewer than 1 in 1 million people, vaccination can trigger a severe allergic reaction called anaphylaxis. Anaphylaxis can be fatal if not quickly treated. According to the World Health Organization (WHO), it usually develops 5 to 30 minutes after injections.

Symptoms of anaphylaxis can include:

Fear of medical procedures involving needles is called trypanophobia. It’s an extremely common fear. In a 2018 review, researchers estimated the prevalence to be 20 to 50 percent in adolescents and 20 to 30 percent in young adults.

The development of phobias is complex and can be caused by a combination of social, psychological, and physiological causes.

According to the WHO, the development of a vaccine phobia may be explained by physiological factors, like:

  • Age. Adolescents are at the highest risk of vaccine anxiety.
  • Sex. Females are more likely to experience anxiety than males.
  • Weight. Lower body weight is associated with a higher risk of fainting.

Vaccine phobia is also influenced by psychological factors, including:

  • personality
  • ability to understand and reason
  • prior knowledge of vaccination
  • underlying anxiety
  • previous experiences

Social factors also play a role, such as:

  • trust in healthcare professionals
  • perceptions of vaccination among people in a community
  • false and misleading news stories
  • experiences of friends and family

In the same May 2021 report published by the CDC, the most reported anxiety-related symptoms were:

More than 98 percent of fainting episodes occur within 30 minutes of injection, according to a 2021 review. After your vaccine, the person who administered the shot will likely tell you to wait around for at least 15 minutes for monitoring.

If you’re with a person who faints, lay the person down with their legs in a raised position until the person is feeling better.

If you develop anxiety-related symptoms after vaccination, you can try taking slow, deep breaths to calm your heart rate. Staying hydrated and having a snack available may also help you ease symptoms such as faintness or lightheadedness.

Many people find it helpful to distract themselves with something such as listening to music, playing a game, or talking.

In two 2018 studies, researchers found that short bouts of exercise before vaccination lowered the number of side effects.

The WHO recommends administering vaccines in a calm, planned, and private environment when possible.

If your child is nervous about vaccines, you may be able to lower their stress by:

  • giving children 2 years and younger something sweet before the shot to help reduce pain
  • breastfeeding babies to help calm and relax them
  • asking the vaccine administrator to use a pain-relieving ointment or spray
  • explaining to your child in simple terms what to expect
  • bringing comforting things for your child, like their favorite toy or blanket
  • distracting your child to pull their attention away from the shot
  • having older children take slow, deep breaths
  • calming infants with hugs and soothing whispers

It’s common to experience mild side effects after getting a vaccine. If side effects appear, they usually go away after 1 or 2 days.

According to the CDC, the most common side effects are:

Rarely, some people may experience more serious reactions. These can include:

Many people find getting a vaccine stressful. This stress can lead to anxiety-related side effects such as fainting, dizziness, or nausea. In very rare cases, vaccines can cause a severe allergic reaction that causes fainting.

For the vast majority of people, vaccines cause no or mild side effects. If you have a history of vaccine-related anxiety, talk with your doctor about ways to manage your anxiety before a vaccine to lower your chances of side effects.