Ask the Experts - Syncopal Episodes Following Vaccination
A 12-year-old female with no significant past medical history had a brief (< 1 minute) syncopal episode about 3 minutes after the placement of measles-mumps-rubella (MMR) shot #2 and hepatitis B #1 and purified protein derivative (PPD). Is this an adverse event that is associated with any of the given vaccines?

Dr Friedman

The sudden onset of hypotonia, hyporesponsiveness, and pallor or cyanosis that occurs within 48 hours after childhood immunizations is called a hypotonic-hyporesponsive episode (HHE). These episodes have been reported to the Vaccine Adverse Event Reporting System (VAERS) almost exclusively with whole-cell pertussis-containing vaccines. They have been reported with the acellular pertussis-containing vaccines as well as non-pertussis-containing ones but at a much lower rate. Most of these events were limited to those under 2 years of age. While syncopal episodes have been reported following vaccine administration to children older than 2 years of age, these episodes most likely represent vasovagal responses to the injection rather than adverse reactions to the antigen being given.

When evaluating data from VAERS, it is important to note that for any reported event, no cause-and-effect relationship has been established. VAERS is interested in all potential associations between vaccines and adverse events. The report of an adverse event to VAERS does not constitute documentation that a vaccine caused the event.

This is true for syncopal episodes following vaccine administration. A study of syncope following immunization demonstrates that females under 20 years of age are more likely to have syncope. The majority of these patients experience syncope within the first 5 minutes after administration. Even more concerning is that some of these episodes resulted in significant head injury. It is important to remember that those patients who seem to be susceptible to vasovagal responses should be monitored for 15-30 minutes in order to prevent head injury.