Hypotonic hyporesponsive episode (HHE)

Hypotonic-hyporesponsive episode (HHE) consists of symptoms similar to fainting found in little children as a rare adverse effect after vaccination.

HHE has been mostly reported in infants who have received their first injectable vaccine.

The exact prevalence of HHE is not known. In an extensive study carried out in Sweden, HHE occurred in 3-4 children out of ten thousand children who had received an acellular pertussis vaccine. 

The exact pathogenetic mechanism is unknown. 

Symptoms of HHE

The symptoms start fairly soon, but no later than 48 hours after vaccination. Children with HHE

  • have pale or bluish skin
  • go flaccid
  • react either poorly or not at all to their environment 

The sudden symptoms can seem scary. The symptoms disappear without treatment, however, and the state does not have after effects. HHE should not be confused with anaphylaxis. 

Treatment of HHE

The child will recover from the HHE symptoms without any special treatment, usually within 1-30 minutes. Sometimes some symptoms take longer.

Follow-up vaccinations

The follow-up vaccinations can be given as normal. Studies show that the risk of recurrence of HHE is very low, in the range of 1% to 3%.