| Hypotonic-hyporesponsive episode (HHE) | |
| Symptoms: | Hypotonia, hyporesponsiveness, pallor, cyanosis.[1] |
| Onset: | 1 minute to 48 hours post-vaccination. |
| Duration: | 1 minute to 14 days (median 10 hours). |
| Causes: | Vaccination, most commonly pertussis vaccine. |
| Diagnosis: | Based on symptoms.[2] |
| Differential: | Vasovagal syncope, seizure |
| Prevention: | Those who experience an HHE after pertussis vaccination may avoid further pertussis vaccines.[3] |
| Treatment: | Usually resolves on its own, management involves checking the airway, breathing, and circulation, and hospitalization for observation and to rule out other conditions.[4] [5] |
| Deaths: | None reported |
A hypotonic-hyporesponsive episode (HHE) is defined as sudden onset of poor muscle tone, reduced consciousness, and pale or bluish skin occurring within 48 hours after vaccination, most commonly pertussis vaccination.[6] An HHE is estimated to occur after 1 in 4,762 to 1 in 1,408 doses of whole cell pertussis vaccine, and after 1 in 14,286 to 1 in 2,778 doses of acellular pertussis vaccine.[2]