Croup Etiology and Immunization Status
Parainfluenza viruses (primarily types 1 and 2) cause the majority of croup cases, and there is currently no vaccine against parainfluenza virus included in standard childhood immunization schedules in any location. 1
Primary Viral Cause
- Parainfluenza viruses are the leading cause of croup, accounting for approximately 74% of all identified viral croup cases, with type 1 being the most common (65% of parainfluenza isolates). 2
- Parainfluenza types 1 and 2 are most commonly associated with the croup syndrome, with type 1 having a 58% propensity to produce croup symptoms when it causes lower respiratory tract infection. 1, 2
- These viruses are second only to respiratory syncytial virus (RSV) as a cause of hospitalization in children under 5 years of age, responsible for up to 11% of all such hospitalizations. 1
Other Viral Causes
- Additional viruses that can cause croup include RSV, influenza viruses, rhinoviruses, human coronaviruses, adenoviruses, and human metapneumovirus. 1
- Influenza-associated croup tends to be more severe than parainfluenza-associated croup and carries higher risk of bacterial tracheitis as a complication. 1
Immunization Status
- No vaccine exists for parainfluenza viruses in the standard childhood immunization schedule. 3
- While influenza vaccine is recommended annually for all children ≥6 months of age and can prevent influenza-associated croup, it does not protect against parainfluenza viruses (the primary cause). 4
- Standard childhood immunizations protect against Streptococcus pneumoniae (PCV13), Haemophilus influenzae type b, pertussis, and influenza—but none of these are the primary cause of croup. 4, 5
Clinical Context
- Croup predominantly affects children 6 months to 6 years of age, with peak incidence in the second year of life. 6, 2
- The disease manifests with inspiratory stridor, barking cough, and hoarseness due to laryngeal/tracheal obstruction from viral-induced vasodilation and hypersecretion. 1, 6
- Boys are 1.43 times more likely to develop croup than girls. 2
Important Caveat
- Although vaccine development strategies for parainfluenza viruses have been investigated (including live attenuated, inactivated, recombinant, and subunit vaccines), none have been successfully implemented in clinical practice to date. 3