Vaccine-associated enhanced respiratory disease (VAERD), or simply enhanced respiratory disease (ERD), refers to an adverse event where an exacerbated course of respiratory disease occurs with higher incidence in the vaccinated population in comparison with the control group. It is one of the barriers against vaccine development and can lead to its failure. Immunologically, VAERD is characterized with an exaggerated Th2 response and eosinophilic pulmonary infiltrations. It is thought to arise as a result of antibody-mediated complement activation followed by weak neutralization.