Vaccine-associated enhanced respiratory disease

Vaccine-associated enhanced respiratory disease , or simply enhanced respiratory disease, is an adverse event where an exacerbated course of respiratory disease occurs with higher incidence in the vaccinated population than in the control group. It is a barrier against vaccine development that can lead to its failure.
Immunologically, VAERD is characterized with an exaggerated Th2 response and eosinophilic pulmonary infiltrations. It may result from antibody-mediated complement activation followed by weak neutralization.
Historical instances of the phenomenon were seen in vaccine candidates for respiratory syncytial virus, SARS-CoV, Middle East Respiratory Syndrome, and some influenza strains. Thus, COVID-19 vaccine clinical research involved monitoring for VAERD because the vaccine target, SARS-CoV-2, belongs to the same viral subfamily as SARS-CoV and MERS. The effect was not shown in phase III clinical trials for Tozinameran or for the Moderna vaccine.