In the pre-licensing clinical trial, Pfizer reported vaccine efficacy to be 66.7 percent against the first episode of RSV-related lower respiratory tract infection with two or more symptoms, and had 85.7 percent efficacy against the first episode of RSV related lower respiratory tract infection with three or more symptoms. The vaccine, however, was reported to have an efficacy of only 33.3 percent in persons with more than one chronic cardiopulmonary condition. Only two RSV related hospitalizations occurred in the clinical trial, both among placebo recipients. A formal evaluation of the vaccine’s efficacy against severe RSV illness was not conducted due to the small number of cases. No RSV-related deaths were reported among trial participants in both the vaccinated and placebo groups.
Preliminary data also reported ABRYSVO’ efficacy to be 62.1 percent against acute respiratory illness. Pfizer, however, failed to complete the analysis of 25.6 percent of swabs submitted for clinical trial participants with acute respiratory symptoms and as a result, the reported vaccine efficacy may be inaccurate.
There is a lack of data on the vaccine’s efficacy against severe lower respiratory tract infection and in persons with immunocompromising conditions and among individuals considered elderly and frail. Data is also lacking on the vaccine efficacy when administrated with other vaccines.
ABRYSVO pre-licensing clinical trials were conducted during the COVID-19 pandemic when rates of RSV illness had significantly decreased, and result in differences between reported efficacy and effectiveness of the vaccine when in use during a season when RSV is circulating at a higher rate.
This vaccine is assumed to offer persistent protection for 7 months but is not considered to have any effectiveness by 24 months.
