Vaccine efficacy in adults in a respiratory syncytial virus challenge study



Although human respiratory syncytial virus (RSV) is an important cause of illness and death in the elderly, no RSV vaccine has been licensed.


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In a phase 2a study, we randomly assigned healthy adults (aged 18-50 years), in a 1:1 ratio, to receive a single intramuscular injection of bivalent prefusion vaccine F (RSVpreF) or placebo . Approximately 28 days after injection, participants were inoculated intranasally with RSV A Memphis 37b challenge virus and observed for 12 days. The protocol-prespecified primary endpoints were: reverse transcriptase – quantitative polymerase chain reaction (RT-qPCR) – detectable RSV infection confirmed on at least 2 consecutive days with at least one clinical symptom of any which grade of two categories or at least one grade 2 symptom of any category, total symptom score from day 1 at discharge, and area under the curve (AUC) of RSV viral load in samples nasal lavage measured using RT-qPCR from day 2 after challenge to unload. Additionally, we assessed immunogenicity and safety.


Once participants were inoculated with the challenge virus, a vaccine efficacy of 86.7% (95% CI, 53.8 to 96.5) was observed for symptomatic RSV infection confirmed by any RNA detectable virus for at least 2 consecutive days. The median RSV viral load AUC (hours × logten copies per milliliter) measured by the RT-qPCR test was 0.0 (interquartile range, 0.0 to 19.0) in the vaccinated group and 96.7 (interquartile range, 0.0 to 675.3) in the placebo group. The geometric mean factor increase from baseline in RSV A neutralizing titers at 28 days post-injection was 20.5 (95% CI, 16.6 to 25.3) in the vaccinated group and 1. 1 (95% CI, 0.9 to 1.3) in the placebo group. More local pain at the injection site was noted in the vaccinated group than in the placebo group. No serious adverse events were observed in either group.


The RSVpreF vaccine was effective against symptomatic RSV infection and virus shedding. No obvious security issues were identified. These results support further evaluation of the RSVpreF vaccine in a Phase 3 efficacy study. (Funded by Pfizer; EudraCT number, 2020-003887-21; number, NCT04785612.)

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Respiratory Syncytial Virus Vaccine Study

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