Vaccine Candidates for RSV

Although respiratory syncytial virus (RSV) infection is typically most prominent during the winter months, the summer of 2021 saw an unusually high incidence of infection, particularly in the Southern United States, prompting the CDC to issue a health advisory warning. This unusual increase in RSV infection is almost certainly linked to the COVID-19 pandemic. In April 2020, RSV activity dramatically decreased, likely due to the public health measures put into place to prevent the spread of COVID-19. RSV activity remained uncharacteristically low through March 2021 but began to rise again at this time, most likely due to the relaxing of public health measures. This resulted in individuals being at an increased risk of RSV because they had much lower exposure during the winter months of 2020.

Today, vaccines are at the forefront of the national consciousness, driven almost exclusively by the COVID-19 pandemic. Now, manufacturers of the highly effective COVID-19 vaccines are harnessing their vaccine technologies in the race to find an effective RSV vaccine.

mRNA-1345 Vaccine
In August 2021, the FDA granted fast track designation to the mRNA-1345 vaccine. Like the COVID-19 vaccine from the same manufacturer, this RSV vaccine candidate utilizes mRNA technology to encode for the prefusion F glycoprotein. The fast track designation is for research in older adults (60 years of age or older). Meanwhile, a phase I study to evaluate the reactogenicity and tolerability of mRNA-1345 remains ongoing in young adults (aged 18-49 years), older adults (aged 65-79 years), and children (aged 12-59 months). Preliminary results from the young adult cohort have been released, which showed an 11-fold increase in neutralizing antibodies from baseline.

Prefusion F Subunit Vaccine
In September 2021, the large, randomized, double-blind, placebo-controlled phase III RENOIR study was initiated to assess safety and efficacy of the bivalent prefusion F subunit vaccine candidate. This trial plans to enroll approximately 30,000 adults aged 60 years or older. Although full results will not be available for some time, the results of an earlier phase IIa study of the prefusion F subunit vaccine demonstrated 100% efficacy in preventing mild or moderate symptomatic RSV infection in adults younger than 50 years of age, and these results prompted initiation of the phase III trial.

Ad26.RSV.preF-Based Vaccine
Most recently, at IDWeek 2021, preliminary results from the phase IIb CYPRESS trial were presented, demonstrating that the Ad26.RSV.preF-based vaccine was 80% effective in preventing severe RSV infections in adults aged 65 years or older. Like the COVID-19 vaccine from the same manufacturer, this RSV vaccine candidate utilizes an adenovirus vector to deliver genetic material that stimulates the production of the viral fusion protein, which in turn elicits antibodies. In addition, the vaccine contains a synthetic version of this same protein, intended to produce a more robust immune response. The vaccine candidate will be further evaluated in adults aged 60 years or older in the randomized, double-blind, placebo-controlled phase III EVERGREEN trial, which was recently initiated.

Additional Approaches
Other RSV vaccine candidates are also in development, including GSK3888550A, a recombinant subunit vaccine comprised of the pre-fusion F protein currently under phase III evaluation in pregnant women. Though not a vaccine, the long-acting monoclonal antibody nirsevimab, is currently under investigation as a passive immunization strategy in infants. Should results from both of these candidates be promising, they may be studied in older adults in the future.

Your Thoughts?
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