Introduction to the IVX-121 RSV Vaccine Trial

Respiratory syncytial virus (RSV) causes significant lower respiratory tract disease across all age groups. Specifically, older adults face a substantial burden of severe disease and hospitalization. Researchers recently evaluated the IVX-121 RSV vaccine, an investigational virus-like particle (VLP) subunit vaccine, in a first-in-human Phase 1/1b randomized trial. This study assessed the safety and immunogenicity of various dose levels in healthy young and older adults.

Novel Mechanism of the IVX-121 RSV Vaccine

The IVX-121 RSV vaccine utilizes advanced VLP technology to display a stabilized prefusion F antigen. This approach mimics the structure of a real virus without containing genetic material. Consequently, the vaccine can trigger a potent immune response while maintaining a high safety profile. During the trial, 90 young adults and 130 older adults received a single intramuscular injection. Participants were randomized to receive low, medium, or high doses, with or without an aluminum hydroxide adjuvant. Furthermore, the trial included a placebo group for rigorous comparison.

Key Safety and Immunogenicity Findings

The trial results showed that adverse reactions were generally mild. Importantly, there were no serious adverse events related to the vaccine. All tested dose levels successfully boosted preexisting RSV neutralizing antibody (nAb) responses. However, the researchers noted no clear dose-response relationship, and the adjuvant did not significantly improve immunogenicity. For both age groups, the vaccine induced nAbs against both RSV A and B strains. These antibody levels remained significantly above baseline for at least six months. A second dose in older adults provided a modest boost for RSV A, though not for RSV B.

FAQs

What is VLP technology in the context of the IVX-121 RSV vaccine?

VLP technology involves creating virus-like particles that look like a virus to the immune system but lack viral DNA or RNA. This allows the vaccine to be non-infectious while eliciting a robust antibody response.

Does the IVX-121 vaccine protect against both major RSV strains?

Yes, the trial demonstrated that IVX-121 induces neutralizing antibodies against both RSV A and RSV B subtypes in young and older adult populations.

Was an adjuvant necessary for the immunogenicity of this vaccine?

The study found that the adjuvant aluminum hydroxide did not appear to improve the immunogenicity of the vaccine, as unadjuvanted doses were equally effective in boosting antibody levels.

Disclaimer: This content is for informational and educational purposes only. It is not intended as medical advice or to replace the professional judgment of a healthcare provider. Refer to the latest local and national guidelines for clinical practice.

References

De Coster I et al. A first-in-human Phase 1/1b randomized trial evaluating the safety and immunogenicity of a respiratory syncytial virus (RSV) virus-like particle subunit vaccine (IVX-121) in healthy young and older adults. Hum Vaccin Immunother. 2026 Dec undefined. doi: 10.1080/21645515.2026.2633021. PMID: 41776715.

Icosavax. Icosavax Reports Positive Durability Data for VLP Vaccine Candidate IVX-121 Against RSV at Six-Month Timepoint. FirstWord Pharma. 2022 Dec 13.

Bandla SS et al. Epidemiology of Respiratory Syncytial Virus in Hospitalised Adult Patients with Respiratory Tract Infections. J Clin Diagn Res. 2025;19(4):DC01-DC05.