A clinical development perspective — 4 November 2026

RSV vaccines are here. hMPV vaccines, not yet.

Licensed RSV vaccines have changed the prevention landscape for older adults. Human metapneumovirus (hMPV), despite its contribution to respiratory morbidity, still has no licensed vaccine. Together, these facts make the next development question both scientifically interesting and commercially relevant: can a combination vaccine establish meaningful protection against each virus, without compromising safety or the RSV component? [1,2]

The combination pipeline has progressed beyond antigen selection. Several programmes now have substantial immunogenicity datasets, while Sanofi has initiated an event-driven Phase IIb study designed to assess clinical disease. Importantly, those are different types of evidence. Neutralising antibody rises support biological activity; they cannot be treated as measured vaccine efficacy against RSV- or hMPV-associated lower respiratory tract disease.

Five programmes, five distinct evidence positions

Figure 1. Selected studies and programme stages as of 4 November 2026. Planned and enrolled sample sizes are not interchangeable. Numbers refer to the indicated total study or cohort, including comparators/placebo.

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Sanofi — bivalent RSV+hMPV protein vaccine, VCA25397

Phase IIb; n=8,000 planned. Adults ≥60 years; placebo-controlled, case-driven clinical study, recruiting. Designed to assess clinical efficacy against RSV and hMPV disease. No efficacy result reported yet. [3]

GSK — investigational hMPV and RSV/hMPV formulations

Phase I/II; n=1,456 planned. Younger adults 18–49 and older adults 60–80; differing formulations and doses. Safety, reactogenicity and immunogenicity; not yet a clinical-efficacy demonstration. [4]

AstraZeneca/Icosavax — IVX-A12 (VLP)

Phase II; n=143 enrolled. Adults ≥60; IVX-A12 n=71 vs licensed RSV vaccine comparator n=72. Peer-reviewed comparative immunogenicity and safety data published 3 November 2026. Clinical efficacy was not evaluated. [5]

Clover — SCB-1022 (RSV+hMPV) / SCB-1033 (RSV+hMPV+PIV3)

Phase II cohort; n=420 enrolled. Adults 60–85 in Australia. Preliminary neutralisation and safety findings reported by the sponsor. The cumulative registered Phase I/II study reports 612 participants, including earlier cohorts. [6,7]

Moderna — mRNA-1365, Profile 2

Phase I; n=375 planned. Adults ≥60; dose escalation/expansion, RSV+hMPV mRNA candidate. Safety, reactogenicity and immunogenicity; recruitment listed as active. This is a new older-adult study, distinct from previous paediatric work. [8]

This is a selected, not exhaustive, pipeline. Nor do these summaries rank products: enrolled and planned sizes reflect different designs, and the relative maturity of clinical evidence matters more than a simple phase label.

The new IVX-A12 Phase II paper: what the data establish

The 3 November 2026 Journal of Infectious Diseases publication is a useful case study. It reports a randomised Phase II study in 143 adults aged ≥60 years, comparing one dose of IVX-A12 (150 µg RSV / 150 µg hMPV; 71 recipients) with licensed RSVPreF3/AS01E vaccine (72 recipients). Safety and immune responses were assessed through approximately six months. [5]

By day 29, geometric mean fold rises (GMFR) in neutralising antibody titres were:

• RSV-A: 3.4 with IVX-A12 versus 6.5 with the RSV-only comparator.

• RSV-B: 2.9 versus 5.4, respectively.

• hMPV-A and hMPV-B: 3.0 and 2.9 with IVX-A12; the RSV-only comparator did not elicit comparable rises against hMPV.

Solicited adverse reactions occurred in 35.2% of IVX-A12 recipients versus 50.0% of comparator recipients; interpretation of the relatively small study must remain appropriately cautious. [5]

These results support a measurable immune response to both target pathogens after IVX-A12. They do not demonstrate that the vaccine prevents hMPV illness or that its clinical RSV efficacy matches the licensed comparator. The trial was not designed to answer either question. This distinction is especially important when discussing a combination product whose two components may perform differently.

A separate Phase IIa study enrolled 264 adults (211 active vaccine, 53 placebo) and evaluated IVX-A12 with and without the MF59 adjuvant. Neutralising responses were observed against both viruses, but MF59 did not improve their magnitude or duration in that study. [9] This is a reminder that adjuvant selection is an empirical, candidate-specific decision; see our related Insight, Same Antigen, Different Adjuvant.

Development context: An IVX-A12 Phase III efficacy trial is registered in the EU Clinical Trials Information System (EUCT 2024-513740-28-00) to assess protection against laboratory-confirmed RSV and hMPV lower respiratory tract disease in adults aged 60 years and older. Trial registration does not establish vaccine efficacy. [10]

Why combination efficacy is more complicated than one antibody panel

1. Each pathogen needs an interpretable outcome. A combined RSV/hMPV respiratory endpoint might increase the number of events, but it could obscure weak efficacy against one component. Virologically confirmed RSV and hMPV disease outcomes must be assessed separately if separate protection claims are sought. Sanofi's Phase IIb is particularly informative because it includes virus-specific clinical endpoints and a 200-participant safety lead-in before the main study cohort. [3]

2. Seasonality influences statistical power. RSV and hMPV circulation patterns overlap but are not identical. Endpoint incidence, geographical diversity, surveillance sensitivity and follow-up windows matter when planning an event-driven trial. An 8,000-participant target is meaningful only in relation to expected pathogen-specific events, not as a stand-alone measure of certainty. [3,11]

3. The target population changes the biology. Age, prior infection, baseline neutralisation and prior RSV vaccination can shape observable vaccine responses. Immunogenicity in relatively healthy trial volunteers cannot be assumed to predict protection against lower respiratory tract disease in frail older adults; it is even less transferable to infant populations.

4. The right question is a development decision. Which formulation should move forward? Is there evidence of interference between RSV and hMPV responses? How durable is immunity? Which clinical endpoint, viral assay and case definition will resolve a go/no-go decision? A programme may be advanced in clinical phase yet still carry a substantial pathogen-specific efficacy uncertainty.

Where human challenge studies can add value

Well-characterised controlled human infection models can complement conventional early clinical studies by providing a defined exposure, dense virological sampling and early clinical readouts. Depending on the candidate, they may help prioritise dose or schedule, explore correlates of protection, or test a mechanistic hypothesis before a large seasonal efficacy programme.

Figure 2. Controlled human infection studies combine defined exposure with repeated virological, clinical and immunological measurements to inform early development decisions. These findings do not establish field efficacy.

The potential contribution of human challenge studies is supported by direct experience in respiratory challenge model development. At ESCMID Global 2025, Nicolas Noulin presented the characterisation of an hMPV human challenge model, developed through collaborative work involving Andrew Catchpole (hVIVO), Guy Boivin (Université Laval), the hVIVO team, and collaborators including Naobios and SGS. [12] The presentation included Alexander J. Mann and other coauthors. These investigations illustrate how controlled infection studies can characterise viral replication, symptomatic disease and immune responses, helping inform endpoint selection and early proof-of-concept strategies.

The limits are equally important. Experimental infection in selected healthy volunteers does not recreate naturally acquired severe lower respiratory tract disease in an older, medically complex population. Challenge data may sharpen a decision; they cannot replace a field study needed to establish real-world clinical protection.

From evidence to a development decision

The field is moving, but the key bridge remains unbuilt for hMPV: an antibody response has to translate into a defensible clinical protection claim. For a combination product, that argument must be convincing for both viruses.

What decision is your programme facing?

• Dose or formulation: What evidence is sufficient to select a combination candidate?

• Clinical endpoints: How can RSV- and hMPV-specific disease outcomes be defined and powered?

• Early proof of concept: Would a human challenge study address a genuine uncertainty before field efficacy?

Explore our clinical science services or discuss a development question with Noulin BioConsulting.

Evidence note (4 November 2026): Study statuses and planned enrolment may change. The Clover results are preliminary sponsor communications, not a peer-reviewed efficacy study. No RSV/hMPV combination candidate in this analysis has provided evidence here of clinically established efficacy against both pathogens.

References

1. World Health Organization. Respiratory syncytial virus immunisation products. Accessed 4 November 2026.

2. CDC. About Human Metapneumovirus. Accessed 4 November 2026.

3. Sanofi. VCA25397: Phase IIb study of a bivalent RSV+hMPV protein vaccine, NCT07810010; EU CTIS 2026-526177-42-00.

4. GSK Study Register. Study 223960 / NCT07628049, Phase I/II hMPV and RSV/hMPV formulations; ClinicalTrials.gov.

5. Oyedele T, et al. A Phase 2 Trial of an RSV/hMPV Combination VLP Vaccine IVX-A12. J Infect Dis. Published 3 November 2026. DOI: 10.1093/infdis/jiag531; NCT06481579.

6. Clover Biopharmaceuticals. Preliminary Phase II SCB-1022/SCB-1033 results, 6 September 2026. Sponsor release; not peer-reviewed.

7. ClinicalTrials.gov NCT06984094; Australian New Zealand Clinical Trials Registry mirror. The registered Phase I/II study covers 612 participants across several parts; the 420-person Phase II cohort is a subset.

8. Moderna. mRNA-1365 (Profile 2), adult Phase I, NCT07764237; Moderna trial locator.

9. Davis M, et al. A Phase IIa randomized trial of RSV/hMPV IVX-A12 ± MF59. Vaccine. 2026;77:128345. PMID: 41785514. DOI: 10.1016/j.vaccine.2026.128345.

10. European Union CTIS. IVX-A12 Phase III study 2024-513740-28-00.

11. CDC MMWR. Human metapneumovirus seasonality and co-circulation with RSV, United States, 2014–2024. 2025.

12. Noulin N, Mann AJ, Veselinski N, et al. Establishing a human challenge model for human metapneumovirus: disease characteristics and immune responses. Oral presentation O0470, ESCMID Global 2025, Vienna, Austria, 11–15 April 2025.