Ziresovir's PICU Pivot: Phase III China Signal Meets Uncharted Severity Tier in US Trial

FDA Greenlights ArkBio's Ziresovir for Phase II RSV Trial

Shanghai Ark Biopharmaceutical (ArkBio) has received US FDA approval to initiate a Phase II clinical trial for its antiviral drug ziresovir (AK0529) in severely ill infants with respiratory syncytial virus (RSV). This randomised, multi-centre, placebo-controlled, double-blind study will focus on hospitalised babies in the paediatric intensive care unit (PICU). Ziresovir previously completed a Phase III trial in China, demonstrating positive results by reducing viral load, respiratory symptom duration, and the risk of recurrent wheezing and asthma over a 24-month follow-up. The World Health Organization also included ziresovir in its 2025 Paediatric Drug Optimization for RSV report.

• The US FDA has approved ArkBio to commence a Phase II clinical trial for ziresovir (AK0529) in severely ill infants with RSV. This randomised, multi-centre, placebo-controlled, double-blind study will specifically target hospitalised babies in the paediatric intensive care unit (PICU), aiming to assess clinical effectiveness by measuring the duration of oxygen therapy and sustained recovery time.
• Ziresovir previously completed a Phase III clinical trial involving children aged one to 24 months in China. This study yielded positive results, showing that the drug effectively reduced viral load, shortened the duration of respiratory symptoms, and lowered the risk of recurrent wheezing and asthma over a 24-month follow-up period.
• Ziresovir has garnered international recognition, being included by the World Health Organization (WHO) in its 2025 Paediatric Drug Optimization for Respiratory Syncytial Virus report. This highlights the drug's potential as a significant therapeutic option for RSV, further supporting its global development efforts.

Addressing the Urgent Unmet Need in Severely Ill Infants with RSV

Recent advances in RSV prophylaxis and vaccination have meaningfully expanded the protected population, yet critical gaps persist — particularly for the most vulnerable infants and under-resourced settings. The field has undergone a paradigm shift from risk-stratified intervention to universal infant protection, while simultaneously recognizing that severe disease burden, therapeutic limitations, and access inequities remain largely unaddressed.

• Universal infant immunization as a new standard: Nirsevimab is now recommended for all infants <12 months, representing a significant expansion beyond the previous high-risk-only framework that historically targeted premature infants (gestational age <33 weeks, or <35 weeks with chronic lung disease) and those with hemodynamically significant congenital heart disease (CHD) or chronic lung disease of prematurity (CLDP).

• Maternal vaccination for passive neonatal protection: Pregnant women represent a key intervention target through maternal immunization with Abrysvo (approved August 2023), enabling transplacental passive immunity transfer to protect newborns in the immediate post-birth window before active prophylaxis can be administered.

• Older adults with cardiopulmonary comorbidities: Adults ≥60 years are a priority population, addressed by two newly approved vaccines — RSVpreF3 (Arexvy) and RSVpreF (Abrysvo) — both authorized in May 2023 as single-dose intramuscular injections, with heightened focus on those carrying underlying cardiopulmonary conditions.

• Therapeutic void in adult RSV infection: Effective treatment options for established RSV infection in adults remain critically limited; ribavirin is the only antiviral currently recommended, and no monoclonal antibody has received regulatory authorization for prophylaxis or therapy in this population.

• Long-term respiratory sequelae post-bronchiolitis: Prevention of post-RSV recurrent wheezing and asthma development remains an unmet clinical need, as early RSV infection can trigger persistent airway inflammation, immune dysregulation, and expiratory airway obstruction that persists beyond clinical recovery.

• Equity gaps in low- and middle-income countries (LMICs): RSV exerts its greatest morbidity and mortality burden in LMICs, where malnutrition-disease interactions compound susceptibility and longer-term sequelae. Ensuring timely, equitable distribution of prophylactic agents — including for First Nations populations at elevated risk — remains a substantial unresolved challenge.

• Children with complex congenital and genetic conditions: Children with genetic syndromes and structural lung or airway abnormalities associated with CHD represent a high-risk subgroup requiring targeted clinical attention, as they face disproportionate risk of RSV-related hospitalization beyond what is captured by standard prematurity-based risk stratification.

Ziresovir's Positive Phase III Results Pave Way for US Trial

The CYPRESS Trial (NCT03982199) evaluated an Ad26.RSV.preF-RSV preF protein vaccine for RSV prevention. In terms of efficacy, vaccine efficacy reached 80.0% (94.2% CI, 52.2–92.9), 75.0% (94.2% CI, 50.1–88.5), and 69.8% (94.2% CI, 43.7–84.7) across three progressively broader case definitions, respectively. RSV A2 neutralizing antibody titers increased by a factor of 12.1 from baseline to Day 15, demonstrating a robust immunogenic response. On the safety front, solicited local and systemic adverse events were more frequent in the vaccine group than in the placebo group (local: 37.9% vs. 8.4%; systemic: 41.4% vs. 16.4%), though the majority were mild to moderate in severity. Notably, serious adverse event rates were comparable between groups (4.6% vaccine vs. 4.7% placebo), supporting an acceptable safety profile.

A separate Phase III study (NCT05842967) investigated a bivalent RSV prefusion F protein-based (RSVpreF) 120-µg vaccine in immunocompromised and renally impaired adults — populations historically underrepresented in RSV vaccine trials. Immunogenicity data showed high geometric mean fold rises in neutralizing antibodies against both RSV A and RSV B one month after the first dose (8.3 and 9.0, respectively), with no meaningful additional increase observed following a second dose (7.5 and 7.8), suggesting that a single dose may be sufficient to confer robust immune responses in these vulnerable populations. From a safety perspective, injection-site pain was the most common local reaction and was more frequent after Dose 2 than Dose 1, while fatigue was the predominant systemic event reported after any dose. Most reactions were mild to moderate. Adverse event rates varied by age cohort: 13.5% (AEs) and 7.3% (SAEs) among adults aged 18–<60 years, compared with 22.4% and 14.0%, respectively, among those aged ≥60 years.

Key Design and Endpoints of the US Ziresovir Phase II Study

The pivotal RSV clinical trial program spans maternal immunization, older adult vaccination, passive immunoprophylaxis, and real-world effectiveness assessments, collectively informing the regulatory and clinical positioning of RSVpreF and nirsevimab. Each study employed distinct randomization schemes, population definitions, and endpoint hierarchies tailored to the target indication and risk stratum.

Ziresovir's Role in the Evolving RSV Treatment Landscape

The RSV treatment landscape has undergone a profound transformation over the past five years, driven primarily by advances in structural vaccinology — most notably the stabilization of the RSV-F glycoprotein in its prefusion (preF) conformation, which represents the principal target of RSV-neutralizing antibodies. This breakthrough unlocked efficient vaccine design after decades of development setbacks rooted in safety concerns, the absence of an absolute correlate of protection, and challenges in antigen-adjuvant optimization. By May 2023, the FDA had approved two protein subunit vaccines — GSK's Arexvy (RSVPreF3) and Pfizer's Abrysvo (RSVpreF) — for the prevention of RSV-related lower respiratory tract disease (LRTD) in adults aged 60 years and older. A third option, Moderna's mRNA-based mRESVIA (mRNA-1345), received FDA approval in May 2024 for the same indication, establishing mRNA technology as a viable platform in the RSV space. In June 2024, the CDC updated its guidance to recommend a single RSV vaccine dose for all adults aged 75 and older, and for those aged 60–74 with risk factors for severe disease — a shift that reflects the maturing real-world evidence base supporting these interventions.

Beyond older adults, the landscape has expanded meaningfully into maternal immunization and infant prophylaxis. Abrysvo is the only RSV vaccine currently indicated for use during pregnancy, with the MATISSE trial demonstrating vaccine efficacy of 82.4% (95% CI: 57.5–93.9) against severe RSV-associated medically attended lower respiratory tract illness within 90 days of birth, declining modestly to 70.0% (95% CI: 50.6–82.5) through 180 days. The vaccine elicited robust immune responses in pregnant participants and facilitated highly efficient transplacental antibody transfer to newborns. On the monoclonal antibody front, nirsevimab (Beyfortus) — an extended half-life anti-RSV monoclonal antibody — has received both EU registration and FDA licensure for infant RSV prevention. Clesrovimab, another half-life-extended neutralizing monoclonal antibody, is currently under late-stage evaluation in the CLEVER and SMART trials for the same indication.

Real-world post-licensure data have broadly validated the clinical trial findings. A pooled analysis covering more than 121.8 million individuals demonstrated vaccine effectiveness estimates of 75.3% (95% CI: 73.7–76.9) against any laboratory-confirmed RSV infection, 74.8% (95% CI: 66.8–82.9) against RSV-related hospitalizations, and 79.8% (95% CI: 68.1–91.5) against severe RSV-associated disease in adults aged 60 and older. However, pharmacovigilance data have introduced an important safety signal: both Arexvy and Abrysvo have been associated with an elevated risk of Guillain-Barré syndrome, reported at 5.2–6.5 cases per million doses for Arexvy and 9.0–18.2 cases per million doses for Abrysvo, prompting the FDA to mandate updated prescribing information warnings for both products. These developments collectively define an RSV treatment environment that is rapidly maturing but still navigating the balance between broad protective benefit and emerging risk considerations.

A New Hope for Severely Ill Infants with RSV

Respiratory syncytial virus (RSV) remains a formidable challenge in paediatric healthcare, responsible for significant hospitalizations, severe illness, and even mortality in infants globally. Beyond the acute phase, early-life RSV infection is a known precursor to long-term respiratory issues, including recurrent wheezing and asthma, placing a substantial burden on families and healthcare systems for years. While recent advancements have brought new preventive options to market, a critical gap has persisted: the lack of an effective antiviral treatment for infants already suffering from established RSV infection.

The US FDA's approval for a Phase II trial of ziresovir (AK0529) in severely ill infants hospitalized in the paediatric intensive care unit (PICU) represents a significant step towards filling this void. Ziresovir, an oral RSV fusion protein inhibitor, has already demonstrated promising results in earlier trials, showing reductions in viral load and improvements in clinical symptoms. Crucially, a Phase III trial in China also indicated a reduced risk of recurrent wheezing and asthma over a 24-month follow-up, suggesting a potential to mitigate the long-term sequelae of severe RSV.

This development positions ziresovir to carve out a distinct therapeutic niche, offering a direct treatment for acute infection where only prevention currently exists. If successful, it could significantly alter the management paradigm for severe RSV, potentially reducing the need for intensive care and alleviating the long-term respiratory burden. However, several considerations warrant attention. The emergence of resistance-associated mutations in a subset of patients in previous trials highlights the need for ongoing surveillance. Furthermore, while generally well-tolerated, the safety profile in the highly vulnerable PICU population will be paramount. Finally, cost-effectiveness analyses suggest that pricing and market access strategies will be sensitive to the drug's efficacy in specific high-risk groups, requiring careful consideration to ensure broad patient access and sustainable adoption. The journey ahead for ziresovir will be closely watched by clinical and strategic pharma teams, as it holds the promise of transforming outcomes for the most vulnerable infants battling RSV.