This is the first installment of a two-part series. The follow-up article was published on August 24, 2023.

An era of renewed hope dawns for infectious disease prevention as multiple vaccines and monoclonal antibody drugs targeting respiratory syncytial virus (RSV) are finally coming to fruition after decades of unsuccessful development. The pediatric population is set to benefit from a long-acting monoclonal antibody drug and passive immunization via vaccination of pregnant individuals (maternal immunization). Meanwhile, two active immunizations have been approved for adults ages 60 and above. With all this innovation becoming commercially available, projections place the RSV vaccine market north of $10 billion by 2030 (1).

The timing is apt, as the RSV season in the U.S. is fast approaching. RSV usually begins circulating in October, peaking in December or January. However, the COVID-19 pandemic disrupted this seasonality, leading to the onset of RSV activity over the summer and peaking in the fall over the past two years. Whether or not the season will kick off early this year remains uncertain, prompting the Centers for Disease Control and Prevention (CDC) to advise healthcare providers to offer the newly approved preventative measures as soon as supply permits (2). 

2022 RSV positivity rates from Ovation’s real-world lab testing data. Due to the COVID-19 pandemic, RSV season peaked earlier than usual last year.

In this article (Part 1), we provide an overview of the RSV prevention landscape, highlighting the unmet needs in RSV prevention, the newly approved RSV vaccines that fill these gaps, and other RSV prevention and treatment candidates currently under clinical investigation. In a follow-up piece (Part 2), we’ll explore how real-world testing and virome sequence data can accelerate the development of RSV vaccines and drugs from discovery through post-approval monitoring and research.

The Pressing Need for Broader RSV Prevention

RSV is a common virus that usually causes mild respiratory symptoms but can trigger life-threatening illnesses such as bronchiolitis and pneumonia in high-risk populations. These include children under five, older adults, adults with underlying chronic heart or lung disease, immunocompromised adults, and those residing in a nursing home or long-term care facility. In the U.S. alone, RSV is estimated to result in around 58,000 to 80,000 hospitalizations and 100 to 300 deaths among children younger than five annually, along with 60,000 to 160,000 hospitalizations and 6,000 to 10,000 deaths in adults 65 years and older (3). RSV is difficult to avoid owing to its highly contagious nature and transmission via nasal secretions that can remain stable for several hours on hard surfaces and hands (4).

The importance of RSV on the health of infants and young children cannot be overstated. It stands as the leading cause of hospitalization in infants in the U.S. (5), with about 1-2% of infants requiring hospitalization, while an additional 20% require treatment in pediatric offices or emergency rooms for acute respiratory symptoms (6). There is also growing evidence to support a strong association between RSV respiratory illness and impaired lung function later in life. Research suggests that children with a history of RSV-associated lower respiratory tract infection (RSV-LRTI) have an estimated two- to 12-fold higher risk of developing pediatric asthma (7).

RSV stands as the leading cause of infant hospitalization in the U.S. (5)

Until recently, the only approved vaccine or drug for RSV prevention was palivizumab (Synagis), a monoclonal antibody recommended by the Academy of Pediatrics (AAP) for administration in high-risk infants and young children (those born prematurely or with certain underlying medical conditions such as chronic lung or heart conditions) (8). While Synagis has effectively prevented RSV-related hospitalizations for high-risk infants and young children (9), its utility remains limited. Around 75% of infants hospitalized for RSV in the U.S. are healthy (6, 10-11), and currently, there is no way to predict which healthy infants will develop severe RSV disease (7). Additionally, Synagis is not indicated for use in adults, even those at higher risk for severe RSV illness. 

To meet this glaring need for safe and effective RSV prevention, several vaccines and one monoclonal antibody drug have recently received the green light from the U.S. Food and Drug Administration (FDA).

Recent FDA Approvals Shaping the Future of RSV Prevention

Adult Populations

In May 2023, both GSK’s RSVPreF3-AS01 (Arexy) and Pfizer’s RSVpreF (Abrysvo) secured FDA approval for use in adults over 60, based on interim results from ongoing Phase 3 trials (12, 13). Both the CDC’s vaccine advisory group and the Advisory Committee on Immunization Practices (ACIP) recommended the new vaccines for people aged 60-64 and those aged 65 and older (2). On June 28, 2023, CDC director Rochelle Walensky, MD, MPH endorsed the ACIP recommendation (14). 

Both Arexy and Abrysvo demonstrated a significant reduction in the risk of developing RSV-associated lower respiratory tract disease (RSV-LRTD), with Arexy reducing the risk of RSV-LRTD by 82.6% and severe RSV-LRTD by 94.1% (12), and Abrysvo demonstrating 65.1% efficacy against RSV-LRTD and 88.9% efficacy against severe RSV-LRTD (13).

While both vaccines were generally safe and well tolerated, the FDA’s advisory panel raised questions about the safety, durability, and co-administration of these new vaccines with other vaccines. In particular, they noted a slightly higher incidence of atrial fibrillation post-immunization and instances of Guillain-Barré syndrome (GBS). Concerns were also voiced about Pfizer’s limited study population, which excluded immunocompromised adults and included a smaller number of participants over 80 years old.

In response, both GSK and Pfizer will carry out pharmacovigilance studies on real-world data. Pfizer will also conduct a post-marketing follow-up trial to investigate the risk of GBS. GSK presented data from a study showing that co-administration of their RSV vaccine with a flu vaccine did not incur any efficacy trade-offs; two further studies are ongoing at GSK, and Pfizer is expected to provide data from a similar study soon (15).

Both Pfizer and GSK are actively conducting early-stage clinical trials with the intention of broadening the population who may benefit from these vaccines. Pfizer has unveiled plans to initiate multiple clinical trials evaluating Abrysvo’s effectiveness in various populations, including healthy children ages 2 to 5; children aged 5 to 18 with underlying medical conditions; adults ages 18 to 60 with underlying medical conditions; and immunocompromised adults ages 18 and over (16). Meanwhile, GSK is advancing with a fully recruited phase 3 clinical trial that aims to prevent severe RSV illness in adults aged 50 to 59, including those with underlying comorbidities (17).

A key competitor, Moderna, is also making advancements with its mRNA candidate (mRNA-1345) for preventing RSV-LRTD in adults aged 60 or above. The candidate is expected to head to U.S. regulators soon, contributing to an increasingly competitive landscape (15). Moderna has presented promising data from an interim analysis of the pivotal ConquerRSV study of approximately 37,000 adults 60 years or older. The vaccine was well tolerated, and the trial met both of its primary endpoints with an 83.7% efficacy against RSV-LRTD and an 82.4% efficacy against severe RSV-LRTD (18). Moderna leveraged Ovation’s RSV Testing Data during this trial to pinpoint areas of active RSV infection and develop a strategic clinical trial enrollment plan.

Denmark-based Bavarian Nordic was working on another competing preventative RSV vaccine (MVA-BN RSV) for adults aged 60 and older; however, their efficacy data failed to meet all the primary endpoints of preventing RSV-LRTD (19). MVA-BN RSV vaccine was differentiated from GSK and Pfizer’s vaccines in that it incorporated five distinct RSV antigens to stimulate a broad immune response against both RSV subtypes (20). 

Pediatric Populations

In the pediatric domain, the FDA recently approved a monoclonal antibody drug, nirsevimab (Beyfortus), developed by AstraZeneca and being brought to market by Sanofi in the U.S. (21). Beyfortus aims to protect infants and toddlers from RSV-LRTD in their first two years of life. A maternal vaccine is also on the horizon, with an expected FDA approval in August 2023. This maternal vaccine, developed by Pfizer, aims to provide in-utero antibodies to fetuses, giving infants protection for the first six months of life (22).

Recent studies indicate a 83% reduction in hospitalizations for RSV-LRTIs and a 76% reduction in very severe LRTIs among infants treated with Beyfortus compared to those who did not receive the treatment (23). In comparison, Synagis, the current standard for RSV immunization, demonstrated only a 55% decrease in hospitalizations (24). Moreover, the adverse events between Beyfortus and placebo were comparable, with most events being mild to moderate in severity (23). Given its impressive efficacy, single-dose administration (in contrast to Synagis, which requires five injections per month during peak RSV season), and cost-effectiveness, experts predict healthcare providers will replace Synagis with Beyfortus as the standard RSV immunization (25).

Turning the focus to maternal vaccination, Pfizer’s vaccine Abrysvo, already FDA-approved for adults over 60, has emerged as a prime candidate. During the third trimester of pregnancy, immunization with Abrysvo prompts the production of antibodies that are subsequently passed to the fetus via the placenta. Consequently, infants are born with RSV antibodies already in their system. A phase 3 trial’s interim analysis reveals promising results: the vaccine exhibited 82% effectiveness at preventing severe illness in newborns for the first three months after birth and a 69% effectiveness rate at preventing severe illness in infants at six months post-birth. In May 2023, the FDA’s Advisory Committee voted in support of approval for this indication (22), and the European Medicines Agency (EMA) echoed this sentiment with a recommendation for approval on July 21, 2023 (26). The FDA, which is not obligated to follow the advisory panel’s recommendations but usually does, has set a Prescription Drug User Fee Act (PDUFA) action date for next month (22).

Despite unanimous approval regarding the efficacy of the maternal vaccine indication of Abrysvo, the safety vote was a more divided 10-4. Premature births were recorded at 5.6% in the vaccine group and 4.7% in the placebo group. While panelists acknowledged this difference was not statistically significant, they underscored the importance of post-marketing surveillance to monitor potential safety signals (15).

Updated: July 27, 2023

RSV Prevention: The Role of F Protein

The new vaccines race their origins back to a groundbreaking discovery in 2013 (27) at the National Institutes of Health (NIH). This pivotal research uncovered how the mechanism of the fusion protein, or F protein, and how to lock its crucial site into place. This novel understanding allowed life sciences companies to accelerate their RSV drug research, yielding more effective results. Pfizer and GSK remarkably shortened the path from first-in-human studies to approval to less than five years (15), substantially reducing the average clinical development timeline for new vaccines, which typically spans ten years (28). 

Barney Graham, an immunologist and virologist at Morehouse School of Medicine and a vital member of the NIH team responsible for this breakthrough explained, “It was the basic research that led to a lot of this [current success]. It’s an example of how one little insight can open up a whole new set of possibilities” (15). This remarkable stride in our understanding of the F protein has indeed opened up a new era in preventing and treating RSV.

Other Promising New Frontiers in RSV Prevention and Treatment

Considering the broader landscape, several exciting early-stage research initiatives are underway to explore the potential of RSV vaccines and therapeutics.

Currently in phase 1 trials, Moderna is investigating mRNA-1345 in children under the age of five who have been exposed to RSV and those who have not (15). Furthermore, Moderna aims to develop a combination vaccine to tackle RSV and other prevalent viral infections such as influenza, COVID-19, and human metapneumovirus (hMPV) (29). Pfizer is also working towards a combination vaccine (PF-07941314) targeting RSV and influenza for adults, which is in a phase 1 clinical trial (30).

Intranasal live attenuated virus (LAV) vaccine candidates are in early-stage development at Sanofi and Meissa Vaccines. Sanofi’s SP0125 is being developed for use in toddlers (31), whereas Meissa’s MV-012-968 is intended for infants (32). Intranasal vaccines, which are non-invasive as they don’t require injections, are designed to produce both mucosal antibodies in the nasal cavity and circulating antibodies in the bloodstream. Their aim is to both prevent serious pulmonary diseases and obstruct viral infection and transmission (33).

Meanwhile, Merck & Co. Inc. is conducting a phase 3 clinical trial of its experimental monoclonal antibody drug, clesrovimab. This drug is designed to prevent RSV infections in infants and children at increased risk of developing severe RSV disease (34). If the FDA grants approval, this drug could pose significant competition for Beyfortus in the market.

RSV Antiviral Treatment Contenders

The prompt and effective treatment of RSV infection, particularly before it escalates to severe disease, is critically important, especially in high-risk populations. The only authorized antiviral treatment for RSV is inhaled ribavirin (35). As a broad-spectrum antiviral drug, ribavirin impedes RSV replication, and clinical benefits have been observed in children treated early in the infection (36). However, the administration of ribavirin is generally not recommended for the majority of hospitalized infants due to its nonspecific anti-RSV activity, potential toxicity risks, and relatively high cost (37). Its use is also not generally advised in adults except for severely immunocompromised persons (38). Therefore, developing safe and effective antiviral drugs for RSV treatment remains an unfulfilled medical need.

Pfizer anticipates that the market for these drugs could potentially exceed $1.5 billion annually. They have acquired sisunatovir, an oral inhibitor designed to block the fusion of the RSV virus to the host cell. A human challenge study in healthy adults revealed that sisunatovir significantly reduced viral load, and it is currently in phase 2 clinical development in infants and immunocompromised adults. Pfizer also has an N-protein inhibitor (PF-07923567 / RV-299) in phase 1 clinical development (39). The company’s two-tiered Abrysvo and sisunatovir approach mirrors their COVID-19 strategy with BNT162b2 vaccine and Paxlovid. In August 2023, sisunatovir was awarded Fast Track designation by the FDA (40).

Enanta Pharmaceuticals is also making strides in this domain, developing an orally administered N-protein inhibitor (EDP-938), currently under investigation in several phase 2 studies. These studies are exploring its use in hospitalized and non-hospitalized children and immunocompromised adults. Additionally, Entada Pharmaceuticals is developing EDP-323, an oral antiviral that selectively targets the RSV L-protein, an essential element for RSV replication. Both EDP-938 and EDP-323 have received Fast Track designation from the FDA (41).

Updated: July 27, 2023

A Groundbreaking Moment and the Road Ahead

The recent RSV prevention approvals are a great example of how scientific progress is possible, even in areas where development has been challenging and approvals largely stagnant for decades. While there are substantial hurdles ahead for life sciences organizations to commercialize and deliver the new shots (and for government bodies, medical professionals, and payers to ensure that these shots get administered), it’s important to celebrate infectious disease prevention researchers and the remarkable impact their work will have on the health of infants, young children, and older adults worldwide. 

Stay tuned for the second part of this series, where we’ll explore how real-world testing and virome sequence data can accelerate the development of RSV vaccines and drugs from discovery through post-approval monitoring and research.

Until then, check out the following resources for more information on Ovation’s work in RSV:

• RSV Testing Data available on AWS Data Exchange
• Case Study: Real-world RSV testing data supports clinical trial enrollment strategies for the ConquerRSV vaccine trial
• About Ovation Testing Data (PDF) 

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