Safety and Immunogenicity of an mRNA-Based Rsv Vaccine and an Rsv/hMPV Combination Vaccine in Children 5 to 23 Months of Age

Matthew D. Snape; Jennifer Moore; Jiejun Du; Caroline Reuter; Necois Penn; Sonia K. Stoszek; Christine A. Shaw

doi:10.20944/preprints202412.0878.v1

Submitted:

10 December 2024

Posted:

11 December 2024

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Abstract

Background: Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) are common respiratory pathogens that cause severe illness in young children. Methods: As part of an age-de-escalation clinical development program, a phase 1 trial of mRNA-based RSV (mRNA-1345) and investigational RSV/hMPV combination (mRNA-1365) vaccines was conducted in infants and children. Participants were randomized in equal numbers to mRNA-1345 (encoding RSV preF), mRNA-1365 (encoding RSV PreF and hMPV F), or placebo, in an observer-blind study. In a stepwise fashion, children 8 to 23 months of age (Part A) received a 3-injection series of mRNA-1345 (30 μg), mRNA-1365 (30 μg), or placebo. After Data Safety Monitoring Board (DSMB) review of these data, infants 5 to 7 months of age (Part B) were similarly randomized to a 3-injection series using a dose escalation approach (starting at 15 μg). The primary study objective was to assess safety/reactogenicity; secondary objectives were evaluation of clinical RSV/hMPV infections and measurement of antibody/cell-mediated immune responses. Results: In children 8 to 23 months of age (Part A), both mRNA-1345 and mRNA-1365 were well-tolerated and induced robust RSV-A and -B neutralizing antibodies (nAbs) and preF-biased binding antibodies (bAbs). RSV-specific cellular responses assessed in a participant subset demonstrated higher type 1 T helper (Th1) than type 2 T helper (Th2) responses. No concerns were identified following active surveillance for respiratory disease through a full RSV season, and age de-escalation progressed to Part B (infants 5 to 7 months of age). Most participants (88.3%) in Part B met the criteria for “RSV-naïve,” and vaccination induced robust nAb and preF-biased bAb responses. In Part B, a protocol-defined study pause was triggered by 2 cases of severe RSV-lower respiratory tract illness (LRTI) in the 15 μg vaccine level recipients, all of whom had received 2 vaccine injections; as of October 2024 (the first RSV season post immunization), RSV-LRTI classified as severe/hospitalized was seen in 2/20, 3/20, and 1/20 of mRNA-1345 15 µg, mRNA-1365 15 µg, and placebo recipients, respectively. The respiratory illnesses in these children have resolved. Conclusions: Vaccination with mRNA-1345 (RSV vaccine) or mRNA-1365 (RSV/hMPV vaccine) increased RSV nAbs in children aged 8 to 23 months, and induced de novo nAb responses in RSV-naïve infants 5 to 7 months old. Vaccination induced a preF-biased bAb response in both age groups and induced RSV-specific Th1 responses in older children. Study dosing and enrollment are paused while immunogenicity assessments and surveillance for respiratory infections are ongoing to better understand the increase in severe/hospitalized RSV LRTI observed among vaccinated, RSV-naïve, young children. Trial registration number: NCT05743881

Keywords:

RSV

;

respiratory syncytial virus

;

hMPV

;

human metapneumovirus

;

children

;

vaccine

;

clinical trial

Subject:

Medicine and Pharmacology - Epidemiology and Infectious Diseases

Trial registration number: NCT05743881

Background

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) are common respiratory pathogens of the Pneumoviridae family that have similar seasonality and overlapping clinical and epidemiologic characteristics, representing significant public health threats to young children [1,2,3,4]. In pediatric populations, RSV is the most common etiological pathogen associated with lower respiratory tract illness (LRTI), followed by hMPV [5]. Each year, RSV- and hMPV-associated LRTI are estimated to lead to a respective 3.6 million and 643,000 hospitalizations and 101,400 and 16,100 deaths in young children [1,2]. Furthermore, LRTI during infancy and early childhood is associated with an increased risk of chronic respiratory disorders, such as asthma, later in life [6,7,8].

There are currently no active immunizations approved for RSV or hMPV in infants or children [9,10,11]. Maternal immunization with the RSVpreF vaccine (Abrysvo; Pfizer, Inc., New York, NY) is recommended for pregnant women in the United States and elsewhere to prevent severe RSV-associated LRTI in their infants younger than 6 months [12,13,14]. Monoclonal antibodies, nirsevimab and palivizumab, are also available as prophylaxis for RSV-LRTI; in the United States, nirsevimab is recommended for infants younger than 8 months born during or entering their first RSV season, and high-risk children aged 8 to 19 months entering their second RSV season [15]. For hMPV, no maternal vaccine or infant prophylaxis are available, although studies have suggested that passively acquired maternal hMPV antibodies may provide transient protection to infants [16,17,18].

Most children are seropositive for RSV by 2 years of age, and for hMPV by 5 years of age, reflecting the ubiquity of these childhood infections [18,19]. Stabilization of the RSV F glycoprotein in the prefusion (preF) conformation has advanced the development of RSV vaccines, given that this conformation is the primary target of potent RSV neutralizing antibodies (nAb)s and is highly conserved across the 2 RSV subtypes (A and B) [20,21,22]. An mRNA-based vaccine encoding membrane-anchored RSV preF (mRNA-1345; mRESVIA; Moderna, Inc., Cambridge, MA) is licensed for the prevention of RSV-lower respiratory tract disease (LRTD) in adults aged ≥60 years [23], confirming the efficacy of this vaccine strategy. In the pivotal phase 3 study in adults at least 60 years of age, mRNA-1345 demonstrated 78.7% and 80.9% vaccine efficacy (VE) against RSV-LRTD with ≥2 and ≥3 signs or symptoms, respectively, through a median follow-up of 3.7 months, with a favorable benefit:risk profile [23,24]. Efficacy was accompanied by an increase in nAb responses in mRNA-1345 recipients, increasing by 8.4-fold (RSV-A) and 5.1-fold (RSV-B) at Day 29 relatively to baseline [25]. The vaccine also induces durable responses, with VE evident through a median of 18.8 months of extended follow-up [26]; nAb responses also remained elevated relative to baseline to 12 months in a phase 1 trial in adults [27]. Additional studies of adults in a phase 1 study of mRNA-1345 demonstrated that vaccination induces RSV-specific T-cell responses (predominantly type 1 T helper (Th1) biased) that remained detectable through 3 months of follow-up [28].

All adults in the mRNA-1345 phase 3 efficacy trial with available data were seropositive for RSV at baseline [25]. mRNA COVID-19 vaccines have also proven efficacious in protecting individuals seronegative for SARS-CoV-2, from infants to adults. Two doses of mRNA-1273 (Spikevax; Moderna, Inc., Cambridge, MA) or BNT162b2 (Comirnaty; Pfizer, Inc., New York, NY) were shown to be efficacious against COVID-19 and licensed in individuals as young as 6 months [29,30,31,32,33].

Clinical development of infant RSV vaccines has proceeded in a careful and stepwise fashion (from adults to seropositive children, and then to seronegative infants) following the occurrence of enhanced respiratory disease (ERD) in a field trial of a formalin-inactivated RSV (FI-RSV) vaccine in the 1960s [34]. In this trial, excess hospitalizations from RSV were observed in previously RSV-naïve infants after receipt of the FI-RSV vaccine. This ERD phenomenon was only observed in RSV-seronegative infants [35]. The protective role of nAbs in infant RSV is clear, and the failure of the FI-RSV vaccine to induce robust nAb responses likely played a role in the ERD phenomenon [35,36]. Other potential mechanisms of the observed vaccine-associated ERD include potential imbalance in type 2 T helper (Th2) responses, infiltration of neutrophils and eosinophils into airways, a failure to elicit cytotoxic CD8+ T cells, low avidity antibodies, immune complex deposition, and complement activation [37,38]. In addition to the stepwise age de-escalation approach to clinical evaluation, thorough investigation of candidate RSV vaccines in well-characterized preclinical models is warranted [39].

After demonstrating safety and immunogenicity in adults, pediatric development of mRNA-1345 began with a phase 1 trial in RSV-seropositive children aged 1 to 4 years. mRNA-1345 was found to be well-tolerated, boosted RSV nAbs, and elicited preF-biased bAbs [40]. The safety and immunogenicity of the hMPV component of mRNA-1365, an investigational combination RSV/hMPV mRNA-based vaccine, was also previously evaluated within a mRNA-based combination hMPV/PIV3 vaccine in phase 1 studies among healthy adults 18 to 49 years [41] and baseline-seropositive children aged 1 to 4 years [42]. Together with nonclinical studies in rodent models, which demonstrated that RSV mRNA vaccines induced robust nAb and preF-biased binding (bAb) responses, elicited a Th1-biased cellular response, and provided dose-dependent protection from RSV challenge without enhanced lung pathology [43], the clinical studies in seropositive children aged 1 to 4 years allowed the progression of mRNA-1345 and mRNA-1365 to clinical studies in RSV-naïve children under 2 years, per established guidelines [44,45,46].

This report summarizes data from a multipart phase 1 trial designed to evaluate the safety and immunogenicity of mRNA-1345 and mRNA-1365 in RSV naïve and experienced children, conducted with oversight from an independent Data Safety Monitoring Board (DSMB). The study initiated with Part A, in which children aged 8 to 23 months were randomized to receive mRNA vaccines or placebo and were observed for a full RSV season. Following DSMB review of the Part A safety and immunogenicity findings, the study age de-escalated to children aged 5 to 7 months (Part B). Findings on mRNA-1345 and mRNA-1365 in children aged 5 to 23 months from Parts A and B of the study are reported herein, with a focus on safety, RSV surveillance, and RSV-specific immune responses. This report summarizes findings for the study through a data cutoff of October 15, 2024.

Methods

Trial Design and Participants

This is a multi-part phase 1 trial in participants aged 5 to 23 months (NCT05743881), with enrollment planned in Australia, Canada, Panama, Poland, South Africa, Spain, the United Kingdom, and the United States. Part A and B are randomized, observer-blind, placebo-controlled, age de-escalation and dose-escalation studies evaluating mRNA-1345 and mRNA-1365 (Figure S1); Part C is an open-label extension evaluating mRNA-1345 administered to participants with, or without, prior exposure to nirsevimab and will be reported separately. The study was designed to age de-escalate and dose-escalate in a stepwise approach gated by DSMB reviews. The trial employed active surveillance for RSV and hMPV disease (including weekly prompts for symptom surveillance during the RSV/hMPV season and visits for investigator assessment) and had conservative pause rules, including for ≥2 cases of severe RSV/hMPV-LRTI (see Supplement for additional details). Additionally, the DSMB reviewed unblinded data on at least a monthly basis after commencement of Part B, with a specific focus on cases of RSV/hMPV-LRTI.

Recruitment in both Parts A and B of the study was timed to fall before the local RSV season to allow for post-vaccination case monitoring. Part A enrolled healthy children aged 8 to 23 months; inclusion and exclusion criteria are shown in the Supplement. Participants across 2 cohorts were randomized in equal numbers via interactive response technology to receive a 3-dose series (administered on Day 1, Day 57 [Month 2], and Day 113 [Month 4]) of mRNA-1345 30 μg (cohort 1), mRNA-1365 30 μg (cohort 2), or saline placebo; 45 participants were planned for each cohort (30:15 [mRNA-1345:placebo] in cohort 1; 30:15 [mRNA-1365:placebo] in cohort 2). After vaccination, the plan was to follow participants through 2 years.

Age de-escalation to Part B of the study was dependent on DSMB recommendations following their review of immunogenicity data and safety data through a complete RSV season from Part A. In Part B, healthy participants aged 5 to 7 months across 4 cohorts were to be enrolled to evaluate escalating dose levels of mRNA-1345 and mRNA-1365 versus placebo. Participants were initially randomized to receive dose levels of 15 μg (cohorts 3 and 4), with dose-escalation to the first 30-µg injections (cohorts 5 and 6) commencing after DSMB review of safety data through 7 days after first 15-µg injections in cohorts 3 and 4. Inclusion and exclusion criteria are shown in the Supplement. Participants were randomly assigned using interactive response technology in equal numbers to receive a 3-dose series (Day 1, Day 57, and Day 113) of mRNA-1345, mRNA-1365, or saline placebo. To enrich for RSV-naïve participants, enrollment occurred before high local RSV activity. Participant follow up was planned through 2 years after vaccination.

The protocol and other relevant documents were approved by the central institutional review board (IRB), and the trial was conducted in accordance with the protocol, applicable laws and regulatory requirements, the International Council for Harmonization Good Clinical Practice guidelines, and the ethical principles derived from the Declaration of Helsinki and Council for International Organizations of Medical Sciences International Ethical Guidelines. Written informed consent for participation in the study was provided for all participants by the participants’ parent(s)/legally authorized representative(s) before the conduct of study procedures.

Trial Vaccines

mRNA-1345 is a lipid nanoparticle (LNP) formulation comprising a single mRNA sequence encoding the RSV F glycoprotein stabilized in the preF conformation [24]. mRNA-1365 is an investigational combination vaccine with an LNP formulation composed of 2 distinct mRNA sequences, 1 that encodes the membrane-anchored RSV F glycoprotein stabilized in the preF conformation, and 1 that encodes the membrane-anchored native hMPV F glycoprotein. Vaccine dose levels that were evaluated were 15 µg or 30 µg; for mRNA-1365, these doses contained half each of the RSV and hMPV encoding components. The placebo consisted of 0.9% sodium chloride (normal saline). Vaccine or placebo was administered as an intramuscular injection in the arm or leg at scheduled time points.

Study Objectives

The primary objective of this phase 1 study was to evaluate the safety and reactogenicity of mRNA-1365 and mRNA-1345, administered as a 3-injection series in healthy children aged 5 to 23 months. Secondary objectives were to evaluate the occurrence of clinical RSV or hMPV infections in study participants, and the antibody- and cellular-mediated immune responses after vaccination.

Safety and Reactogenicity Assessments

Safety assessments included solicited local and systemic adverse reactions (ARs) within 7 days after injection; unsolicited adverse events (AEs) within 28 days after injection; and serious AEs (SAEs), AEs of special interest (AESIs), medically attended AEs (MAAEs), and AEs leading to study discontinuation throughout the study. The AESIs for this study were defined as thrombocytopenia; new onset or worsening of Guillain-Barre Syndrome, acute disseminated encephalomyelitis, Bell’s palsy, or seizures; anaphylaxis; myocarditis/pericarditis; and disease caused by RSV or hMPV. The occurrence of solicited ARs were recorded daily by the participants’ parent(s)/legally authorized representative(s) via an eDiary and graded on a 1 to 4 scale based on the grading scale for pediatric participants enrolled in preventative vaccine clinical trials (Pediatric Internal Standard; Table S1).

Study staff contacted the participants’ parent(s)/legally authorized representative(s) on a regular basis to monitor for symptomatic RSV or hMPV infections (weekly during the RSV/hMPV seasons; monthly outside the seasons) and requested the immediate reporting of new/worsening symptoms or difficulty breathing or wheezing (Supplement), with in-person visits and nasal swabs taken within 5 days of respiratory symptom onset. Protocol- defined endpoints were the number and percentage of participants with respiratory tract illness (RTI), LRTI, severe LRTI, very severe LRTI, and hospitalizations associated with RSV or hMPV; these were recorded throughout the study and were classified based on World Health Organization (WHO) standardized case definitions for RSV (Table S2). In a post hoc analysis, the number of participants meeting any of the latter 3 categories (severe LRTI, very severe LRTI, and hospitalizations) were combined into a “severe/hospitalized RSV disease” category in order to optimally identify participants with clinically significant RSV-LRTI.

Immunogenicity Assessments

Blood samples for antibody-mediated immunogenicity were scheduled on Days 1, 29, 85, and 141 in Part A and on Days 1, 85, 141, and 365 in Part B. Protocol-defined endpoints included serum nAb titers against RSV (subtypes A and B) and hMPV (subtypes A and B), and bAb IgG concentrations against RSV and hMPV preF and postfusion (postF) conformations (Supplement). This article reports RSV nAb and bAb immune responses up to Day 141 (Part A) and up to Day 85 (Part B; cohorts 3 and 4 only). Blood samples for cell-mediated immunogenicity (CMI) were collected at Days 1 and 85 from a subset of participants ≥12 months of age at enrollment in Part A; because of blood volume constraints, no cell-mediated immunogenicity assessments were planned for participants <12 months). Assessments were performed for RSV- and hMPV-specific cellular responses (Supplement).

Statistical Analysis

No formal hypotheses were tested; the number of participants enrolled was considered sufficient to provide a descriptive summary of the safety and immunogenicity of mRNA-1345 and mRNA-1365. In Part A, with 30 participants receiving each vaccine (mRNA-1345, mRNA-1365, or placebo), the study had 99% probability to observe at least 1 participant with a specific solicited AR, assuming the solicited AR rate was 15% in the mRNA-1345 and mRNA-1365 groups. In Part B, if the underlying RSV severe LRTI rate was 1% per year in placebo recipients and 20% per year in mRNA vaccine recipients, approximately 40 placebo recipients and 80 mRNA vaccine recipients (pooled mRNA-1345 and mRNA-1365) would provide ≥80% power to detect the RSV severe LRTI rate difference between the placebo group and the pooled mRNA vaccine groups, at a 2-sided 5% alpha level.

In each study part, safety analyses were descriptive and were presented by vaccine group using the safety population and solicited safety population (see Supplement). Statistical details on immunogenicity are provided in the Supplement. In Part A, immunogenicity was analyzed in the per-protocol population, consisting of all randomly assigned participants who received study injection; complied with the vaccination schedule, the timing of blood collection for immunogenicity (including a baseline and at least 1 postvaccination assessment); and had no major protocol deviations that impacted the immune response. In Part B, immunogenicity was analyzed in the full analysis population, consisting of all randomized participants who received vaccination. Since RSV serostatus was unknown at the time of enrollment, a post- hoc analysis was performed to define baseline RSV-experienced versus RSV-naïve participants. RSV exposure status was defined based on postF bAb concentrations at baseline using a threshold of 1800 arbitrary units (AU)/mL for participants aged 5 to 7 months at enrollment (Part B), and 200 AU/mL for participants aged 8 to 23 months (Part A). These values were determined based on the assessment of baseline antibody concentrations in this study and other RSV clinical trials conducted by the sponsor in pediatric and adult populations, with the higher threshold in younger participants reflecting the persistence of transplacentally acquired maternal antibodies.

Results

Part A

Participants

A total of 90 participants aged 8 to 23 months in cohorts 1 and 2 were enrolled between February and June 2023, 25 of whom were from the United States and 65 from Panama. The median age was 16.5 months, 54.4% of participants were male, 25.6% were White, and 85.6% were Hispanic or Latino. These participants were randomized in equal numbers to receive a 3-injection series of mRNA-1345 (30 μg), mRNA-1365 (30 μg), or placebo; 85 (94.4%) received all 3 injections (mRNA-1345, n = 29; mRNA-1365, n = 29; placebo, n = 27) (Figure 1). Demographics and other baseline characteristics were well-matched between groups (Table S3). As of the data cutoff date (October 15, 2024), the median (range) study duration from the first injection was 516 (3-596) days. Three participants discontinued study intervention due to withdrawal of consent (mRNA-1365, n = 1; placebo, n = 2), while 1 placebo participant was withdrawn due to a physician decision related to poor compliance with study activities. One participant discontinuation of the study vaccine was reported due to an AE; this was in a 15-month-old placebo recipient with a history suggestive of seizures. The physician withdrew this participant because of an SAE of grade 2 seizure on Day 26 after the first injection, which resolved the same day and was not considered related to the study injection. At baseline, 36 of the 85 participants (42.4%) with available results had RSV postF bAb GMCs <200 AU/mL and were therefore considered RSV-naïve.

Figure 1. Participant disposition – Part A: 8 to 23 months (cohorts 1 and 2). Disposition of participants as of the October 15, 2024, data cutoff date. ^aNo participants in the mRNA-1345 30-µg group discontinued the study intervention. One participant discontinued the study due to physician decision. ^bOne participant in the mRNA-1365 30-µg group discontinued the study intervention due to withdrawal of consent by parent/guardian. Three participants discontinued the study due to lost to follow-up (n=1) or withdrawal of consent by parent/guardian (n=2). ^cFour participants in the placebo group discontinued study intervention due to withdrawal of consent by parent/guardian (n=2), physician decision (n=1), or adverse event (n=1). Four participants discontinued the study due to withdrawal of consent by parent/guardian (n=2) or physician decision (n=2). .

Safety

Solicited local ARs within 7 days after vaccination were experienced by 24.1%, 23.3%, and 9.7% of participants after first injection of mRNA-1345, mRNA-1365, and placebo, respectively. Solicited systemic ARs were experienced by 31.0%, 30.0%, and 29.0% of participants, respectively. There was no trend towards increased reactogenicity with subsequent dosing. Most solicited ARs within 7 days after any injection were grade 1 or 2 in severity, and the most common local and systemic solicited ARs were injection site pain and irritability/crying (Figure S2). No grade 3 or 4 solicited local ARs were reported; one grade 4 solicited systemic AR of fever was reported on Days 1 to 3 after the first injection of mRNA-1365; the report coincided with an AE of influenza infection in the participant.

Up to October 15, 2024, 7 participants had each experienced 1 SAE: 4 in the mRNA-1345 group, 1 in the mRNA-1365 group, and 2 in the placebo group. One of these SAEs was a hospitalization for RSV pneumonia, and is included in the RSV case surveillance section (see below). One participant in the mRNA-1345 group had an SAE of hMPV pneumonia. The remaining 5 SAEs (gastroenteritis, n=1; bronchiolitis with no pathogen identified, n=1; human rhinovirus bronchiolitis, n=1; cervical adenopathy, n=1; and seizure disorder, n=1 [described above]) were considered unrelated to injection. In addition to the seizure disorder and RSV/hMPV respiratory infections, there were 2 AESIs (mRNA-1345, n=1; placebo, n=1), both of which were febrile convulsions and were not related to study injections. No deaths were reported.

RSV Case Surveillance

Per the protocol, participants in Part A were followed through a full RSV season before age de-escalating to Part B. Through March 2024 (end of the first RSV season), no cases of severe/hospitalized RSV had occurred in participants in Part A. Respiratory surveillance is ongoing, and as of October 15, 2024, a total of 38 cases of symptomatic RSV infections were identified (Table 1). Among baseline RSV-experienced participants (n=49), symptomatic RSV infections were reported in 5 of 13 (38.5%), 7 of 19 (36.8%), and 6 of 17 participants (35.3%) in the mRNA-1345, mRNA-1365, and placebo groups, respectively. Among baseline RSV-naïve participants (n=36), symptomatic RSV infections were reported in 6 of 14 (42.9%), 6 of 9 (66.7%), and 8 of 13 participants (61.5%) in the mRNA-1345, mRNA-1365, and placebo groups, respectively. No symptomatic RSV infections occurred in the 5 participants with unknown RSV-naïve/-experienced status. As of the data cutoff, 1 baseline RSV-naïve participant was hospitalized with respiratory symptoms. This 24-month-old participant received mRNA-1365 and had a positive test for RSV/rhinovirus/enterovirus co-infection during their second season of RSV surveillance (in August 2024, after the prespecified DSMB decision to de-escalate to Part B). The participant received supplemental oxygen and was discharged from the hospital within 2 days.

Table 1. Summary of symptomatic RSV infections and severe/hospitalized RSV by RSV-naïve and RSV-experienced participants – Part A: 8 to 23 months (cohorts 1 and 2; safety population)^a,b.

	mRNA vaccines
	mRNA-1345 30 µg (n=29)	mRNA-1365 30 µg (n=30)	Either vaccine (n=59)	Placebo (n=31)
RSV-naïve (n=36)
Participants, n	14	9	23	13
Symptomatic RSV (all severity), n (%)	6 (42.9)	6 (66.7)	12 (52.2)	8 (61.5)
Severe/hospitalized RSV, n (%)	0	1 (11.1)^c,d	1 (4.3)	0
RSV-experienced (n=49)
Participants, n	13	19	32	17
Symptomatic RSV (all severity), n (%)	5 (38.5)	7 (36.8)	12 (37.5)	6 (35.3)
Severe/hospitalized RSV, n (%)	0	0	0	0

Abbreviations: DSMB, data safety monitoring board; LRTI, lower respiratory tract illness; RSV, respiratory syncytial virus. ^aPercentages are based on the number of participants in the safety population. ^bRSV-naïve participants were defined as having postF IgG <200 AU/mL at baseline and RSV-experienced participants were defined as having postF IgG ≥200 AU/mL at baseline. ^cCase of hospitalized RSV-LRTI occurring in a recipient of mRNA-1365 30 µg in August 2024 (second RSV season), after DSMB decision to age de-escalate to infants aged 5 to 7 months. ^dCo-infection with human rhinovirus/enterovirus.

Immunogenicity

Antibody Responses

Serum samples collected at baseline and at 1 month post-injection 1 (Day 29), 1 month post-injection 2 (Day 85), and 1 month post-injection 3 (Day 141) were tested for RSV-specific antibodies across study groups (Figure 2A-B; Table S4). Among baseline RSV-experienced participants, a single injection of mRNA-1345 induced a Day 29 nAb geometric mean fold rise (GMFR) from baseline (95% CI) of 58.4 (29.7-114.8) and 38.8 (17.5-86.2) for RSV-A and -B, respectively. Similarly, a single injection of mRNA-1365 in RSV-experienced participants induced a Day 29 nAb GMFR (95% CI) from baseline of 44.3 (20.4-96.3) and 23.4 (7.8-70.2) for RSV-A and -B, respectively. Subsequent injections did not meaningfully further increase nAb titers, and nAb responses remained elevated through Day 141. A similar pattern was observed for bAb responses (Figure 2C-D; Table S5).

Figure 2. Neutralizing antibody titers against RSV-A (A), RSV-B (B), and binding antibody concentrations against RSV preF (C) and postF (D) – Part A: 8 to 23 months (cohorts 1 and 2; per-protocol population). Study injections occurred at Day 1, Day 57, and Day 113. RSV-experienced is defined as baseline RSV postF bAbs ≥200 AU/mL. RSV-naïve was defined as baseline RSV postF bAbs <200 AU/mL. 95% CI was calculated based on the t-distribution of the log-transformed values or the difference in the log-transformed values for GM titer/concentration, then back-transformed to the original scale for presentation. Numbers indicated below each time point are those with non-missing antibody data. Data cutoff date for nAb data: October 7, 2024. Data cutoff date for bAb data: October 18, 2024. Abbreviations: bAb, binding antibody; nAb, neutralizing antibody; postF, postfusion F; preF, prefusion F; RSV, respiratory syncytial virus.

Among participants considered RSV-naïve at baseline, vaccination with mRNA-1345 or mRNA-1365 induced nAb responses after a single injection (Day 29), which continued to increase with subsequent injections. Among recipients of mRNA-1345, the Day 141 nAb GMFR (95% CI) from baseline was 1240.3 (655.7-2346.3) and 302.5 (98.1-932.4) for RSV-A and -B, respectively; geometric mean titers (GMTs) at Day 141 were comparable to maximal nAb GMTs for RSV-experienced participants. Similarly, for RSV-naïve participants receiving mRNA-1365, the Day 141 nAb GMFR (95% CI) from baseline was 184.5 (73.2-465.2) and 7.3 (1.1-47.3) for RSV-A and -B, respectively. A similar pattern was observed for bAb responses in this group (Figure 2C-D; Table S5). In both baseline RSV-experienced and RSV-naïve populations, a preF biased response was observed, with a higher preF/postF ratio in RSV-naïve participants than RSV-experienced through Day 141.

Cell-Mediated Immunogenicity

RSV-specific cellular cytokine responses representative of Th1 (interleukin [IL]-2, tumor necrosis factor [TNF] α, and interferon [IFN] γ) and Th2 (IL-4, IL-5, and IL-13) were measured in a subset (n=25) of Part A participants aged 12 to 23 months. Prior to vaccination, baseline RSV-experienced participants had detectable Th1 responses and minimal Th2 responses (Figure 3). Among these participants, mRNA-1345 and mRNA-1365 vaccination increased Day 85 RSV-specific cytokines with a Th1 bias, robustly increasing TNFα and IFNγ concentrations, while IL-5 concentrations were low, and minimal to no IL-4 or IL-13 were detected. RSV-naive participants similarly had a robust RSV-specific Th1 response, with a minimal Th2 response. Compared with RSV-experienced participants, a higher percentage of RSV-naive participants had detectable IL-5 concentrations following vaccination; however, among participants with measurable IL-5 concentrations, IL-5 levels were similar between RSV-experienced and RSV-naïve participants.

Figure 3. RSV F-specific Th1 and Th2 responses – Part A: 8 to 23 months (cohorts 1 and 2; subset of per-protocol population). Study injections occurred at Day 1, Day 57, and Day 113. Th1-specific responses were defined as RSV-F-specific IL-2, TNFα, and/or IFNγ and Th2-specific responses were defined as RSV-F-specific IL-4, IL-5, and/or IL-13. Results are DMSO corrected and values less than LLOQ/2 were imputed as LLOQ/2. Dashed lines represent LLOQ/2. Each color/symbol combination represents an individual participant. Abbreviations: DMSO, dimethyl sulfoxide; IFN, interferon; IL, interleukin; LLOQ, lower limit of quantification; RSV, respiratory syncytial virus; Th1, type 1 T helper; Th2, type 2 T helper; TNF, tumor necrosis factor.

Part B