Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease (LRTD) in infants, resulting in significant morbidity and mortality globally. The high incidence of severe LRTD (bronchiolitis and pneumonia) in infants under one year of age, and the continued burden of disease into the second year, underscores the urgent need for an effective RSV vaccine. Past attempts, such as with formalin-inactivated RSV vaccines, have been hampered by the risk of vaccine-induced enhanced respiratory disease (ERD). ERD is characterized by a Th2-biased immune response, a high ratio of non-neutralizing to neutralizing antibodies, and a lack of CD8+ T cell responses. Therefore, an ideal RSV vaccine should induce high titers of neutralizing antibodies, a Th1-biased CD4+ T cell response, and CD8+ T cell responses. Viral vectored vaccines, such as those using adenoviral vectors, are promising candidates because they can induce both humoral and cellular immune responses. Chimpanzee adenoviruses (ChAds), like ChAd155, offer advantages due to their lower seroprevalence in humans compared to human adenoviruses. The ChAd155-RSV vaccine candidate, encoding RSV fusion protein (F), M2-1, and nucleocapsid protein (N), aims to achieve this balanced immune response. Preclinical studies highlight the limitations of existing animal models in predicting human RSV vaccine efficacy. The bovine RSV (bRSV) model in calves closely mimics the pathogenesis and clinical manifestations of severe pediatric RSV disease, making it a valuable translational model for preclinical RSV vaccine evaluation, particularly regarding ERD.

The literature extensively documents the global burden of RSV infection in infants, emphasizing the urgent need for a safe and effective vaccine. Studies on previous RSV vaccine attempts, particularly formalin-inactivated RSV vaccines, highlighted the potential for ERD, a serious adverse effect characterized by exacerbated disease upon subsequent infection. The mechanisms underlying ERD are not fully understood, but studies suggest a role for Th2-biased immune responses, antibody isotypes, and deficient cytotoxic T cell responses. The development of viral vector-based vaccines has offered a promising approach, with adenoviral vectors, especially chimpanzee adenoviruses (ChAds) showing potential due to their lower seroprevalence in the human population. The superior suitability of the bovine RSV model in calves for preclinical evaluation of RSV vaccines compared to murine models is also widely discussed in the literature, emphasizing its capacity to reliably predict clinical efficacy and safety in humans.

Two placebo-controlled studies were conducted in calves. Study 1 assessed the efficacy of single and two-dose ChAd155-RSV vaccination regimens in seronegative, colostrum-deprived calves, with challenges after both short (4 weeks) and long (16 weeks) durations of immunity (DOI). Study 2 evaluated the two-dose regimen's efficacy in the presence of pre-existing bRSV antibodies (by feeding colostrum) after a short DOI, and a single dose regimen after a long DOI. Calves were vaccinated intramuscularly with 5 × 10⁹ viral particles of ChAd155-RSV or placebo (phosphate-buffered saline). Challenges were performed using aerosolized bRSV (strain Odijk, subtype A). Clinical symptoms (general illness, fever, respiratory rate, URTD), lung pathology (macroscopic consolidated lung area (CLA), microscopic histopathology scores), and viral loads in nasal and bronchoalveolar lavage (BAL) samples were assessed. Neutralizing antibody (nAb) responses against human RSV A Long were measured by standard neutralization assay. Statistical analyses were performed using appropriate methods (t-tests, ANOVA, mixed models) to compare vaccine groups with placebo groups.

In Study 1, two doses of ChAd155-RSV provided near-complete protection against clinical symptoms after a short DOI and complete protection after a long DOI. A single dose showed comparable short-term protection and significant protection in the long-term. Study 2 demonstrated that the two-dose regimen effectively protected calves even with pre-existing bRSV antibodies. In both studies, vaccination significantly reduced viral loads and lung pathology. The nAb responses were low after the first dose but were significantly boosted by the second dose and subsequent challenge. Even with lower nAb titers after a single dose, protection was significant, suggesting a contribution from cellular immunity. The presence of pre-existing antibodies did slightly dampen the vaccine-induced nAb response, but it did not compromise clinical efficacy.

The findings demonstrate that ChAd155-RSV is immunogenic and highly efficacious in preventing bRSV-induced disease in calves. The vaccine elicited a robust neutralizing antibody response, enhanced by a second dose, and provided sustained protection even after a 16-week interval. The efficacy of a single-dose regimen, particularly in the short term, is promising for vaccine development. The absence of ERD and the maintenance of efficacy in the presence of pre-existing antibodies are crucial observations. The combination of humoral and likely cellular immune responses contributes to the vaccine's effectiveness. The bovine model accurately reflected the clinical manifestations and efficacy of the vaccine, further supporting the development of ChAd155-RSV for infants.

The ChAd155-RSV vaccine demonstrated robust immunogenicity and efficacy against bRSV infection in a calf model. The two-dose regimen offered superior protection, while even a single dose provided significant benefits. The absence of ERD and the effectiveness in the presence of pre-existing antibodies are particularly encouraging. These results strongly support the continued clinical development of ChAd155-RSV as a potential pediatric RSV vaccine. Future studies could explore optimal dosing strategies and investigate the contribution of cellular immunity to the observed protection.

The study used a relatively small sample size due to the logistical constraints of working with calves. While the calf model is valuable, it does not perfectly replicate all aspects of human RSV infection. The absence of a positive control ERD-inducing vaccine (bovine FI-RSV) limited the comprehensive assessment of ERD risk, although the observed lack of exacerbated disease in vaccinated calves is reassuring.