Advanced Live Attenuated Vaccines for the Prevention of Respiratory Syncytial Virus Infections in Young Children

(See the Major Article by Karron et al., on pages 82–91.)

Respiratory syncytial virus (RSV) infections are associated with significant disease burden across the world, especially among children. It is estimated that RSV causes 33 new million episodes of acute lower respiratory tract infection in children <5 years of age, and approximately 120 000 deaths annually. Among infants, RSV represents the leading cause of hospitalization worldwide, and the second cause of death in low- and middle-income countries [1, 2]. After >60 years of research, there are still no licensed RSV vaccines. A formalin-inactivated vaccine developed in the 1960s was evaluated in RSV-naive infants, who after subsequent exposure to natural infection developed enhanced respiratory tract disease that resulted in increased hospitalizations and 2 deaths [3]. This experience discouraged efforts to develop RSV vaccines, especially those designed for young children.

To overcome these challenges, it has been proposed that a live attenuated vaccine administered intranasally might represent a safe strategy to immunize young children against RSV. Over several decades, investigators have developed and tested several RSV live attenuated vaccine candidates in young children. Initially, these vaccine candidates were developed using traditional techniques of passage under suboptimal conditions to attenuate the virus. Studies have illustrated the difficulties of selecting vaccine candidates that were well tolerated without causing significant symptoms when administered to young children and at the same time showed adequate replication to induce a protective immune response, all without reversion of the attenuating mutations. In the last 2 decades, however, the incorporation of reverse genetics to target specific genes with a role in disease pathogenesis has allowed the development of vaccine candidates purposefully designed to achieve the desired attenuation, immunogenicity and genetic stability, leading to more consistent progress.

In the present issue of the Journal of Infectious Diseases, Karron et al [4] report the results of the first-in-human study of one promising live attenuated vaccine candidate RSV/ΔNS2/Δ1313/I1314L. This vaccine contains 2 unique elements: (1) deletion of the NS2 gene and (2) deletion of codon L1313 of the polymerase protein (L) and a stabilizing missense mutation, I1314L. These changes provide the virus with important properties. The NS2 gene is an interferon antagonist, and it enhances shedding of the infected epithelial cells, likely contributing to small airway obstruction [5]. Deletion of the NS2 gene results in increased interferon responses that diminish virus replication, reduce damage to the respiratory tract epithelium, and, possibly, enhance of adaptive immune responses. The L1313 deletion also provides attenuation of viral replication and mild temperature sensitivity. Because this vaccine virus was attenuated and immunogenic in animal experiments [6, 7], the investigators advanced their studies to seropositive and seronegative children. In this phase I study, they administered a single dose of the vaccine (randomized in all groups 2:1 vs placebo) initially to seropositive children, aged 12–59 months (n = 15) at a dose of 10⁶ plaque-forming units (PFUs), and subsequently to seronegative children, aged 6–22 months, at 2 doses: 10⁵ (n = 22) and 10⁶ (n = 30) PFUs.

Among seropositive children (n = 15), upper respiratory tract symptoms developed in 2 and 1 cough in 1, and in both rhinovirus was detected at the time of illness. Importantly, none of the vaccine recipients shed vaccine virus. In addition, none of the seropositive children demonstrated a ≥4-fold increase in anti-RSV F or neutralizing antibodies. These findings confirmed the attenuation of the vaccine and are similar to those observed with other live attenuated vaccines when tested in seropositive children [8].

The results in seronegative participants were substantially different: rhinorrhea, pharyngitis, cough, and febrile illness were commonly observed. In the 10⁵-PFU dose cohort, respiratory tract or febrile illnesses developed in 11 of 15 vaccine recipients (73%) and 4 of 7 placebo recipients (57%). Among the 10⁶-PFU dose recipients, the rates were 11 of 20 vaccine recipients (55%) versus 8 of 10 placebo recipients (80%), and lower respiratory tract infections developed in 2 children on days 33 and 45 after placebo administration. Respiratory tract viruses other than the vaccine virus were detected in 10 of 22 symptomatic vaccine and 7 of 12 placebo recipients, respectively. Nasal wash samples obtained within 3 days of illness in 22 symptomatic vaccinees showed vaccine virus (n = 5), other respiratory tract viruses (n = 5), a combination of vaccine and other viruses (n = 5), or no viruses (n = 7). The vaccine virus was detected by mean of polymerase chain reaction in 11 of 15 in the 10⁵-PFU cohort and 18 of 20 in the 10⁶-PFU cohort. Analyses of the viruses shed by the vaccine recipients confirmed the stability of the ΔNS2 and Δ1313 deletions and the I1314L mutation.

In terms of antibody responses, in the 10⁵-PFU cohort, neutralizing and anti-RSV F antibodies developed in 8 of 15 children, and in the 10⁶-PFU cohort, neutralizing antibodies developed in 16 and anti-RSV F antibodies in 17 of 20 children. There was an excellent correlation between neutralizing and anti-RSV F antibodies, but there was no correlation between the magnitude of viral shedding detected in nasal wash samples and the subsequent antibody titers. Vaccinees were followed up for 2 subsequent respiratory seasons and there was no evidence of enhanced disease in any participant. Cases of RSV-associated medically attended acute respiratory tract infections were identified during the surveillance periods. Other viruses were identified, however, in 50% of presumed cases in vaccine recipients and 33% in placebo recipients, so the causative pathogen remains unclear. Karron et al [4] mentioned that the rates of medically attended acute respiratory infection associated with RSV were low compared with those observed in population-based epidemiologic studies.

Importantly, these studies indicated increased antibody responses after each surveillance season. During the first season, vaccinees showed neutralizing antibody titers 14-fold higher than those seen in placebo recipients. This is particularly encouraging, because it indicates induction of memory responses associated with this live attenuated vaccine. During the second RSV season surveillance, Karron et al also observed memory responses, indicating adequate priming; however, a mild RSV lower respiratory tract infection developed in 1 vaccinee. This observation led them to suggest the possibility of offering a booster dose for the second RSV season.

Overall, these results are very encouraging. As already indicated by the investigators, one particular aspect needs to be carefully analyzed as larger numbers of subjects are evaluated. It concerns the frequency and nature of respiratory tract symptoms observed in some of the vaccine recipients, and the extent to which they are related to the vaccine or caused by other respiratory tract viruses. Nevertheless, the data are promising and clearly indicate that a live attenuated vaccine remains an attractive strategy to immunize against RSV for infants aged ≥6 months.

Alternative active vaccination strategies that use other viruses as vectors for expressing the RSV F protein alone, or with additional RSV proteins, are also under development. These approaches use adenovirus vectors, a weakened strain of vaccinia virus, or Sendai virus [9, 10]. Clinical trial data in children have not yet been published, but it will be interesting to compare the protection afforded by these vectored vaccines to that from live attenuated RSV vaccines.

The present study [4] is particularly relevant, because efforts to develop RSV vaccines targeting different patient populations have flourished in the last few years (as reviewed in [10, 11]). Advances in understanding RSV pathogenesis and immunity, especially studies that have resolved the structure of the RSV F protein, in its 2 forms of prefusion and postfusion, have fundamentally advanced the field [12]. Additional studies have shown the multiple epitopes of RSV F and the importance of the epitopes uniquely found in prefusion F (pre-F) protein for inducing potent neutralizing antibodies [13]. These discoveries have facilitated the design of novel vaccine candidates with the capacity to induce high titer neutralizing antibodies [14]. In parallel studies, other investigators have developed new, more potent, monoclonal antibodies directed against those recently identified highly neutralizing epitopes in the pre-F protein [15, 16].

These advances suggest that it may be possible to protect young infants during their highest-risk period, the initial 3–6 months of life, via passive immunization. Two approaches are being actively pursued and have already shown promising results: (1) maternal immunization with the goal of inducing high levels of neutralizing antibodies that can be transferred across the placenta and protect the young infant [9, 17] and (2) administration to newborns and young infants of potent neutralizing monoclonal antibodies with extended half-lives that provide antibody titers adequate to protect the infant during the initial RSV season with a single dose [15, 16]. The expectation is that these 2 approaches can and will provide adequate protection to young infants in the first few months of life. Nevertheless, we need additional strategies to protect children aged ≥6 months who also experience significant RSV-associated illness and represent a large population in need. A live attenuated vaccine offers an attractive strategy to protect this population, emphasizing the importance of the results reported by Karron et al [4] for the RSV/ΔNS2/Δ1313/I1314L vaccine.

Notes

Financial support. This work was supported by the National Institutes of Health (grants AI112524 to O. R., AI112524, AI095684, and AI093848 to M. E. P., and AI112524 to A. M.,), Fondo de Investigacion Sanitarias, Spain (grant FIS PI16/00822 to R. R. F.), the Cystic Fibrosis Foundation (M. E. P.), and the Bill & Melinda Gates Foundation (O. R.).

Potential conflicts of interest. O. R. has received research grants from Janssen, fees for participation in advisory boards from Sanofi/Medimmune, Merck, and Pfizer, and fees for lectures from Pfizer and Merck. R. R. F. has received fees for participation in advisory boards and lectures from AbbVie. M. E. P. has received research grants from Janssen and Pfizer and fees for a lecture from Pfizer. A. M. has received research grants Janssen, fees for participation in advisory boards from Janssen and Roche, and fees for lectures from AbbVie. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.