January 05, 2021
Respiratory syncytial virus (RSV) vaccine development has progressed rapidly in recent years, and there is hope that an efficacious vaccine soon may be approved.
Louis Bont, MD, PhD, provided an overview of the most recent developments in the complex RSV vaccine landscape at the annual meeting of the European Society for Paediatric Infectious Diseases (ESPID), held virtually this year.
RSV imposes significant burden worldwide, with 33 million patients, 3 million hospitalizations, and at least 120,000 deaths, reported Bont of the Wilhelmina Children's Hospital, University Medical Centre, Utrecht, the Netherlands. Of those deaths, more than 50% are in infants younger than 5 months, and "about 99% of the children dying from RSV live in low- and middle-income countries."
"There are high-risk populations, such as children with prematurity, congenital heart disease, lung disease, and Down syndrome, but about 73% of all children who are hospitalized for RSV infection were previously healthy children," Bont explained. "So, we need to find a solution for all children to prevent RSV infection."
As observed by Nienke Scheltema in a Lancet Global Health article, population distributions of RSV infection mortality show that, regardless of whether children have comorbidities or they are previously healthy, most children die at a very young age, Bont explained. These data suggest "that a maternal vaccine or an antibody prophylaxis approach from birth onwards or during the first RSV season is the solution for the problem."
The path to developing an RSV vaccine has now narrowed its focus to a structural element of RSV, the prefusion F protein. This shift started with the discovery by Jason McLellan (Science, 2013 [two papers]) that there are two variants of the RSV F-fusion protein: the very stable postfusion conformation and the prefusion active conformation, a metastable protein that exists for a "fraction of a second," Bont said.
"The interesting thing is that epitopes that are visible at the prefusion, metastable state ... induce highly neutralizing antibodies, whereas epitopes at the postfusion conformation do not," Bont explained. "So, by stabilizing the prefusion state, we start inducing neutralizing antibodies that will protect against severe RSV infection, and this is the basic concept of all the vaccine developments currently ongoing."
These RSV vaccine developments fall into five approach types: live-attenuated or chimeric vaccines, vector-based vaccines, monoclonal antibodies, particle-based vaccines, and subunit or protein-based vaccines.
One breakthrough, which was presented at last year's ESPID meeting, is the monoclonal antibody nirsevimab. In addition to being nine times more potent than the broadly used antibody palivizumab, it is also more stable; whereas many antibodies have a half-life of 3 weeks, nirsevimab has a half-life of 100 days. "The idea is that a single injection at the start of the RSV season protects children in the first RSV season of their life, a dangerous episode for them." Bont explained. The originators, AstraZeneca and Sanofi Pasteur, have "the vision that every child on this planet should receive a single injection with this antibody in the first season," he explained.
Studies of nanoparticle-based maternal vaccines have also revealed interesting results: Although a phase 3 trial investigating such vaccines didn't achieve its primary endpoint, "interestingly, 15% of all RSV infections were mild, and only 2% were very severe and leading to hypoxemia," Bont noted. "But if we look at vaccine efficacy, we see the opposite -- the vaccine was not very efficacious to prevent mild disease, but very efficacious to prevent severe hypoxemia; actually, this is exactly what you would like to see in a vaccine."
Investigations into live-attenuated and vector-based vaccines have been promising as well, Bont shared. Studies of live-attenuated vaccines suggest they have a future and that we can move onto their next phase of clinical development, and a study investigating adenoviral vector-based vaccines has demonstrated safety, efficacy, and immunogenicity, though it has also shown that we should anticipate some side effects when using them.
Simple subunit vaccines for RSV are also being explored -- a study of DS-Cav1, a stabilized prefusion F subunit protein candidate vaccine, has shown that it has a superior functional profile, compared with previous pre-F subunit vaccines. However, it seemed to be more efficacious against strains of RSV A than strains of RSV B, the dominant strain.
Bont also discussed exciting work by Sesterhenn et al, in which they used a computer-based program to develop their own vaccine. Using their in-depth knowledge of the RSV prefusion F protein and a computer program, Sesterhenn et al developed a trivalent vaccine, produced it, and showed -- both in vitro and in monkeys -- that such vaccines can work up to the level of preclinical in vivo experiments.
"We can now make vaccines behind our computer," Bont declared. "And the system doesn't only work for RSV vaccines, but also for other pathogens -- as long as you have an in-depth molecular knowledge of the target epitope," he added.
Joanne Wildenbeest, MD, PhD, at the Utrecht University, the Netherlands commented: "Lower respiratory tract infections due to RSV are among the leading causes of death worldwide in children under the age of 5, especially young infants. The recent advances in the development of a vaccine and passive immunization are important steps toward the goal to reduce childhood mortality due to RSV worldwide. Since RSV-related mortality is mainly seen in developing countries, it is important that, once a vaccine has been approved, it will also be made easily available to these countries."
Bont reported being on the board of nonprofit foundation ReSViNET; investigator-initiated studies with the Bill & Melinda Gates Foundation, AbbVie, MedImmune, and MeMed; participation with Pfizer, Regeneron, and Janssen; and consultancy with GlaxoSmithKline, Ablynx, Novavax, and Janssen.