Discussion
infections
RSV INFECTION, LRTI, INFECTIONS
This study of infants with confirmed RSV, including LRTI and respiratory illnesses requiring hospitalization, demonstrated that nirsevimab effectively neutralized both RSV A and B infections among participants enrolled in the Phase 2b and MELODY trials, regardless of the presence of naturally occurring amino acid substitutions, and including RSV infections that did not meet protocol defined endpoints. Genotypic analyses showed that most substitutions occurred at a low frequency and in non-EC portions of RSV F rather than in antigenic sites, indicating that the nirsevimab binding site remains conserved and thus RSV remains susceptible to nirsevimab.No major RSV A binding site variant substitutions occurred in participants with MA RSV LRTI and hospitalization for RSV LRTI. However, two participants who met an exploratory RSV endpoint had RSV A that contained a binding site substitution (K209R) that did not affect nirsevimab susceptibility. While RSV B isolates had more binding site substitutions through 511 days post-dose, all but three substitutions identified in two participants were potently neutralized by nirsevimab. Additionally, of all the substitutions observed in RSV B in both placebo and nirsevimab recipients, only two occurred in the binding site at high frequency (I206M and Q209R) in the Phase 2b study, with one additional substitution, S211N, observed in the MELODY study. All these frequently observed binding site substitutions were effectively neutralized by nirsevimab. Recent surveillance studies have shown these substitutions to be highly prevalent in circulating RSV (28–>51% in 2021)This study has shown that major variant substitutions within the nirsevimab binding site have not led to a reduction in potency of nirsevimab. In fact, RSV B variants with the I206M:Q209R and Q209R substitutions exhibited increased susceptibility to nirsevimab. A potential mechanism to explain the increased susceptibility of RSV B to nirsevimab in the presence of the Q209R substitution was proposed based on molecular modeling; the Q209R substitution demonstrated improved electrostatic interaction with nirsevimab, thereby increasing the effectiveness of locking the F protein in the pre-fusion structure and preventing viral entry. This analysis highlights that not all binding site substitutions decrease the potency of monoclonal antibody prophylactics such as nirsevimab, and indeed changes in electrostatic interactions can be beneficial by increasing potency.Limitations of this study include the small number of cases in each group that did not allow for any between-group statistical analyses. Additionally, although the primary cohort of MELODY was followed through two seasons, there were few cases of RSV past Day 150; however, this would be expected as infants are most susceptible to having an MA RSV infection during their first RSV season. With regards to qRT-PCR, limitations include differential times from symptom presentation to clinical site visits for central laboratory assessments and the possibility of higher inoculating doses of RSV being responsible for the breakthrough infections.In summary, nirsevimab neutralized both RSV A and B subtypes obtained from participants enrolled in two pivotal clinical trials. The nirsevimab binding site was highly conserved in RSV A, and few binding site substitutions were observed in RSV B. Furthermore, microneutralization data showed that the prevalent Q209R substitution in RSV B resulted in increased susceptibility to nirsevimab and modeling provided a potential mechanism based on increased electrostatic interaction between nirsevimab and the binding site.No resistance-associated substitutions in the F protein of RSV A isolates were identified. Although rare, resistance substitutions were identified in the F protein of RSV B in two participants in Phase 2b trial; these changes were not observed in subsequent clinical trials or molecular surveillance studies. Overall, >99% of RSV F protein sequences from the Phase 2b and MELODY trials remained susceptible to nirsevimab, and no resistance-associated substitutions were identified beyond 150 days post-dose as levels of nirsevimab waned.