Effectiveness and efficiency of pre-season administration of long-acting monoclonal antibodies for infants born to RSV vaccinated mothers: a modelling study

Background: A maternal vaccine (MV) and a long-lasting monoclonal antibody (la-mAB) have been licensed to protect children against RSV. Given the swift waning of their protection, we evaluated the added benefit of seasonal la-mAB administration for children born to vaccinated mothers shortly after the RSV season in Germany. Methods: We fitted an age- and birth season-structured catalytic model to cross-sectional seroprevalence data of RSV antibodies using a Bayesian framework to estimate the timing of RSV infections in children <5. We then estimated the incidence of severe outcomes in the absence of immunisation in children <1. Finally, we estimated the impact of the MV and of additional la-mAB administration, accounting for the waning of protection. Results: We estimate that children would, on average, be 7 months old (mo) at their first infection, those born in the autumn being the youngest at first infection (4 mo). Together with the children born in the winter, they account for 46% of all RSV hospitalisations and 62% of RSV ICU admissions in unimmunised <1 yo. MV would prevent a total of 776 (473-1,122) hospitalisations per 100,000 vaccinees and 73 (51-94) ICU admissions per 100,000 vaccinees, predominantly among autumn-born children. The summer birth cohort would benefit most from additional la-mAB administration, preventing an additional 46% (10-63) of ICU admissions compared to MV alone, corresponding to an additional 25 (4-45) ICU admissions prevented per 100,000 immunised children. Conclusion: Compared to MV alone, the impact of MV+la-mAB on RSV hospitalisations and ICU admissions would likely be modest.