The benefits and risks of maternal RSV vaccination on mortality in South Africa: A modeling study

Maternal respiratory syncytial virus (RSV) vaccine, RSV prefusion F protein vaccine (RSVpreF (Abrysvo)), was found to be safe and efficacious in the MATISSE trial. However, post-hoc stratified analyses identified an excess of preterm births in the intervention arm in two upper-middle-income countries, most prominently in South Africa. This study weighs the potential benefits and risks in mortality associated with maternal RSV vaccination in South Africa, assuming the increased risk of preterm births observed in the trial was caused by vaccination.

We compared the estimated RSV-associated infant deaths averted by vaccination (benefits) and neonatal mortality potentially associated with vaccine-associated risk in preterm birth (risks) in South Africa. The benefit model estimated the South African RSV disease burden in 2011-2016 and waning vaccine protection during infancy. The risk model estimated excess neonatal mortality using gestational age (GA)-specific mortality data from the Drakenstein Child Health Study and the GA-specific birth distribution in South Africa in the MATISSE trial, but did not incorporate the mortality risk found in the MATISSE vaccine trial in which no excess deaths occurred. The benefit model estimated that vaccination would reduce RSV-associated infant deaths by 31 (95% credible interval (Crl): 27, 35) per 100,000 live births born to vaccinated mothers in South Africa. Using the number of infants born to mothers vaccinated at 24-36 GA weeks in the MATISSE trial, if the association in South Africa between vaccination and preterm birth is actually causal, the risk model suggested that neonatal deaths would increase by 44 (95%CrI: -43, 210), totaling 1.4 (95%CrI: -1.4, 6.9) excess neonatal deaths for every infant RSV death prevented. When this was changed to the number of infants born to mothers vaccinated at 27-36 GA weeks in the MATISSE trial, the predicted risks dropped and in 97% of the simulations the benefits outweighed the risks. The outcome was sensitive to the GA window that we used to determine which infants to include in the analysis. The study was limited by only considering mortality associated with RSV disease and preterm birth.

If RSVpreF increases preterm birth risk, and if this increases neonatal mortality, then the benefit-risk analysis failed to show that the direct benefits of vaccination in reducing RSV-associated infant mortality would substantially outweigh the risks of preterm birth-associated neonatal mortality in South Africa with vaccination from 24 GA to 36 GA weeks. There was large uncertainty in the analyses due to small numbers of preterm births. With vaccination from 27 GA weeks, the benefit-risk analysis favored vaccination. RSVpreF vaccination has the potential to be safe and effective when used from the third trimester.