Objectives: This preliminary study assessed the cost effectiveness of immunizing low-risk infants against RSV in October-born (in-season) infants and April-born (out-of-season) infants at the start of the RSV season.

Methods: Baseline healthcare utilization by chronological age and calendar month used existing literature (Rainisch, 2019). Reductions in RSV-events came from clinical trial results on healthy preterm infants (Griffin, 2020). Adverse events included visit-related exposures. Current pediatric vaccine prices, $10.60 (low price) and $181.30 (high price), proxied for product prices.

Results: Immunization reduced RSV-associated lower respiratory tract infection (LRTI) hospitalizations by 78.4%, averting 278 hospitalizations in the April-cohort and 1,031 in the October-cohort per 100,000 infants. Total illness-related healthcare costs and DALYs averted were substantially lower in the April-cohort compared to the October-cohort ($3,973,270 vs $11,757,389 and 11.8 vs 43.3 DALYs, respectively). Inclusion of product-related adverse events made immunization in April-cohort counterproductive (net DALYs:-166.4), while reducing net benefits by 16% in October-cohort. For the October cohort, immunization was marginally cost-effective at the higher price (incremental cost-effectiveness ratio [ICER] $170,041 per DALY averted, just below 3 times US gross domestic product per capita), and cost saving at the lower product price (ICER -$293,700/DALY). Immunization for the April-cohort was strongly dominated, with lower health and higher costs than no immunization. Even at a lower product price, immunization in the April-cohort was counterproductive and ICER unfavorable.

Conclusions: RSV risk declines with increasing chronological age, so infants born earlier in the year face lower risk when entering the RSV season. Benefits of RSV immunization differ between birth cohorts. For infants born in October and immunized during the birth hospitalization, protection is cost-effective and potentially cost saving. However, for those born in April, immunization potentially presents more harms than benefits and would not be recommended. Useful extensions could focus on other birth months and probabilistic sensitivity analyses.