Scenario Projections of Respiratory Syncytial Virus Hospitalizations Averted Due to New Immunizations

Abstract

Importance: In 2023, new immunization strategies became available for preventing respiratory syncytial virus (RSV) hospitalizations in infants and older adults. Modeling studies to understand the population-level impact of their use are important for public health planning.

Objective: To estimate the number of hospitalizations averted in 2023 to 2024 due to new RSV immunization strategies and provide scenario projections for future seasons.

Design, setting, and participants: This decision analytical model examined RSV hospitalizations in King County, Washington, from October 7, 2023, through April 26, 2025. The population of King County was disaggregated into infants younger than 6 months, infants aged 6 to 11 months, children aged 1 to 4 years, children/adults aged 5 to 59 years, adults aged 60 to 74 years, and adults aged 75 years or older.

Exposures: Respiratory syncytial virus vaccination for adults aged 60 years or older, maternal RSV vaccination, and long-acting monoclonal antibodies (nirsevimab) for infants younger than 8 months.

Main outcomes and measures: The proportion of RSV hospitalizations averted in adults aged 60 years or older and infants younger than 1 year were estimated using an RSV transmission model calibrated to RSV hospitalizations.

Results: The RSV transmission model simulated the population of King County, which includes approximately 2.3 million individuals, with 23 700 infants younger than 1 year and 446 500 adults aged 60 years or older. During the 2023 to 2024 RSV season, 21.2% of adults aged 60 to 74 years, 32.7% of adults aged 75 years or older, and 33.0% of infants were protected through active or passive immunization. A total of 125 (95% projection interval [PI], 77-192) RSV hospitalizations were averted, with most of the benefit observed in infants younger than 6 months (28.6% [95% PI, 26.9%-30.5%] reduction from baseline) and adults aged 75 years or older (14.8% [95% PI, 14.3%-15.5%] reduction from baseline). For the 2024 to 2025 season, optimistic scenarios of high immunization coverage (50% in older adults and 80% in infants) projected reductions of 29.8% (95% PI, 29.1%-30.8%) in adults aged 75 years or older and 68.8% (95% PI, 66.0%-71.7%) in infants younger than 6 months compared with a counterfactual scenario with no immunizations. Targeting infants eligible for catch-up doses of nirsevimab early in the season increased the proportion of RSV hospitalizations averted in infants aged 6 to 11 months from 31.7% (95% PI, 29.4%-33.9%) to 40.4% (95% PI, 39.0%-42.1%). If vaccine protection in adults aged 75 years or older waned by 50% in the second year after immunization, the proportion of RSV hospitalizations averted was projected to decrease to 22.2% (95% PI, 21.7%-23.0%).

Conclusions and relevance: In this decision analytical model of RSV immunizations, the results suggest a modest reduction in RSV-diagnosed hospitalizations during the 2023 to 2024 season due to limited availability of immunization products, particularly for infants. A higher uptake earlier in the season may lead to substantial reductions in RSV hospitalizations in the 2024 to 2025 season.

Figure 1.. Reported Respiratory Syncytial Virus (RSV) Hospitalizations, Immunizations, and Model Projections in King County, Washington, 2023 to 2024 Season

A, Observed RSV hospitalizations are after adjusting for RSV testing (eMethods in Supplement 1). Dashed line indicates median; shaded areas, 95% projection intervals (PIs).

Figure 2.. Sensitivity Analyses Exploring Alternative Scenarios for Infants in the 2024 to 2025 Respiratory Syncytial Virus (RSV) Season

Using the optimistic scenario (scenario A in Table 1) for the 2024 to 2025 season, we explored alternative immunization strategies in infants, assuming that the same total number of infants were immunized (80% of eligible infants). In scenario A2, all conditions are the same as scenario A except that the duration of protection for monoclonal antibodies (nirsevimab) is extended from 180 to 270 days. In scenario A3, the total coverage is the same but all catch-up doses of nirsevimab are administered by the end of November. In scenario A4, all immunization coverage is achieved through nirsevimab (no maternal immunization). In scenario A5, the catch-up doses of nirsevimab stay the same, but all newborn immunization is achieved through maternal vaccination (no nirsevimab birth doses). The insets show the percent reduction under each immunization scenario for infants younger than 6 months and aged 6 to 11 months compared with the counterfactual scenario of no immunizations. Error bars indicate the 95% projection interval.

Figure 3.. Sensitivity Analysis Exploring Alternative Scenarios for Older Adults in the 2024 to 2025 Respiratory Syncytial Virus (RSV) Season

In our main analysis, we assumed that the vaccine protection in older adults was durable for 2 years and that the protection (assuming a vaccine effectiveness of 75%) was only against hospitalization given infection and did not protect against infection. In these sensitivity analyses, we considered how our estimates would change if protection from the vaccine was reduced by 50% in the second year and if the protection was a combination of protection against infection and hospitalization given infection (50% and 50% for a combined relative risk of 0.25; vaccine effectiveness, 75%). Error bars indicate 95% projection intervals.