At a glance
Overview
In August 2024, ÐÇ¿ÕÓéÀÖ¹ÙÍø released its respiratory disease season outlook to provide decision-makers with information to assist in public health preparedness during the 2024-2025 respiratory virus season (October to April), such as when hospitals might experience the most demand. We note that our outlook was intended to offer potential scenarios for how the season would unfold and was not intended as a prediction or a forecast. We also produced three updates throughout the season. In this evaluation, we compare the initial outlook and subsequent updates to observed data to evaluate how well our assessment of the data, expert opinion, and scenario modeling captured overall trends.
Combined peak hospital demand
In our August outlook, experts estimated that the 2024-2025 respiratory virus season would likely have a similar or lower number of combined peak hospitalizations due to COVID-19, influenza, and RSV, compared to the 2023-2024 season. This assessment of the data remained the same in subsequent updates to the outlook.
During the 2024-2025 respiratory virus season, the combined peak hospitalization rate occurred in the weeks ending February 1 and February 8, 2025, with 19.3 hospitalizations per 100,000. This combined peak hospitalization rate was similar (within 20%) to that of last season (21 hospitalizations per 100,000 during the week ending December 28, 2023), which experts had estimated was the most likely scenario (54% chance of occurring). Additionally, the COVID-19 and RSV peaks were both lower than the 2023-2024 peaks, aligning with experts' assessment that these peaks would most likely be similar to or lower than the previous season's.
The combined peak hospitalization rate was strongly driven by influenza, which peaked the same weeks (Figure 1). This influenza season was classified as a high-severity season overall as well as for all age groups (children, adults, older adults), and was the first high-severity season since the 2017-2018 season. This flu season differed substantially from experts' assessment that the influenza peak would most likely be similar to or lower than the previous season's. Based on currently available data from FluSurv-NET, the cumulative hospitalization rate for this season was the highest observed since the 2010-2011 season.
COVID-19 scenario modeling
Initial outlook
We developed two COVID-19 hospitalization demand scenarios for the 2024-2025 respiratory season in our original outlook, based on inferred COVID-19 weekly hospitalizations for the 2024 summer using two different surveillance systems: hospitalization data from a participating in population-based surveillance (Scenario A) or emergency department visit data (Scenario B). The scenarios incorporated assumptions around prior immunity, vaccination rates, and levels of immune escape of potential new variants. In Scenario A, the peak weekly COVID-19 hospitalization rate was predicted to be between 7.2 and 9.7 hospitalizations per 100,000, occurring in late December, and in Scenario B, the peak weekly COVID-19 hospitalization rate was predicted to be between 4.2 and 5.8 hospitalizations per 100,000, occurring in mid-January.
Observed data indicates that the peak COVID-19 hospitalization demand occurred during the week ending January 4, 2025, with a peak hospitalization rate of 4.2 hospitalizations per 100,000A. Our initial scenarios provided useful bounds for what could occur during the season. The observed peak was at a lower magnitude, aligning with Scenario B—likely reflecting the higher levels of immunity in the population from the larger 2024 summer wave (Figure 2).
December outlook update
One key uncertainty in our original outlook was whether a new SARS-CoV-2 variant would emerge during the 2024-2025 respiratory season. In our initial August outlook and the October update, we assumed, based on the historical timing of major variants emerging, that a variant with high immune escape properties would emerge by November. However, by December, it was clear that no such variant had emerged. In our December update, using updated data on the summer wave and vaccination rates, we detailed two additional scenarios for what could occur with and without the emergence of a major new variant with high immune escape. If no new immune-escape variant emerged, modeling results indicated that a substantial winter wave was either not expected or expected to be much smaller than the wave from the previous summer, with a peak occurring in December or January. As of May 22, 2025, no new variant with high immune escape properties has emerged during the 2024-2025 season. Nationally, the winter wave was smaller than the previous summer wave, aligning with modeling results for our scenario without a new immune-escape variant in our December update.
Vaccine effectiveness estimates
Experts estimated vaccine effectiveness (VE) for the respiratory season in our original outlook, assessing that effectiveness would be similar to prior seasons for all three diseases.
Influenza
For influenza, experts estimated, with moderate confidence, that VE against hospitalization would likely be between 33% and 50%, with a midpoint of 41%, which was used in our averted burden modeling. Interim estimates for seasonal influenza VE this respiratory season are in line with expert estimates; among adults ≥18 years in two different networks, VE was 41% and 55% against hospitalization.
RSV
Unlike influenza and COVID, the RSV immunizations are not updated annually. Because all of the current RSV immunization products were new in 2023-2024 season, immunization efficacy estimates for last season were based on clinical trial results. Experts estimated, with moderate confidence, that for older adults, effectiveness of RSV vaccination against hospitalization would likely be between 73% and 88%, which was used in our averted burden modeling. Experts estimated that maternal RSV vaccination effectiveness against infant hospitalization would be between 55% and 73.5%, with moderate confidence, and that infant immunization effectiveness against infant hospitalization would be between 85% and 95%, with moderate confidence.
The first real world studies of effectiveness were conducted during the 2023-2024 season and estimated VE against hospitalization for adults 60 and older to be and in two different studies, and infant immunization effectiveness to be 90% against hospitalization for infants in their first RSV season.
COVID-19
Vaccine effectiveness estimates were used in the scenario modeling for COVID-19 for the outlook, with VE assumed to be similar to that of last season (43-52%). Experts assessed with moderate confidence that COVID-19 VE against hospitalization would be between 40-60%, with a midpoint of 50%. These results are in line with interim estimates for the 2024-2025 respiratory season, which showed a VE against hospitalization of 45-46% among immunocompetent adults ≥65 years.
- The peak COVID-19 hospitalization rate of 4.2 hospitalizations per 100,000 is from COVID-NET, a COVID-19 surveillance network using data from a subset of hospitals nationwide. This peak differs from the peak COVID-19 hospitalization rate from the National Healthcare Safety Network (NHSN) shown in Figure 2 (5.5 per 100,000).