For children younger than 5 years old, RSV is the main drive — approximately 2,000,000 outpatient/ED visits and about 75,000 hospitalizations annually. RSV disease ranges from upper respiratory tract infections, eg, in older children and healthy adults, to more severe lower tract disease in young children and the elderly. Premature infants and high-risk groups are particularly prone to severe disease.1 Up to 300 pediatric RSV deaths occur yearly. “Normal” RSV seasons start in mid-November, peak in late December-January, and end after April. Note: More drawn out seasons occur in southern latitudes, eg Texas or Florida. But lately RSV seasons have been anything but normal.
2015-2016 to 2022-2023
RSV data from the Pediatric Health Information System (PHIS), collected at over 49 US children’s hospitals during 2015 to early 2023, show how crazy RSV seasons have been lately.2 The involved months, intensity, and duration of four prepandemic seasons were pretty “normal” (Figure 1). The 2019-2020 season started normally, peaked in January 2020, and was slowing as expected by February. But when SARS-Cov-2 restrictions kicked in during mid-March, RSV detections tanked to almost nothing (ditto other respiratory viruses). A near 14-month RSV hiatus meant that the 2020-2021 RSV season never materialized. However, RSV was not done with us in 2021. It rebounded in May with weekly hospitalizations peaking in late July; this “rebound season” lasted 9 months, not dropping to baseline until February 2022 (Figure 1).
I guess we should have expected a post-pandemic “disturbance in the Force,” as Yoda once said; but I sure didn’t see a prolonged summer/fall/early winter RSV season coming. It was like two “normal” seasons mashed up into one late-but-long season. Not to be outdone, the 2022-2023 RSV season started early (September) and hospitalizations skyrocketed to peak in November at over twice the peak number from any year since 2015, overloading hospitals (influenza and SARS-Cov-2 seasons were co-circulating). The season terminated early though (March 2023).
Okay, so RSV seasonality/intensity were weird post pandemic, but was anything else different? Some 2021-2023 data suggest more RSV disease in older children, rather than the usual younger than 18 month-olds going through their first winter.3 More medically attended RSV in older ages (2-4 years of life) may have been due to the pandemic year without RSV circulation distorting herd immunity, ie older children remained RSV naive. Other data suggest the apparent increase was really just more frequent multiplex viral testing in older children triggered by SARS-CoV-2 co-circulation.4 More data are needed to decide.
CDC 2023-2024 RESP-NET data
The 2023-2024 winter surge (Figure 2), as measured by RESP-NET’s cumulative RSV, influenza and SARS-CoV-2 hospitalization rates for 0- to 5-year-olds,5 shows that all three viruses’ seasonal months were normal-ish: late October 2023 start, late December-early January peak, and mid-May 2024 return to baseline. RSV season was approximately 22% less severe by area-under-the-curve calculations compared with 2022-2023, but still worse than prepandemic years.6 
One wonders if the 2022-2023 RSV season might have been worse but for use of the limited supply of nirsevimab.7
Viral Parade
Now we ready ourselves for the 2024-2025 respiratory surge, wondering what nature has in store for us. Will the usual “respiratory virus parade” occur? Will rhinovirus and parainfluenza prevalence bump after a few weeks of schools being in session, adding to the now-usual summer/fall SARS-CoV-2 surge? Note: Twenty-seven states as of Aug. 16 had high SARS-CoV-2 detection in wastewater. Will RSV and influenza start sometime in October/November, peak in January (along with rising SARS-CoV2 activity), followed by a second parainfluenza bump as SARS-CoV-2, influenza, and RSV drop off in April/May? Further, will RSV and influenza seasons be more or less severe than the last 2 years?
Prediction
The overall 2024-2025 respiratory season will be less severe than the past 2 years and hopefully than recent prepandemic years. What is the blueprint for a milder season? First, herd immunity to non-RSV and non-influenza viruses (parainfluenza, rhinovirus, metapneumovirus, adenovirus) in older children should be normalized after 2 years back to usual social activity. So, I expect no mega-seasons from them. The emerging SARS-CoV-2 virus (LB.1) is immunologically close to its recent still-circulating ancestors (KP.2, KP.2.3, KP.3 and KP.3.1.1), so existing SARS-CoV2 herd immunity along with recommended booster vaccine uptake should keep the lid on SARS-CoV2.
Dr. Christopher J. Harrison
Influenza Could Be the Bad News
Which type will dominate? Will a drift/shift occur or vaccine-mismatch reduce vaccine effectiveness? Can we get at least half the population influenza vaccinated, given the vaccine fatigue permeating the US population? The influenza season now underway in the Southern Hemisphere usually helps us predict our season. The Australian May-August 2024 experience (still on an upward trajectory for severity in mid-August) saw no drift/shift or vaccine mismatch. However, this 2024 season has been as severe as 2022 (their worst in a decade). That said, more than 95% has been type A (mostly H1N1 but H3N2 increased in July). So, if our overall 2024-2025 respiratory season is not milder, influenza is the most likely culprit. To reduce chances of influenza being the fly-in-the-ointment, we need to be particularly proactive with seasonal influenza vaccine which is back to the traditional trivalent formulation (one H1N1, one H3N2, and one B type).8 All of this could go out the window if avian influenza becomes more transmissible, but that seems unlikely at present.
Mild RSV Season?
RSV season should be blunted because of the increased use of both the remarkably effective CDC-recommended maternal RSV vaccine9 (one dose during pregnancy weeks 32 through 36, administered September through January) and of nirsevimab (up to 90% reduction in hospitalizations and ED visits).10 (See Figure 3.)
I also expect residual disease to occur mostly in younger than 18 month-olds (the “normal” aged population experiencing their first winter), who received no passive immunity (mother RSV unvaccinated and child did not receive nirsevimab). Some disease will still occur in high-risk infants/children. However, unlike active vaccination strategies, a competent immune system is not required to benefit from passive antibody, whether transplacental or directly administered.