IDWeek Studies Tie RSV, Flu, and COVID-19 to Harm After Acute Illness

About 9% of adults hospitalized with respiratory syncytial virus (RSV) died in the hospital or within 30 days of discharge across 8 seasons of US surveillance, according to preliminary data presented during the “Tri-Demic: Combating the Triple Threat of COVID, Flu, RSV” oral abstract session on October 22, 2026, at IDWeek in Washington, DC.^1,2 The session also covered long-term symptoms after infection, pediatric virus trends, influenza burden without vaccination, and COVID-19 vaccine effectiveness in older adults.

Tessa Andermann, MD, MPH, associate professor of medicine in the Division of Infectious Diseases at the University of North Carolina (UNC) School of Medicine, and Kathleen Gensheimer, MD, MPH, a medical officer at the FDA, moderated the session.

Deaths After RSV Hospitalization

Michael Melgar, MD, a medical epidemiologist at the CDC, presented on behalf of Rebecca C. Woodruff, PhD, MPH, an epidemiologist at the CDC, who could not attend.² The analysis used the RSV Hospitalization Surveillance Network (RSV-NET), the RSV platform of the CDC-funded Respiratory Virus Hospitalization Surveillance Network (RESP-NET), which covers 170 counties in 14 states, more than 300 acute care hospitals, and about 11% of the US population.

Across 8 RSV seasons from 2014 to 2024, the network identified 18,301 hospitalizations among adults aged 18 years or older with laboratory-confirmed RSV. About 9% of those adults died during hospitalization or within 30 days of discharge. About 4% died in the hospital, and about 4% died within 30 days after discharge. Almost 2% of all hospitalizations ended in death within 30 days after discharge to hospice, and almost 3% ended in death within 30 days after discharge to a setting other than hospice.

Melgar said the latter group is the most concerning. “These patients were likely discharged home or to some higher level of care but were probably not expected to die within the month of the hospitalization with RSV infection,” he said.

About 81% of adults who died were aged 65 years or older, and about one-third were aged 85 years or older. About 89% had at least 1 chronic condition, and about half had 3 or more. Compared with adults who died in the hospital, a greater proportion of those who died after discharge were aged 65 years or older, and fewer had underlying conditions or required intensive care or invasive mechanical ventilation.

Only about 10% of the adults who died had RSV listed anywhere on the death certificate. That share was about 15% for in-hospital deaths, about 7% for post-discharge deaths after hospice discharge, and about 4% for post-discharge deaths after discharge elsewhere. Melgar noted that earlier CDC analyses of deaths during or within 30 days after hospitalization found influenza listed on about 37% of death certificates (adults and children) and COVID-19 on about 72% (adults).

The results are preliminary and reflect all-cause deaths in the surveillance area, which may not generalize nationally. Melgar said the findings support prioritizing RSV vaccination for eligible adults and strengthening discharge planning and early follow-up. Melgar reported no conflicts of interest.

Pediatric Respiratory Virus Trends

Carol Yohanna Godoy Collazos, MD, a pediatric infectious diseases fellow at the University of Miami/Jackson Memorial Hospital, presented work completed at Boston Medical Center.³ The descriptive analysis covered children aged 0 to 59 months who underwent clinician-driven testing for RSV, influenza, human metapneumovirus, and parainfluenza across a multicenter Massachusetts health system. It included complete respiratory seasons from 2017 to 2025 plus 45 of 52 weeks of the 2025-2026 season, with more than 435,000 pathogen-specific tests and more than 51,000 positive results.

Testing for RSV increased several-fold after the pandemic. RSV accounted for most detections in infants younger than 6 months, while influenza made up a growing share of detections with increasing age and was more prominent among children aged 24 to 59 months. RSV peak timing shifted after the 2021-2022 rebound and then moved progressively later toward winter. Recent RSV proportions of hospitalization or pneumonia appeared lower, possibly reflecting testing patterns. Among positive tests, human metapneumovirus was associated with the highest proportions of hospitalization or pneumonia, and influenza with the lowest.

“We need to interpret this very cautiously because all these proportions could be affected by testing practices,” Godoy Collazos said.

The data were aggregated test results rather than patient-level records; individual RSV prevention status was unavailable, and the final season was partial. Godoy Collazos reported no disclosures.

Influenza Burden Without Vaccination

Hiral Shah, PhD, director and vaccine modeling lead in global modeling and evidence synthesis at GSK in London, presented a counterfactual analysis that estimated US influenza burden if the protection from current vaccines were removed.⁴ The model used publicly available CDC data from 2010 to 2025, excluding the COVID-19 pandemic year, on symptomatic illness, hospitalization, and death, along with monthly vaccine coverage and vaccine effectiveness by outcome. Shah and 5 of the 7 coauthors are affiliated with GSK, and the other 2 are with Maple Health Group.

Shah reported that current vaccination programs avert about 5.3 million symptomatic cases, 141,000 hospitalizations, and 11,000 deaths per year on average. Without vaccination, the model estimated about 33.3 million symptomatic cases, 538,000 hospitalizations, and 42,000 deaths annually, compared with about 28 million cases, 397,000 hospitalizations, and 30,000 deaths with vaccination. That equals a reduction of roughly 16% for symptomatic illness and 26% to 27% for hospitalizations and deaths. The largest reductions in hospitalizations and deaths occurred among adults aged 65 years or older.

Shah described the approach as a simple calculator that supports economic modeling and helps identify gaps in protection. The analysis did not quantify uncertainty and did not account for transmission dynamics or health inequities, and the estimates are national. Shah said a follow-up study will refine the estimates and quantify the uncertainty around them.

Long-Term Symptoms After 3 Infections

Avantika Srivastava, MS, a research scientist at the City University of New York (CUNY) Institute for Implementation Science in Population Health who is also affiliated with the CUNY School of Public Health and Health Policy, presented findings from Project PROTECTS, a prospective community-based cohort of US adults aged 18 years or older.⁵ The study was a collaboration between CUNY and Pfizer and was funded by Pfizer. Participants used at-home rapid antigen tests to identify SARS-CoV-2, influenza, and RSV infections, then completed monthly symptom questionnaires through 180 days.

The team operationalized a case definition for post-acute sequelae of respiratory infection (PAS-RI). Participants met the definition if they reported 2 or more symptoms in at least 3 months of a 4-month window and had not returned to their usual health by the end of that window. From a cohort of 6115 participants, the analysis included 1267 with infections from July 2024 to January 2026: 773 with SARS-CoV-2, 385 with influenza, and 109 with RSV.

PAS-RI prevalence at day 180 was similar across pathogens, at 14% for influenza, 16% for SARS-CoV-2, and 17% for RSV. At month 6, participants with PAS-RI continued to report fatigue, cognitive symptoms, and respiratory symptoms, and all of the participants with RSV and PAS-RI, a small subgroup, reported fatigue. Over half of participants with PAS-RI still reported 8 or more symptoms at month 6. The cohort was highly vaccinated, with about 61% of influenza infections and 40% of SARS-CoV-2 infections occurring in participants up to date on vaccination, compared with about 14% for RSV.

Srivastava noted that the 15-symptom list may raise the chance of reporting 2 or more ongoing symptoms, that high symptom burden also appeared in participants who did not meet the definition, and that the symptom scale was built around COVID-19. Rapid antigen tests for RSV have shown low sensitivity in adults in meta-analyses,^6,7 which could affect the unadjusted prevalence comparisons, although the specific tests used in the study were not described. Srivastava said later analyses will adjust the estimates.⁵

COVID-19 Vaccine Effectiveness in Older Adults

Nevena Vicic, MSc, director of global medical affairs for infectious disease at Moderna, presented real-world data from 3 US analyses of 2025-2026 COVID-19 vaccines, including the first evaluation of Moderna's mRNA-1283 vaccine.⁸ Vicic is a Moderna employee, and the studies were funded by Moderna. Slides noted that Moderna-affiliated authors hold stock or stock options in the company and that the Medicare analysis was run by Mathematica employees who received Moderna funding.

The first analysis used Veradigm electronic health record and claims data on adults aged 65 years or older vaccinated between August 28, 2025, and January 24, 2026. About 233,000 recipients of mRNA-1283 and about 423,000 recipients of Pfizer-BioNTech's BNT162b2 were each matched 1:1 with unvaccinated adults. The mean age was 76 years, and nearly 85% had received a vaccine the prior season.

Effectiveness of mRNA-1283 against COVID-19 hospitalization was 59.3% in adults aged 65 years or older and 66.9% in those aged 75 years or older. Effectiveness against medically attended COVID-19 was 42% and 50.2%, respectively. For BNT162b2, effectiveness was 48.3% against hospitalization and 41.2% against medically attended COVID-19 in adults aged 65 years or older. The confidence intervals for the 2 vaccines overlapped, and median follow-up was about 70 days.⁹

“They should not be interpreted as any kind of head-to-head comparison between the 2 vaccines,” Vicic said, noting that each analysis had its own comparator cohort.⁸

A second, unpublished analysis used Optum claims data and a target trial emulation, a design that mimics a randomized trial using observational data, of more than 384,000 mRNA-1283 recipients aged 18 years or older, with follow-up through April 30, 2026. Effectiveness against hospitalization was 60.3% overall, 61.4% in adults aged 65 years or older, and 60.4% in those aged 75 years or older, and remained positive through 180 days of follow-up. Effectiveness against medically attended COVID-19 was 30% through 180 days.

A third, unpublished analysis matched nearly 1.4 million Medicare fee-for-service recipients of mRNA-1283 1:1 with a similar number of unvaccinated beneficiaries and found 62.8% effectiveness against COVID-19 hospitalization and 46.8% against medically attended COVID-19 for mRNA-1283. Exploratory estimates were about 26% for all-cause hospitalization, 26% for cardiorespiratory hospitalization, and 27% for major adverse cardiovascular event hospitalization, which Vicic said should be interpreted cautiously because of possible residual confounding. Median follow-up in this analysis was 82 days.

References

1. Andermann T, Gensheimer K, moderators. Tri-Demic: combating the triple threat of COVID, flu, RSV. Oral abstract session presented at: IDWeek 2026; October 22, 2026; Washington, DC.
2. Woodruff RC, Milucky J, O'Halloran A, et al. 30-day mortality among adults hospitalized with laboratory-confirmed respiratory syncytial virus infection. Presented at: IDWeek 2026; October 22, 2026; Washington, DC.
3. Godoy Collazos CY, Campbell JI, Pelton SI. Burden and severity of common pediatric respiratory viruses across years in a multicenter Massachusetts cohort. Presented at: IDWeek 2026; October 22, 2026; Washington, DC.
4. Jansen I, Rua T, Smith N, et al. Estimating the burden of influenza in the absence of vaccination in the US. Presented at: IDWeek 2026; October 22, 2026; Washington, DC.
5. Srivastava A, Penrose K, Robertson M, et al. Post-acute sequelae following SARS-CoV-2, influenza, and RSV infection among adults in the prospective community-based Project PROTECTS cohort. Presented at: IDWeek 2026; October 22, 2026; Washington, DC.
6. Chartrand C, Tremblay N, Renaud C, Papenburg J. Diagnostic accuracy of rapid antigen detection tests for respiratory syncytial virus infection: systematic review and meta-analysis. J Clin Microbiol. 2015;53(12):3738-3749. doi:10.1128/JCM.01816-15
7. Onwuchekwa C, Moreo LM, Menon S, et al. Underascertainment of respiratory syncytial virus infection in adults due to diagnostic testing limitations: a systematic literature review and meta-analysis. J Infect Dis. 2023;228(2):173-184. doi:10.1093/infdis/jiad012
8. Vicic N, Bogdanov A, Hensler H, et al. Effectiveness of 2025-2026 mRNA-1283 and BNT162b2 COVID-19 vaccines against COVID-19-related hospitalizations and medically attended COVID-19 among adults aged ≥65 years in the United States. Presented at: IDWeek 2026; October 22, 2026; Washington, DC.
9. Vicic N, Bogdanov A, Hensler H, et al. Interim effectiveness of 2025-2026 mRNA-1283 and BNT162b2 COVID-19 vaccines against COVID-19-related outcomes among adults aged ≥65 years in the United States. Infect Dis Ther. 2026;15(9):2307-2318. doi:10.1007/s40121-026-01395-4