Two new studies on respiratory syncytial virus (RSV) find that older adults, those residing in rural areas, those with certain conditions, and nursing home residents had higher rates of hospitalization and that most household transmission is from children to adults.
### Patients 75 and older accounted for 76% of hospitalizations
For the first [study](https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2025.30.10.2400364), published late last week in _Eurosurveillance_, researchers in Spain analyzed electronic medical records to determine the RSV hospitalization rate of four cohorts involving a total of 165,000 people aged 60 years and older in the Navarre region of northern Spain over four respiratory virus seasons (2016-17 to 2019-20).
"Knowing how often this infection requires hospitalisation and what the risk factors are, is essential to plan preventive measures and recommendations for the new RSV vaccines," the study authors wrote.
On average, RSV caused 84.7 hospitalizations per 100,000 people older than 60 years over all four seasons.
Average annual hospitalization rates were similar among men and women (92.2 vs 78.4 of 100,000 people), but differences by age were relevant. People 75 years and older made up 38.6% of the study population but accounted for 76.1% (414) RSV hospitalizations. The rate was higher than 100 per 100,000 in adults 75 years and up, especially among those ages 85 to 89 years (243.9/100,000) and 90 to 94 (268.7/100,000), and in rural versus urban residents (105.4 vs 68.9/100,000).
### **Priority groups for vaccination**
RSV hospitalization rates were 18.8 and 131.1 per 100,000 among people without and with a high-risk condition, respectively. Factors tied to the highest rates of RSV hospitalization were blood cancers (399.0/100,000), nursing home residence (366.7/100,000), functional dependence (348.1/100,000), and chronic obstructive pulmonary disease (COPD; 269.7/100,000). The rate was also higher than 200 per 100,000 among patients with asthma, cardiovascular disease, chronic kidney disease, and dementia.
> A first priority population group for preventive measures, because of their very high rate of RSV hospitalisation, ...includes people 60 years and older with haematological cancer or nursing home residence, people older than 74 years with COPD or functional dependence, and people older than 84 years with asthma or cardiovascular disease.
The combination of age category and each specific high-risk condition led to important differences in the average yearly RSV hospitalization rate. The rate was lower than 40 per 100,000 in people younger than 85 years without high-risk conditions, but in some age-groups, people with blood cancer, nursing home residence, functional dependence, COPD, asthma, cardiovascular diseases, and diabetes reached a rate higher than 300 per 100,000.
"A first priority population group for preventive measures, because of their very high rate of RSV hospitalisation (>0.3% per year), includes people 60 years and older with haematological cancer or nursing home residence, people older than 74 years with COPD or functional dependence, and people older than 84 years with asthma or cardiovascular disease," the researchers wrote.
"This strategy would involve immunising only 13% of people aged 60 years and older, but would address half of RSV hospitalisations in this age group," they added. "The prevention of these hospitalisations would appreciably reduce the likelihood of hospital resources being overwhelmed during the weeks with high RSV circulation and would probably prevent a high proportion of RSV-related deaths."
### **Household infection fueled by kids younger than 12**
To determine factors influencing RSV household transmission, as well as genomic diversity, in the United States, University of Washington at Seattle-led researchers collected nasal swabs, surveys, and antibody data from 3,100 people living in 200 households from June 2022 to May 2023. They also performed whole-genome viral sequencing.
> Viral genome sequencing demonstrated that multiple household infections within a 14-day period are likely due to within-household transmission.
The [findings](https://academic.oup.com/cid/advance-article-abstract/doi/10.1093/cid/ciaf048/8078353) were published late last week in _Clinical Infectious Diseases_.
RSV was diagnosed in 310 (10%) participants living in 200 (20%) households. Most (94%) primary infections were symptomatic. In total, 37 cases of potential secondary transmission were identified within 14 days of an index case (10%). The median ages of participants with index or secondary infections were 6 and 35 years, respectively, with 89% of index patients aged 6 months to 12 years.
Risk factors for RSV transmission included index case viral detection 1 week or more after an index case and a household contact age of 12 years or younger. Of 120 sequenced specimens, the main strains were A.D.5.2 (37 cases) and A.D.1 (30).
Sequenced viruses from households with two or more RSV infections were similar when they occurred within 14 days of the primary infection (mean pairwise difference, 4; 17 households), compared with those occurring after 14 days (mean pairwise difference, 137; 2 households).
"Most RSV household transmission occurs from infants and young children to adults," the authors wrote. "Viral genome sequencing demonstrated that multiple household infections within a 14-day period are likely due to within-household transmission."