- Top of page
- Funding Sources
In adults 50 years and older, we found substantial rates of hospitalization associated with both RSV and HMPV detection, about 10–20 per 10 000, which is very similar to annual influenza-associated hospitalization rates reported for this age group.[1, 6-11] ED visit rates for RSV and HMPV in older adults were approximately 2 times higher than hospitalization rates. In adults aged 18–49 years, rates of hospitalization associated with RSV and HMPV were about 2 per 10 000 and ED rates were 5–6 times higher.
There have been few previous studies of the rates of hospitalizations and ED visits due to RSV in adults and almost no prior estimates for HMPV. Two studies estimated RSV hospitalization rates based on models of seasonal trends in cardio-respiratory hospitalizations and influenza and RSV surveillance data. Muloolly, et al., estimated annual RSV hospitalization rates among adults without identified high-risk conditions to be 0·2, 0·9, and 10·6 per 10 000 persons aged 18–49, 50–64, and ≥65 years. Among adults with high-risk conditions, rates were estimated to be 1·2 (95% CI [−0·9, 3·4]), 7·2 (95% CI [1·9, 12·4]), and 44·0 (95% CI [31·0, 57·0]) per 10 000 persons 18–49, 50–64, and ≥65 years, respectively. The rates we determined for all adults were similar to their estimates for those with high-risk conditions in adults aged <65 years, and in between their estimates for low and high risk for those aged 50–64 years. Zhou, et al., using a model similar to that of Muloolly, estimated average annual RSV hospitalization rates over 15 consecutive years to be 1·28 and 8·61 per 10 000 adults aged 50–64 and 65 years and older, respectively. These rates are comparable to those estimated by Muloolly for low risk persons, but considerably lower than our rates for all adults.
The RSV hospitalization rate of 11/10 000 for adults 50 years and older is similar to that of 15/10 000 for the same age group, reported in our prior three-year study, which also used direct measurement of RSV. Estimated RSV hospitalization rates in both modeling studies were considerably lower. RSV is difficult to diagnosis in adults because antigen testing sensitivity is so poor. Therefore, estimation of disease due to RSV is likely low if based on historical clinical laboratory testing. Our RSV hospitalization rates for adults 50 years and older were similar to influenza hospitalization rates during the same period. Falsey, et al., reported that a similar proportions of adult patients hospitalized with acute respiratory illness (ARI) had influenza and RSV detected when using RT-PCR to diagnose RSV; however, they did not estimate hospitalization rates.
Little is known about HMPV in adults. The virus was first described in 2001 and was originally thought to cause respiratory infections only in children. Subsequently, significant disease in adults with mortality as high as 50% in frail elderly residents has been documented. We were able to show that in adults, hospitalization rates increased significantly with age. This new knowledge provides evidence that HMPV causes significant morbidity at the extremes of age.
Because this study was performed during the H1N1 influenza pandemic year, comparison of rates of ED visits and hospitalizations due to pandemic influenza can be directly compared to RSV and HMPV ED visit and hospitalization rates. Using similar methods, Jules, et al.,  estimated 14·3 hospitalizations per 10 000 adults aged ≥50 years for influenza during May 2009 through March 2010, and Self, et al.,  estimated 36 ED visits per 10 000 (95% CI [10·5, 77·7]) adults aged ≥50 years during the same time period. Both studies found rates that approximate the rates we estimated for both RSV- and HMPV-associated illness.
Our study has several limitations. Our sample size was small, making it difficult to draw comparisons of the clinical presentations associated with each virus. In addition, because this study took place during the novel influenza A H1N1 pandemic and was only 1 year, rates may not be representative of all years. However, annual hospitalization rates for adults 50 years and older were similar to those we had reported for the prior 3 years for RSV. Furthermore, this study only included the middle Tennessee region and relied on the assumption that the percent of infections in persons we enrolled would be similar to percent in persons with acute respiratory illness ED and hospital discharge diagnoses. Despite these limitations, studies such as this that use direct patient testing to estimate rates are needed to help determine the validity of modeling studies.
RSV and HMPV are associated with a significant number of ED visits and hospitalizations in adults ≥50 years of age, especially in those ≥65 years of age. Previous studies have shown that older adults with influenza do not always display typical symptoms, making the sensitivity of clinical diagnosis low.[18, 19] As the clinical presentations of RSV and HMPV are similar to that of influenza, awareness and clinical suspicion, in addition to the use of sensitive molecular diagnostic methods, are needed to detect and distinguish these infections. In addition, rates of hospitalization associated with RSV and HMPV were higher than those due to influenza in adults ≥65 years of age that we reported in our previous 3-year study. This is likely due in part to high vaccination of the older population in the United States. To date, efforts to limit the deleterious effects of RSV and HMPV infection have involved mainly infection control measures, including good hand hygiene practices. With such a high burden of illness, there is significant need for vaccines and treatment for both RSV and HMPV.
In summary, during 2009–2010, when pandemic influenza was circulating, hospitalization rates for RSV and HMPV in those ≥50 years of age were similar to those reported for influenza, about 10 per 10 000. ED rates were approximately 2 times higher than hospitalization rates. For those aged 18–49 years, hospitalization rates were about 2 per 10 000 with ED rates five to six times higher.