Respiratory syncytial virus infection-associated hospitalization in adults: a retrospective cohort study

RSV is the most common cause of serious lower respiratory tract infection in infants and young children, but immunity to RSV is incomplete and infections recur throughout life [1]-[10]. In adults, RSV was long thought to cause primarily mild upper respiratory tract infections. Estimates of adult hospitalization, morbidity and mortality attributable to RSV have varied, likely due to a lack of routine testing for respiratory viruses in adult acute respiratory illness and the poor sensitivity of many test methods when performed [11]. Over the last several decades, however, studies have shown that RSV causes more severe infection in the elderly, and in adults with immune compromising and chronic cardiopulmonary conditions [3],[4],[9],[12]-[19]. There now is growing evidence that rates of health care utilization, hospitalization, morbidity and mortality among adults with RSV infection may be similar to those observed with influenza infection [9],[20]-[25].

More recent studies also suggest that RSV causes greater morbidity in younger and healthier adult populations than previously appreciated [4],[26]-[28]. A prospective study of RSV infections in previously healthy working adults aged 18–60 found that 26% had LRTI symptoms and 38% required time away from work during their illness [4]. In studies of previously healthy military recruits, 11-14% with respiratory symptoms were found to have RSV and most were unwell enough to require time away from duty [26],[27]. Previously healthy adults have also been shown to have altered airway reactivity for weeks following RSV infection [29].

In order to better understand the burden of adult disease due to RSV, we undertook a retrospective study of hospitalization due to RSV infection in adults in four acute care hospitals in Toronto, Canada, during a winter season in which routine testing for RSV was added to all specimens submitted for respiratory virus testing.

This study contributes to the growing body of evidence documenting that RSV is a serious pathogen in adults. In our cohort, adult hospitalization associated with RSV infection was not uncommon, and it was associated with a substantial rate of complications and mortality. Although the majority of adults were older, more than one-third were less than 65 years of age.

To complement the findings of this study, we conducted a literature search using MEDLINE (1946- December 2013), EMBASE (1947- December 2013), and PubMed (through December 31, 2013) databases. The references for each publication were reviewed to search for additional studies. After excluding studies with nosocomial cases, those focused specifically on ICU patients, or those that did not specifically describe RSV infections, we identified six previous cohort studies of RSV hospitalization in adults (Table 3) [9],[20],[21],[25],[28],[30]. One of these cohorts had data in more than one publication [9],[31].

Despite substantial differences in time, geography, and means of diagnosis of RSV infections, the combined results provide a consistent and cohesive view of the characteristics of these patients and their disease (Tables 3 and 4). The majority of patients in these studies were older, and both underlying pulmonary and cardiac disease were common. There were however, differences in the distribution of underlying conditions. For instance, the proportion of immunosuppressed patients was noticeably greater in our study and in that of Widmer et al. than that of Lee et al [21],[25]. This difference may be due to differences in patient populations cared for in study hospitals, definitions of immunosuppression, or criteria for RSV testing. In our study subjects were considered immunosuppressed if they had a hematologic malignancy, solid tumor on chemotherapy, chronic renal failure, liver cirrhosis, or if they were on immunosuppressive therapy (see Table 1). Lee et al. do not describe the conditions considered immunocompromising, and Widmer et al. appear to have included all malignancies, as well as splenectomy and HIV/AIDS [21],[25].

Adult patients with RSV infection often experience symptoms of nasal congestion, productive cough, and low-grade fever that may or may not be followed within a few days by dyspnea and wheezing. Rates of runny nose or nasal congestion (22% versus mean of 58%) and wheezing (38% versus mean of 64%) were lower in our study compared to other studies [4],[9],[20],[21],[25],[28]. This may be a limitation of our retrospective study design as treating clinicians may have failed to document these symptoms. Rates of other symptoms were similar to those observed in previous studies in which they could not facilitate differentiation from other respiratory illnesses.

Adult patients with RSV infection often present with an exacerbation of underlying cardiac or pulmonary disease, such as asthma, COPD, or CHF [32]-[34]. In our study, the admitting diagnosis for 23 (27%) patients was recorded as an exacerbation of underlying lung disease or cardiac disease.

Lower respiratory tract complications including pneumonia were common in our study and in previous studies. Radiographic appearance of pneumonia associated with RSV infection has been noted to be variable, ranging from the absence of detected abnormality to small and unilateral pulmonary infiltrates, symmetrical or asymmetrical bilateral pulmonary infiltrates, or lobar consolidation [7],[20],[31]. This variability was also seen in our study.

In our cohort potential co-pathogens were identified in 13% of cases, similar to the rate observed in other published studies [9],[25],[28]. Vikerfors et al. observed much higher rates of co-pathogens, but their positive cultures for co-pathogens included results from nasopharyngeal samples rather than sputum or blood cultures as collected in other studies [20],[25],[30],[32]. Widmer et al. report no co-infections in their cohort, although it is unclear whether pathogens present in non-sterile site specimens would have met criteria for co-infections in their patients [21].

Requirement for invasive mechanical ventilation, ICU care, mortality rates, and hospital lengths of stay were substantial and consistent across studies [9],[20],[21],[25],[28],[30]. Vikerfors et al. observed a lower case fatality rate, which may be a result of the lower prevalence of underlying illness in their cohort [30].

Antibiotics were commonly prescribed in our cohort, as was the case in two other large studies of adult patients hospitalized with RSV [20],[25]. While co-infections have been noted to occur, these numbers highlight the challenges of optimal use of antibiotics when viral respiratory infections have been diagnosed, but bacterial pathogens not identified. Due to difficulties with diagnosis of co-existent bacterial infections, the most effective way to reduce this overuse may be through prevention of RSV infection via enhanced infection control measures, development of effective prophylaxis or, as noted in a recent commentary, vaccination [35].

Advantages of our study are that we have included adults of all ages, and that testing was performed by RT-PCR. Previous testing for RSV has been done using antigen immunoassays or culture, which lack sensitivity, or using acute and convalescent serology, which cannot identify fatal cases, and may also lack sensitivity in immunocompromised patients or the frail elderly [11],[36]-[38]. Most previously published studies using RT-PCR have also included only older adults [9],[21],[25]. Our study demonstrates that a substantial fraction of adult patients with RSV infection are not elderly; however, virtually all did have chronic underlying illnesses.

This study has several limitations. It took place in only one geographic location during only one season, and may therefore not be generalizable to other seasons or locations. Testing of patients was not systematic in all hospitals, and we may have failed to identify some infected patients. Our use of RT-PCR for diagnosis may have resulted in some false positive tests, although the Simplexa Flu A/B & RSV assay (Focus Diagnostics, Cypress, CA) used for diagnosis has good performance characteristics and high specificity [39],[40]. Finally, RSV infection may have been incidental and the requirement for hospital admission due to other causes; this may be a particular issue in immunocompromised patients, who may shed RSV for prolonged periods of time [18].

In conclusion, RSV infection in hospitalized adults is associated with substantial morbidity and the need for extended hospital stay, ICU care and mortality in adults of all ages with chronic underlying conditions. The data produced from this study are useful in supporting existing data from other areas, and reinforce that RSV infection in hospitalized adults is a global issue. There is need for ongoing research and development of effective RSV prophylaxis, vaccines and treatments.