Our study, which assessed the presence of 10 common respiratory viruses in a cohort of children evaluated for acute respiratory disease at a single medical center, found that children with only a single virus detected were more likely to have severe illness as measured by inpatient and ICU admissions, hospital stays greater than 3 days, and need for supplemental oxygen than children who had multiple respiratory viruses detected. In contrast, we found that rates of fever and abnormal radiographic findings were similar between children with single and multiple virus illnesses. The relationships in our study between multiple virus detection and clinical disease were consistent across age groups and both healthy and chronically ill children. Notably, we found the highest prevalence of multiple virus infection among children 6 to 23 months of age. While younger infants (birth to 5 months of age) are undergoing exposure to their first viral season and are likely experiencing a primary infection, these older children (6 to 23 months) may be more likely to have had previous exposures to these respiratory viruses in an earlier season. Perhaps the increased severity(22
) and heightened immune response during a primary infection in the youngest children may discourage colonization by a second viral pathogen, leading to lowered prevalence of multiple viruses in this youngest group.
The relationship between viral load and viral coinfection differed by virus. For example, RSV, FluA, hMPV, and PIV3 were present at consistently high quantities regardless of the presence of other viruses in the sample. By contrast, the viral load for PIV 1 and AdV differed substantially depending on whether single or multiple viruses were detected. These associations may offer insight as to which virus predominates in a multiple virus illness. Notably, 82% of coinfections consisted of one virus from the group with consistently high viral load (RSV, Flu A, hMPV, or PIV3) combined with an alternate virus (CoV, PIV 1, or AdV). This may suggest a possible model for virus coinfections that include one predominant virus and one virus that is present at a lower quantity and does not confer increased severity. The presence of low-quantity CoV and AdV in a coinfection merits further study, especially given the finding that the prevalences of these two viruses in asymptomatic individuals are second only to rhinovirus(16
Similar to our findings, Canducci et al. reported a lower prevalence of multiple virus illnesses in the youngest infants and an increased severity of disease (as measured by increased hospital stay and prevalence of hypoxia) in RSV-only illnesses compared to those with both RSV and hMPV detected(4
). However, our results are in contrast to several studies which found increased hospital admissions (3
), intensive care unit admissions(9
), and duration of hospitalization and need for supplemental oxygen(6
), or, recently, increased presence of fever(14
) for illnesses with multiple virus detections compared to those with single virus detections. Some of these differences may be due to variation in the age ranges studied(3
) or the restriction to specific respiratory illnesses (5
) or to RSV or RSV/hMPV coinfection in particular (6
). Other studies have reported no association between respiratory illness severity and multiple virus detections(7
), including very recent reports(15
It is perhaps counterintuitive that the presence of multiple species at a single timepoint is not associated with more severe disease. An immune response to an infection with the first virus could modify the disease severity of a subsequently acquired virus, potentially by the induction of interferon and other anti-viral response modifiers. While our sample size was too small to determine whether our results reflect the number of viruses detected or the characteristics of specific virus combinations, we found our estimates to be largely consistent when stratified by virus, although not statistically significant.
Although our study is among the largest to evaluate this question using clinical data so closely tied to the time of virus testing (within 24 hours), there are several study limitations. Our study design precluded identifying incident viral infections, limiting our ability to define acquisition of each virus in relation to symptom onset. Similarly, the course of the illness prior to presentation and collection of the respiratory specimen could not be reliably verified. Although viral load generally decreases from illness onset(25
), our study was able to compare viral loads at the time of illness symptoms that warranted medical attention. The total number of viral coinfections was likely underestimated because we did not evaluate the presence of rhinovirus and bocavirus, viruses known to be associated with prolonged viral shedding and detection during both symptomatic and asymptomatic periods(24
). Thus, some cases of single infection in our study could be classified as multiple infection in studies which include these viruses. We were unable to determine rhinovirus viral load due to the large number of serotypes present, making it difficult to differentiate asymptomatic shedding from active infection. We believe these particular pathogens are better studied in prospective settings where baseline viral shedding can be examined (24
). We also did not assess potential bacterial pathogens, although the comparable rates of antibiotic use between the two study groups () indicated that suspected or documented bacterial infection is an unlikely confounder.
In our study population of children evaluated for acute respiratory infection, we observed that multiple virus combinations of RSV, hMPV, PIV, Flu A, and AdV were more common in children 6 to 24 months of age. Lessened illness severity was observed among multiple virus illnesses. We determined the relationship between viral quantity and coinfections to be virus-specific, and we hypothesize that viral load may serve as an important clue as to which virus in a mixed-infection may have a greater influence on the clinical severity of the illness.