Among 502 hospitalized children with ILI in Stockholm during the H1N1 pandemic in 2009, 61.6% had a respiratory virus detected by PCR and 16.5% were infected with H1N1. We assessed the impact of co-infections among these children over time and in relation to severity of disease. Although the retrospective study design has limitations with clinical data not being systematically collected, the strength of the study was the uniquely high sampling coverage with extended viral PCR-analyses in admitted children, as a result of the hospital policy during the H1N1 pandemic. The study thus provides a rather complete assessment of the viral panorama in hospitalized children with ILI in Northern Stockholm during this six-month period. Interestingly, among the positive samples only 26.9% were positive for H1N1, suggesting that several other respiratory viruses caused ILI.
Co-infections with two or more viruses were detected in 14.6% of the positive samples, and 14.5% of the H1N1 infected children were positive for an additional virus. Some viruses were more often found in combination with another virus e.g. HBoV and HCoV, while H1N1, HEV, HRV, PIV2 and RSV were mostly detected as single respiratory viral infections. Furthermore, when H1N1-negative children where compared to H1N1-positive children, HAdV, HCoV, HEV, HRV, RSV and PIV1 where more often detected in H1N1-negative children which suggest an association of these viruses with ILI. Although attempts have been made to attribute diseases to individual viruses by quantitative assessments of the PCR results (CT-values) 
the single time point measurements cannot distinguish which virus that causes the disease. Our results indicate that the causal role of other respiratory viral infections needs to be considered when estimating morbidity attributed to H1N1.
Co-infections might be a result of acquiring two viruses concurrently. Nonetheless, detection of two viruses might rather be a result of a combination of a newly acquired virus together with an asymptomatic infection or shedding from a recent symptomatic infection. HRV is commonly detected in asymptomatic individuals and viral shedding is known to occur for several weeks after an infection 
, and can be prolonged in asthmatic children 
. Co-infections were here mainly detected in children 1–6 years of age, indeed the age group attending daycare in Sweden and known to be at high risk of respiratory viral infections. The increasing frequency of co-infections during the study period might also represent accumulation of persistent viral shedding during the autumn.
The children infected with H1N1 were older than children infected with HCoV, HEV, HRV and RSV. H1N1-positive children where also more often treated in the PICU compared to children with other viral infections indicating the pathogenicity of H1N1 in this population. Detection of additional viruses in H1N1-positive children were, as opposed to previous findings 
, not associated with the severity of disease. One explanation could be the low rate of RSV infections in our material since the study period included only the beginning of the RSV season and hence none of the H1N1-positive children were co-infected with RSV. In South America the two epidemics of H1N1 and RSV coincided with a reported high morbidity and a high frequency of severe disease 
. Unfortunately, bacterial cultures where not systematically performed during the study period. This might bias our results as it has been reported that both influenza and HRV are associated with increased risk for bacterial infections, such as Streptococcus pneumoniae
The importance of viral interference for the spread of respiratory disease needs further understanding, as does the role of co-infections in terms of severity of disease. Although a few epidemiological studies have assessed the role of viral interference during the H1N1 pandemic and pointed out HRV as a potential protective factor for H1N1 infection, the results and conclusions drawn have been diverging 
. As opposed to previous reports of low prevalence of H1N1 (or even reduction of H1N1) when HRV was prevalent 
, our data indicated co-circulation of the two viruses and 6.0% of the children with H1N1 were also positive for HRV. Our observation is in line with a recent report from Beijing 
The high detection of HRV during the study period might reflect seasonality of HRV disease, but could also be an effect of increased sampling due to the fear of influenza A(H1N1)pdm09 during the pandemic. Indeed, the frequency of HRV-detection seems to closely follow the sampling frequency in children at our hospital throughout the year (unpublished data).
The viruses detected in these hospitalized children are likely to only partly reflect the spread of different viruses in the catchment area during the study period. Our data can therefore not be used to address whether viral interference prevented the spread of H1N1 in Stockholm. It might be tempting to use hospital data to calculate the observed number of co-infections in relation to expected numbers based on the assumption that infections with different viruses occur independently of each other within the total number of hospital specimens 
. However, these estimates do not take into account the incidence in the catchment population nor the hospitalization rate of the children infected by the respective viruses.
Interestingly, in a recent intervention study, children vaccinated with inactivated trivalent influenza vaccine carried a higher risk of future infections with respiratory picornaviruses compared to children who received placebo 
, suggesting possible interactions between influenza and other viruses. Sweden adopted countrywide vaccination against H1N1 reaching a coverage of approximately 60% of the population, with the first doses distributed in the middle of October 2009 
which might have affected the distribution of respiratory viruses in the studied population.
Our data indicate that in addition to H1N1, ILI during the pandemic was associated with a large number of other respiratory viruses. Viral co-infections in children with H1N1 were not associated with severity of disease; moreover the findings of additional viruses in H1N1-positive children need to be taken into account when attributing morbidity to H1N1. To further investigate interaction of respiratory viruses, population-based prospective studies with longitudinal sampling would be highly informative. Moreover the biological mechanism in viral interference needs to be better understood.