The Malaysian Ministry of Health reports that respiratory infections are one of the principal causes of hospitalization (9.4% in 2009), and pneumonia is one of the ten principal causes of death (10.4% in 2009) in public hospitals . However, as diagnostic capacity for respiratory viruses is extremely limited, little is known about the epidemiology of viral RTIs in Malaysia, which are well known to have high financial and clinical impact [11–13]. This retrospective study of respiratory viruses at a teaching hospital over the past 27 years is the most comprehensive study carried out in Malaysia. Our findings support a previous local study carried out over a year, which showed that RSV was the most commonly detected respiratory virus, followed by parainfluenza viruses, influenza viruses and adenovirus . We found that this trend has remained for the past three decades.
In our laboratory, the average virus detection rate (annually) by IF and/or virus isolation was 25.4% in all respiratory samples tested. This considerably underestimates the true burden of respiratory viruses, as lack of resources precluded the routine use of more sensitive diagnostic methods such as shell vial culture  and molecular detection methods, and testing of a wider range of viruses. Respiratory virus detection rates of more than 50% can be achieved with molecular detection [15, 16]. Other studies in tropical countries have reported that emerging respiratory viruses such as metapneumovirus (5.3-5.4%) [17, 18], coronaviruses (0.6%) , bocavirus (8.0%)  and human rhinovirus C (12.8-30%) [20, 21] also contribute substantially to morbidity.
Most studies, including those in Asia, show that the most common causes of respiratory viral infections are RSV and rhinoviruses [3, 22–24]. Our laboratory does not routinely detect rhinoviruses, but RSV was the most frequently detected respiratory virus, particularly in infants less than one year old. This suggests that maternal antibodies were ineffective in preventing RSV infections. Older children may be less prone to RSV infection due to the maturity of their immune system or natural immunity obtained through repeated infections of RSV. The burden of RSV in young infants in tropical countries, including Malaysia, emphasizes the likely global benefits of developing a safe and effective vaccine for RSV. Our data also showed a slight male preponderance (59.8%) in children with respiratory viral infections, consistent with other studies .
In temperate countries, respiratory viral infections have clear seasonal variations with most cases occurring during winter. Possible explanations for this include seasonal variations in host immune response to infection , climatic factors such as ambient temperature and low relative humidity which increase viral survival in the environment , and changes in host behaviour which increase contact with others. Seasonal trends are more variable in the tropics, with some studies showing that respiratory virus infections occur all year round, while some show clearer seasonality.
Located in Southeast Asia, Kuala Lumpur (latitude 3°N) has a mean annual temperature of 27.4°C, constant high relative humidity (> 71.6%), and heavy rainfall throughout the year. In our study, the common respiratory viruses were detected throughout the year. The RSV annual epidemics are strongly seasonal, while seasonality is less clear for influenza, parainfluenza, and adenovirus, which may be due to the smaller number of cases. For adenovirus, most studies show sporadic occurrence without any seasonal trend [28, 29]. In the tropics, influenza occurs all year-round , similar to our data. In Singapore, parainfluenza virus type 3 was the most commonly detected parainfluenza virus type, with seasonal peaks in February-March similar to our results .
In Kuala Lumpur, RSV peaks at the end of the year. In contrast, in neighbouring Singapore (latitude 1°N) and Lombok, Indonesia (latitude 8°S), RSV peaks around March-August [29, 30]. In most RSV studies carried out in temperate countries, where temperature varies widely between seasons, temperature is highly inversely correlated to RSV cases [4, 31]. We also found a negative correlation between RSV and temperature in tropical Kuala Lumpur, where the much reduced temperature variation may explain the weaker correlation (correlation coefficient of -0.116). As previously observed in Indonesia , we also found that the number of rain days was significantly associated with RSV cases, but not rainfall. Malaysia experiences brief, intense showers of rainfall, as well as prolonged episodes of light rainfall. Therefore, the number of rain days may have a greater influence than the absolute amount of rainfall on behaviours such as children staying indoors, thus increasing close contact and indoor transmission of respiratory viruses .
While humidity was inversely correlated with RSV in our study, as with respiratory infections in Singapore , a positive correlation was reported in Lombok, Indonesia . There are well recognized inconsistencies in reported associations of respiratory infections with meteorological factors in different settings . Clearly, seasonality of respiratory viruses in the tropics cannot be explained by climatic factors alone, as associations vary widely between geographic locations . There are likely to be multiple, poorly understood interactions between climatic, environmental and behavioural factors, and complex interplay between different circulating viruses and population immunity. The local epidemiology of respiratory viruses needs to be determined for each site, for effective planning of interventions such as potential vaccines.
This study has several limitations. As the data was collected retrospectively, there was some missing data. We were unable to obtain dates of admission for the earlier patients, and thus could not differentiate between nosocomial and community-acquired viral infections. However, we have previously found that nosocomial cases made up 25/157 (15.9%) influenza cases from 2002-2007  and 17/146 (11.6%) RSV cases from 1989-2010 (Khor CS, Sam IC and Chan YF, unpublished observations). These nosocomial rates of respiratory viral infections are similar to previously published rates of 12.1-13.8% [36, 37], thus supporting the likelihood that the majority of infections seen in our study were community-acquired. Over 27 years, there may have been changes in physicians' practices, such as criteria for taking specimens and admitting patients, and types of samples collected. Furthermore, with a relatively low IF sensitivity of 55.8% using virus isolation as the gold standard, there is likely to be considerable underdiagnosis of viral infections compared to molecular methods . Nevertheless, despite these limitations, these unusually extensive records kept over 27 years provide valuable insight into current and historical respiratory virus epidemiology in a tropical Southeast Asian country, particularly with the limited available facilities for virus diagnosis. To extend these findings, we are currently carrying out prospective studies using molecular methods to detect a wider range of respiratory viruses.