A recent multicenter, prospective registry study found that adenovirus was the third-leading cause of viral infection among community-acquired pneumonia (CAP) patients in China, after influenza viruses and respiratory syncytial virus . An increasing number of HAdV outbreaks have been reported in China in recent years [7, 12, 14, 18]. But because there is no national surveillance system for HAdV in China and there is often no difference in symptom clustering between viral and bacterial infections, the diagnosis of HAdV is often underestimated .
The present study recorded the epidemiological distribution of circulating HAdV strains among hospitalized patients with respiratory tract infections (RTIs) between 2017 and 2019 in Guangzhou City, China. In this study, the overall positive rate of HAdV was 2.4%, which is consistent with the positive rate (2.0%-6.1%) found in hospitalized patients with acute viral respiratory infection in other recent reports [3, 19–21]. However, two previous studies conducted in the Northern part of China showed a higher HAdV prevalence (10.4%-20.1%) than our study [22, 23]. These findings demonstrate that HAdV prevalence may differ by geographic locations. Such differences in HAdV prevalence could be influenced by a number of factors, including sample type, small sample size, sampling period, study duration, and patients’ demographic information.
Regarding patient demographic data, there was no significant difference observed in HAdV detections between genders in this study. This is consistent with the findings of previous studies conducted in China [3, 20]. However, this was in contrast to some previous reports that male children were more likely to be infected with HAdV [24, 25]. The highest HAdV-positive rate was observed among children between the ages of 5 and 10 years, which is similar to the findings of a recent study in the same city . However, most previous reports found that the HAdV infections occurred more often among children under five years of age [19, 26–28].
Our study revealed that although HAdV infections were detected throughout the year, the prevalence peaked in the summer. This is consistent with findings of Chen et al  during 2012–2013. But it was not consistent with that of studies conducted in Northern China  and Mexico . In Tanzania and Switzerland, HAdV infections were observed during all seasons of the year with no clear seasonality demonstrated [21, 27]. These difference in HAdV prevalence between seasons are interesting and bear future study as seasonal risk could influence future employment of HAdV vaccines which are in development in China .
In this study, seven HAdV types were identified: HAdV-B3, HAdV-B7, HAdV-B55, HAdV-C1, HAdV-C2, HAdV-C5, and HAdV-E4. Among these types, HAdV-B3 and HAdV-B7 were most prevalent which is consistent with other reports in Asia [26, 31, 32].
The majority (94.6%) of HAdV types detected in this study were of species B. HAdV-B (e.g., 3, 7, and 55) has been continuously reported to be associated with more severe acute respiratory disease than other HAdVs species [2, 14, 18]. Only seven patients were found to have HAdV-C and only eight patients were found to have HAdV-E infection during the study period.
Previously, three types of HAdV-C (e.g., 1, 2, and 5) were identified in China, although studies of HAdV-C species have been limited . HAdV-C species viruses were identified as the primary pathogens responsible for respiratory tract infections among hospitalized children, particularly among infants under two years of age . Our study is consistent with this in that more than half of HAdV-C-positive patients were observed in children less than two years old. Previous research suggests that recombination events are commonly observed among HAdV-C types . As recombinant HAdV strains have caused epidemics, it seems prudent to monitor for changes in HAdV-C types in China.
Although our study provides crucial molecular evidence regarding the epidemiology and clinical features of HAdV infections in Guangzhou, China, it has several limitations. First, we did not study outpatients and they could have had a different distribution of HAdV types. Second, study samples were first identified using a commercial assay which is not thought to be as sensitive as the qPCR we employed. Hence, we likely only captured data on the specimens with higher viral titers. Thus, the true prevalence of HAdV among hospitalized patients may have been higher and the distribution of HAdV types different. Third, we were unable to rule out a number of possible viral coinfections that were not surveilled for with the commercial product (e.g., parainfluenza 4, rhinoviruses, enteroviruses, etc.). Hence, some of the clinical findings we observed could be due coinfections with viruses other HAdV.
At present, there are no specific antiviral drugs or vaccine available in China for the treatment or prevention of HAdV infections. However, effective antivirals [34, 35] and vaccines [30, 36] seem to be on the near horizon. Hence, conducting type-specific surveillance for HAdV is an important need for China. Nationwide, periodic HAdV surveillance could alert Chinese public health officials of the emergence of pre-pandemic or particularly virulent strains and help them mitigate the threat. For instance, currently, there seems to be a worldwide increase in the threat of HAdV type four which could be mitigated with the Teva, Inc. HAdV four live vaccine .