Substantial pathogen epidemiological studies of respiratory virus infection in children have reported that respiratory viruses were detected in more than 50–70% (11–15). Children easily have viral respiratory infections because of the prematurity of immune function and respiratory function physiologically and anatomically.
However, a report that viruses were detected in 20% of severely affected adult patients (ICU inpatients) has been reported so far about the examination of the pathogen epidemiology of respiratory virus infection in adults (10).
When considering respiratory virus epidemiology from social and regional points of view, it is important to observe both children and adults in a certain restricted community and during the same period.
Our hospital is located in West Tokyo, Japan, designated as an infection and emergency medical center institute in the south of north-Tama area, which covers approximately 1 million residents.
During the designated period from December 2020 to November 2021, a seasonal outbreak of RSV and PI3 infection was found, as well as a pandemic of SARS-CoV-2 infection (Fig. 1).
It is well known that a seasonal outbreak of flu was not found internationally in the 2020–2021 season, probably because of the changes in public life and behavior that promote mask wearing and hand washing in addition to the restriction of people flow against the global pandemic of SARS-CoV-2 (16–19). To date, no seasonal outbreak of influenza has been found in the 2020–2021 and early 2021–2022 seasons in Japan (19). In our cohort, no influenza virus was detected.
In Japan, a substantial lockdown was performed from April to August 2020, just before the regular outbreak season of RSV, and the seasonal outbreak of RSV was not found in the 2020 season; however, it came back in 2021 as a larger wave than the standard regular wave (20). However, the closely related virus hMPV did not prevail for two consecutive years. In our study, hMPV was also not detected at all.
The differences in the impact on a seasonal outbreak between flu and RSV may be explained by the fact that the main target of RSV infection is a child under 2 years old, and it is quite difficult to prevent contact and droplet transmission of the virus in the nursery without social lockdown. In contrast, school children, who are the main target of Flu infection, can obey and maintain hygiene management and procedures individually.
Most of the RSV and PI3 were detected in the children under 5 years old; however, a small number of adult patients infected with RSV or PI3 were found throughout all ages. Of note, there is an interesting difference between the outbreak of RSV and PI3. In the former, it occurred simultaneously both in children and adults. In contrast, PI3 infection in children preceded three weeks earlier than the outbreak in adults. There was no difference in the infected age group between RSV and PI3 (Fig. 2-b). Considering that the incubation periods of both viruses are the same of 4 to 6 days and that they both have an envelope and are similarly sensitive to alcohol, it is difficult to explain this difference from the virological point of view. Complicated social factors might be involved in this discrepancy.
For reference, we retrospectively analyzed the previous seasonal outbreak of Flu (2014–2015, 2015–2016, 2016–2017, 2017–2018) by using the laboratory database in our hospital, in which patients with rapid antigen positivity for influenza A were investigated. There was no difference in the cumulative ratio of Flu A-infected patients between children and adults as of RSV in this study (supplemental Fig. 1).
In this sense, it is also interesting to see the cumulative ratio of SARS-CoV-2-infected patients in the 5th wave of the pandemic in Tokyo from the public database (21) (supplemental Fig. 2). More than 200,000 were infected during the 5th wave in Tokyo, and infected patients aged between 20 s and 50 s preceded the infected children under 10 years old, which accounted for 5.8% of the total infected patients by 10 to 14 days. This indicates that virus transmission from adults to children in the community occurred in the 5th wave of SARS-CoV-2 infection in Tokyo as a whole.
RV/EV infection is observed during the year, and it was not affected during the pandemic era of SARS-CoV-2 (22, 23), which was also true in our observation. Considering that a peak of RV/EV infection was found in children under 5 years old and adults of 20s to 30s years old (Table 1), virus transmission may occur in the family between the parents and the children. EV/EV has no envelope and is not sensitive to alcohol, which indicates resistance to the prevention of viral transmission by standard precautions. These findings may explain some of the reasons why RV/EV infection persists despite intensified social standard precautions for viral infection during the SARS-CoV-2 pandemic.
The limitation of this study is that we could not validate the clinical evidence of viral transmission between children and adults in detail. Additionally, the criteria for FARP testing were not tightened. Principally, FARP testing was performed at admission for the patients who presented fever and respiratory symptoms. However, it was also performed for adult outpatients who visited the emergency unit and presented with fever and/or respiratory symptoms. Furthermore, FARP testing was applied even for adults with mild cold symptoms to exclude SARS-CoV-2 when SARS-CoV-2 prevailed explosively as a social infection control strategy. In this sense, positivity of virus detection in adults in our study may be lower than that of previous reports. From another perspective, our study may cover community-acquired respiratory virus infections in adults more widely than previous reports. However, the viral detection rate in children was approximately equivalent to that in previous reports. (11–15) Additionally, the multiple viral detection rate was similar to that of previous reports(11, 12, 14, 15). This may be explained by the fact that FARP testing was principally applied to children who required admission, as in previous reports.
The multiplex PCR method is a powerful tool to detect respiratory viruses in children and adults and is also useful to monitor the trend of viral infection and transmission in the community.