The present study provided four main findings. First, despite experiencing their peak seasons, dramatic reductions were observed in the numbers of patients with PCF and GAS pharyngitis after the 10th week in 2020 as prompt responses to the national school closure. Furthermore, the rapid decreases in human mobility in transit stations from the 14th week enhanced the reductions in numbers of patients with PCF and GAS pharyngitis throughout 2020, with the smallest weekly numbers among the examined years since 2015. Second, no seasonal peaks after the 20th week were observed in the numbers of patients with all PIDs identified in 2020. In particular, the numbers of patients with HFMD and RSV infection, which have single seasonal peaks during the 30th–40th weeks in average years, were consistently minimized without any surge trends during the peak seasons. Third, the approximately 20% decrease in human mobility (in transit stations) appeared to have contributed to consistent reductions in the numbers of patients with PIDs. Finally, human mobility in transit stations had the highest correlations with reduced numbers of patients with PIDs compared with those in groceries and pharmacies and parks. To our knowledge, this is the first study that demonstrate that human mobility may be a potential factor affecting reductions in droplet-transmissible PIDs.
In Japan, the government formally started to close schools nationwide from the 10th week in 2020. The school closure continued until the State of Emergency was officially lifted in the 19th week. The numbers of patients with PIDs just in the middle of a peak season (PCF and GAS pharyngitis) were clearly reduced from the 10th week, coinciding with the start of the nationwide school closure. The numbers of patients with PCF and GAS pharyngitis were consistently decreased during the period of school closure (10th–19th weeks) with much smaller numbers than those identified in the previous years. Even after the 20th week when schools reopened throughout Japan, the numbers of patients with PCF and GAS pharyngitis identified in 2020 retained the large reductions and presented the smallest numbers since 2015. Furthermore, the numbers of children with HFMD, RSV infection, and herpangina identified after school reopening were also greatly reduced after the 20th week, even during the annual peak seasons. A study focusing on SARS-CoV-2 infection indicated that the numbers of children with COVID-19 did not rapidly increase after school reopening, while easing of restrictions on large-scale gatherings had a major influence on rapid increases . After the 20th week in Japan, the fact that school children adhered to social distancing and standard infection control and prevention measures, such as thorough hand-washing and mask-wearing, may have resulted in the moderate increases in numbers of patients with PIDs as well as COVID-19. A study conducted before the SARS-CoV-2 pandemic reported that hand-washing by children and their caregivers had a significant protective effect against community-acquired HFMD and herpangina . Other studies documented that increased compliance with social distancing measures can be a cost-effective strategy to mitigate the transmission of infectious diseases [29, 30]. These studies support our explanations for the present findings.
The highest correlation coefficients were found for the associations between numbers of patients with PIDs and human mobility in transit stations. The primary users of transit stations are typically adults. Therefore, the present findings indicate that greater reductions in the movements of adults with less potential human-to-human contacts may contribute to large decreases in the numbers of children with droplet-transmissible PIDs. For these reasons, our findings suggest that parent-to-child transmission may be a primary factor associated with increasing numbers of patients with PIDs, rather than cross-transmission among children. For example, RSV infection among children was greatly reduced without an annual peak season during 2020. Young infants are the dominant patients for RSV infection-associated hospital visits and hospitalizations . Previous studies indicated that parents most often introduced RSV into households, leading to infection in infants [31–34], which could support our explanations for the present findings. The present results also suggested that an approximately 20% decrease in human mobility in transit stations (for adults) may contribute to reductions in the numbers of patients with droplet-transmissible PIDs.
The study had some limitations. First, children with PIDs who lacked severe signs and symptoms would have refrained from visiting hospitals during the study period to avoid the potential risk for hospital-related asymptomatic transmission of SARS-CoV-2 , and this may have affected the decreased numbers of patients with PIDs identified throughout 2020. Second, the human mobility reports typically represented the activity of adults because the reports were based on data from Google users [24–26]. Therefore, the study did not include accurate information on the movements of children. Finally, our data only reflected weekly data on patients with PIDs from limited hospitals, although approximately 3,000 hospitals throughout Japan participate in the surveillance of PIDs. The small samples comprising only 52 weeks of data may have introduced statistical inaccuracy to the correlation coefficients between numbers of patients with PIDs and human mobility.