Changing Patterns of Infectious Diseases Among Hospitalized Children in Hokkaido, Japan, in the Post-COVID-19 Era, July 2019 to June 2022 : The Pediatric Infectious Disease Journal

Secondary Logo

Journal Logo

COVID Reports

Changing Patterns of Infectious Diseases Among Hospitalized Children in Hokkaido, Japan, in the Post-COVID-19 Era, July 2019 to June 2022

Fukuda, Yuya MD*,†; Togashi, Atsuo MD, PhD; Hirakawa, Satoshi , MD, PhD; Yamamoto, Masaki MD, PhD; Fukumura, Shinobu , MD, PhD; Nawa, Tomohiro , MD; Honjo, Saho , MD§; Kunizaki, Jun , MD; Nishino, Kouhei , MD; Tanaka, Toju , MD, PhD**; Kizawa, Toshitaka , MD, PhD††; Yamamoto, Dai , MD, PhD‡‡; Takeuchi, Ryoh , MD, PhD§§; Sasaoka, Yuta , MD¶¶; Kikuchi, Masayoshi , MD‖‖; Ito, Takuro , MD***; Nagai, Kazushige , MD, PhD†††; Asakura, Hirofumi , MD‡‡‡; Kudou, Katsumasa , MD§§§; Yoshida, Masaki , MD¶¶¶; Nishida, Takeshi MD‖‖‖; Tsugawa, Takeshi , MD, PhD

Author Information
The Pediatric Infectious Disease Journal 42(9):p 766-773, September 2023. | DOI: 10.1097/INF.0000000000003982



Many reports have reported a reduction in respiratory infectious diseases and infectious gastroenteritis immediately after the coronavirus disease 2019 (COVID-19) pandemic, but data continuing into 2022 are very limited. We sought to understand the current situation of various infectious diseases among children in Japan as of July 2022 to improve public health in the post-COVID-19 era.


We collected data on children hospitalized with infectious diseases in 18 hospitals in Japan from July 2019 to June 2022.


In total, 3417 patients were hospitalized during the study period. Respiratory syncytial virus decreased drastically after COVID-19 spread in early 2020, and few patients were hospitalized for it from April 2020 to March 2021. However, an unexpected out-of-season re-emergence of respiratory syncytial virus was observed in August 2021 (50 patients per week), particularly prominent among older children 3–6 years old. A large epidemic of delayed norovirus gastroenteritis was observed in April 2021, suggesting that the nonpharmaceutical interventions for COVID-19 are less effective against norovirus. However, influenza, human metapneumovirus, Mycoplasma pneumoniae, and rotavirus gastroenteritis were rarely seen for more than 2 years.


The incidence patterns of various infectious diseases in Japan have changed markedly since the beginning of the COVID-19 pandemic to the present. The epidemic pattern in the post-COVID-19 era is unpredictable and will require continued careful surveillance.

Infectious diseases are the most common conditions seen in the field of pediatrics, and there are a wide variety of epidemic diseases that are often difficult to control. In December 2019, an outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing coronavirus disease 2019 (COVID-19), was reported in Wuhan, China, and rapidly spread all over the world.1 Two and a half years later, the effects of the COVID-19 epidemic have far surpassed those of the disease itself, representing a great impact on society.

The first COVID-19 patient in Hokkaido Prefecture, Japan, was recognized on January 28, 2020, and the epidemic spread rapidly thereafter.2 The importance of nonpharmaceutical interventions (NPIs), such as wearing masks, physical distance from others, washing hands, and disinfecting hands with alcohol were emphasized.3 Due to the increasing number of COVID-19 patients, both the national and Hokkaido governments repeatedly issued requests for people to refrain from unnecessary outings and gatherings, and for businesses to shorten their hours as a “state of emergency” or “semi-state of emergency,” even though these requests are not legally binding.4 Furthermore, to prevent the spread of COVID-19 among children, all primary, junior high, and high schools and special support schools in Hokkaido Prefecture were closed from February 28 to May 31, 2020.5 Although NPIs remain in place in Japan, there have been 7 waves of COVID-19 in Japan, and no prospect of COVID-19 convergence by the date this article was submitted (Fig. 1).2

The number of patients newly diagnosed with COVID-19 in Hokkaido Prefecture, Japan. This result is based on newly diagnosed COVID-19 patients reported to the Hokkaido government by public health centers and hospitals in Hokkaido Prefecture, and made publicly available on the Hokkaido government website. The data include both pediatric and adult patients, and there are no specific criteria for the diagnostic testing methods used. The vertical axis shows the number of patients, and the horizontal axis shows days. The red and yellow shaded areas indicate the period of “state of emergency” and “semi-state of emergency.” The arrow indicates a school closure period from February 28 to May 31, 2020.

Beginning July 1, 2019, we conducted the survey “Hokkaido Pediatric Infectious Diseases Surveillance” (HPIDS) to gather data about inpatient children with infectious diseases in the 18 hospitals in Hokkaido Prefecture, Japan (see Figure, Supplemental Digital Content 1, Hokkaido Prefecture is in the northernmost part of Japan, with a population of approximately 5.2 million (4.1% of the total population of Japan). Shortly after the survey started, COVID-19 spread in Hokkaido Prefecture. We have previously reported that various infectious diseases among pediatric patients decreased drastically shortly after COVID-19 began to spread.6 Specifically, when we defined July 2019 to February 2020 as “pre-COVID-19,” and July 2020 to February 2021 as “post-COVID-19,” we found that (1) patients with influenza and respiratory syncytial virus (RSV) decreased drastically post-COVID-19 (from 308 and 795, respectively, pre-COVID-19 to 0 and 3 post-COVID-19); (2) adenovirus (respiratory infection) decreased to 60.9% (46 cases to 28) of pre-COVID-19 levels; (3) norovirus, rotavirus, and adenovirus gastroenteritis decreased markedly post-COVID-19 to 27.8% (97 cases to 27), 2.6% (38 to 1), and 13.5% (37 to 5) of their pre-COVID-19 levels, respectively; and (4) Kawasaki disease (KD) decreased post-COVID-19 to 31.7% (161 cases to 51) of pre-COVID-19 levels. However, there is no guarantee that these non-SARS-CoV-2 infectious diseases will remain at these low levels for years. Understanding the current situation of various infectious diseases among children will have great significance for public health in the post-COVID-19 era.

In this study, we report the changes in the number and age distribution of various infectious diseases among children in Japan for the 3 years from July 2019 to June 27, 2022.


Study Period and Institutions

We aggregated data from patients 0–15 years old who were newly admitted to the 18 hospitals in Hokkaido Prefecture, Japan, from July 1, 2019, to June 27, 2022. We defined the 8-month period from July 2019 to February 2020 as the “pre-COVID-19 period,” the 3-month period from March to May 2020 during the school closure period as the transition period, and the 25-month period from June 2020 to June 2022 as the “post-COVID-19 period.” The staff of each hospital entered the data every week using Microsoft Excel and emailed the data file to the central staff.


We continued to investigate the 10 infectious diseases presented in the previous study.5 These included data for RSV, influenza A, influenza B, human metapneumovirus (hMPV), Mycoplasma pneumoniae, adenovirus (respiratory infection), norovirus gastroenteritis, rotavirus gastroenteritis, adenovirus gastroenteritis, and KD. We aggregated each disease by sex and by 6 age groups (0 years old, 1 year old, 2 years old, 3–6 years old, 7–12 years old, and 13–15 years old). In this study, we did not examine the number of patients with COVID-19.

Inclusion Criteria

For RSV, influenza virus, hMPV, M. pneumoniae, and adenovirus (respiratory infection), we included patients with positive rapid antigen detection tests or polymerase chain reaction (PCR) tests. For norovirus gastroenteritis, rotavirus gastroenteritis, and adenovirus gastroenteritis, we included patients with positive rapid antigen detection tests. When more than 2 types of pathogens were detected, if the attending physician determined it to be a coinfection, multiple items were counted. For KD, we included patients who met the diagnostic criteria outlined in the Japanese diagnostic guideline for KD (the 6th revised edition).7 Patients who met the diagnostic criteria for KD but were diagnosed with multisystem inflammatory syndrome in children (MIS-C) based on the Japanese MIS-C clinical consensus statement were excluded.8

Data Analysis

The hospitalization rates per 100,000 person-years were calculated using Poisson regression, with exact 95% confidence intervals based on the number of hospitalizations associated with each infectious disease and the size of the target population in the hospitals’ catchment areas.9 We compared the age group of patients with RSV, adenovirus (respiratory infection), norovirus gastroenteritis, adenovirus gastroenteritis, and KD in the pre-COVID-19 and post-COVID-19 periods using χ2 tests and residual analysis. We calculated the adjusted standardized residuals to identify significant cells after a statistically significant χ2 test. P values of <0.05 were considered significant. The statistical analyses were conducted in R Statistical Software (version 4.0.2; R Foundation for Statistical Computing, Vienna, Austria).

Ethical Approval

This study was approved by the Ethics Committee of Sapporo Medical University (Institutional ethical clearance number: 312-3349). All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration (as revised in 2013).


In total, 3417 patients with 10 infectious diseases were hospitalized during the study period (July 1, 2019, to June 27, 2022) (Table 1). The changes in the number of patients with each disease during the study period are shown in Figure 2A–H. Patients with RSV, influenza A, influenza B, hMPV, M. pneumoniae, and rotavirus gastroenteritis decreased drastically after the school closure period (February 28 to May 31, 2020). Among these, only RSV re-emerged in 2021.

TABLE 1. - Patient Diagnoses During the Study Period
Diseases Period Total
Pre-COVID-19 (8 mo)* Transition Period (3 mo) Post-COVID-19 (25 mo)
Total 1841 103 1473 3417
(1380.8 [1318.4–1445.3]) (206.0 [168.1–249.8]) (353.5 [335.7–372.0])
Respiratory infectious diseases
 RSV 795 6 871 1672
(596.3 [555.5–639.2]) (12.0 [4.40–26.1]) (209.0 [195.4–223.4])
 Influenza A 269 2 0 271
(201.8 [178.4–227.4]) (4.0 [2.2–20.5])
 Influenza B 39 6 0 45
(29.3 [20.8–40.0]) (12.0 [4.4–26.1])
 hMPV 201 7 5 213
(150.8 [130.6–173.1]) (14.0 [5.6–28.8]) (1.2 [0.4–2.8])
Mycoplasma pneumoniae 158 3 5 166
(118.5 [100.7–138.5]) (6.0 [1.2–17.5]) (1.2 [0.4–2.8])
 Adenovirus (respiratory infection) 46 11 126 183
(34.5 [25.3–46.0]) (22.0 [11.0–39.4]) (30.2 [25.2–36.0])
Infectious gastroenteritis
Norovirus gastroenteritis 97 10 205 312
(72.8 [59.0–88.7]) (20.0 [9.6–36.8]) (49.2 [42.7–56.4])
Rotavirus gastroenteritis 38 3 2 43
(28.5 [20.2–39.1]) (6.0 [1.2–17.5]) (0.5 [0.1–1.7])
Adenovirus gastroenteritis 37 6 56 99
(27.8 [19.5–38.2]) (12.0 [4.4–26.1]) (13.4 [10.2–17.4])
Kawasaki disease 161 49 203 413
(120.8 [102.8–140.9]) (98.0 [72.5–129.6]) (48.7 [42.2–55.9])
The numbers in parentheses indicate the hospitalization rates per 100,000 person-years, and the numbers in square brackets indicate the 95% confidence intervals.
*Pre-COVID-19 period refers to the 8 months from July 2019 to February 2020.
Transition period refers to the 3 months from March to May 2020.
Post-COVID-19 period refers to the 25 months from June 2020 to June 2022.

Trends in the number of patients over the study period. A: RSV. B: Influenza. C: hMPV and Mycoplasma pneumoniae. D: Adenovirus (respiratory infection). E: Norovirus gastroenteritis. F: Rotavirus gastroenteritis. G: Adenovirus gastroenteritis. H: Kawasaki disease. The vertical axis shows the number of patients, and the horizontal axis shows weeks. The red and yellow shaded areas indicate the period of “state of emergency” and “semi-state of emergency.” The arrow indicates a school closure period from February 28 to May 31, 2020. The gray shaded area indicates the trends in the number of patients newly diagnosed with COVID-19 in Hokkaido Prefecture (shown in Fig. 1).

Only patients with RSV showed significant changes in the age distribution from the pre- to post-COVID-19 period [χ2(5) = 17.311, P = 0.004, V = 0.102] (Fig. 3A). There were no significant changes in the age groups of patients with adenovirus (respiratory infection), norovirus gastroenteritis, adenovirus gastroenteritis, or KD (P = 0.630, 0.248, 0.962, and 0.908, respectively) (Fig. 3B–E).

Age distribution of each infection. A: RSV. B: Adenovirus (respiratory infection). C: Norovirus gastroenteritis. D: Adenovirus gastroenteritis. E: Kawasaki disease. The percentages of patients 0 years old, 1 year old, 2 years old, 3–6 years old, 7–12 years old and 13–15 years old for each disease are shown. Vertical bars colored in red and blue are data in the pre-COVID-19 and post-COVID-19 period, respectively.

Respiratory Diseases


There were 1672 patients with RSV during the study period (Table 1). RSV tended to decrease from the end of 2019, and few patients were hospitalized during the time from April 2020 to March 2021 (Fig. 2A). However, patients with RSV re-emerged from the end of April 2021, and gradually increased from May, reaching a notable peak of 50 patients/week in August 2021. After that, the number of patients gradually decreased to 2–3 patients/week toward June 2022.

Patients less than a year old were hospitalized the most in both pre- and post-COVID-19 periods (Fig. 3A). The proportion of patients 3–6 years old was significantly lower in the pre-COVID-19 period and higher in the post-COVID-19 period (10.9% [87/795] and 17.0% [148/871], P = 0.001) (see Table, Supplemental Digital Content 2,

Influenza A and B

In total, 271 and 45 patients with influenza A and B were hospitalized during the study period (Table 1). Influenza A increased from the end of November 2019, and the largest cluster of patients was in December (Fig. 2B). After that, incidence decreased sharply, and no patients were observed after May 2020. Influenza B peaked in mid-February 2020, but the incidence subsided in about a month, and there were no patients with influenza B after April 2020.


There were 214 patients with hMPV during the study period (Table 1). About 5–10 patients with hMPV were hospitalized every week from July 2019 to February 2020 (Fig. 2C). After the school closure period (March–May 2020), the incidence decreased drastically, and only 5 patients with hMPV were hospitalized from June 2020 to June 2022.

Mycoplasma pneumoniae

In total, 166 patients with M. pneumoniae were hospitalized during the study period (Table 1). M. pneumoniae increased from July 2019 and peaked in November with almost 10 patients per week (Fig. 2C). After that, M. pneumoniae tended to decrease, and only 5 patients were hospitalized after the school closure period (March–May 2020).

Infectious Gastroenteritis

Norovirus Gastroenteritis

During the study period, 312 patients with norovirus gastroenteritis were hospitalized (Table 1). In 2020, norovirus gastroenteritis peaked in January and February, with 6–10 patients per week (Fig. 2E). In 2021, there were only 1–2 patients per week in January, but a large peak exceeding 10 patients per week emerged in April. From July 2021 to June 2022, no major epidemic was observed, but at least 1 patient was hospitalized each week. The number of patients increased from February 2022 through July.

Rotavirus Gastroenteritis

In total, 44 patients with rotavirus gastroenteritis were hospitalized during the study period (Table 1). After the school closure period (March–May 2020), the incidence decreased, and only 2 patients were hospitalized from June 2020 to June 2022, both of whom had not been vaccinated against rotavirus (Fig. 2F).


This is a multicenter survey to comprehensively describe the trends in various infectious diseases among hospitalized children before and after the COVID-19 epidemic in Japan, from July 1, 2019, to June 27, 2022. There are several reports showing the reduction of various infectious diseases immediately after the COVID-19 epidemic,10–14 but those following into 2022 and beyond are very limited. We previously reported, using surveillance data from July 2019 to February 2021, a dramatic reduction in many infectious diseases among children from the pre- to the post-COVID-19 era in Japan.6 Surprisingly, by extending the survey period to June 2022, we found notable changes in the numbers and age distributions of infectious diseases in pediatric patients from previous reports.

First, RSV, which had decreased drastically after the school closure period (March–May 2020) and had hardly been seen for a year, formed a marked epidemic, reaching 50 patients/week in August 2021 (Fig. 2A). In the pre-COVID-19 era, RSV was seen almost year-round in Hokkaido Prefecture from 2017, and there had not been a major epidemic in August like that of 2021 in the last decade.15 Therefore, this dramatic out-of-season resurgence of RSV in 2021 was unpredictable. The National Institute of Infectious Diseases, Japan, reported similar RSV epidemics in 2021 throughout Japan.16 Other countries such as the United States, Australia, Chile, South Africa, and Canada also demonstrated a marked decrease in RSV detection in the early phases of the COVID-19 pandemic.10,11 Similar to our results, the resurgence of RSV was observed in the United States, Israel, and China from April to August 2021, and in Australia in December 2020.17–20 In the United States and Israel, RSV increased after relaxing COVID-19 restrictions, which underscores the high effectiveness of NPIs such as masking and social distancing in preventing the spread of respiratory pathogens. However, the resurgence of RSV in Hokkaido Prefecture in 2021 was observed during a period when a “state of emergency” and “semi-state of emergency” had been declared, suggesting that factors other than relaxation of NPIs may have been involved. RSV is primarily transmitted via close contacts and airborne droplets,21–23 and mainly presents young children who have difficulty in performing appropriate infection control measures.24 Moreover, RSV can be carried asymptomatically,25 and young children who are unaware of their RSV infection may contribute to viral transmission within households or nurseries. In the post-COVID-19 era in Japan, a lack of exposure to RSV over a certain period may have resulted in children with increased susceptibility to the virus. These factors may have played a significant role in the resurgence of RSV, which was not observed in other respiratory infectious diseases.

Another interest is the change in age group of patients with RSV (Fig. 3A). The re-emergence of RSV in the post-COVID-19 period included more older children (3–6 years old) than in the pre-COVID-19 period (17.0% and 10.9%, respectively). The above reports from China and Australia also mentioned that patients in the RSV re-emergence were significantly older than in pre-COVID-19.19,20 In general, more than 80% of children experienced the RSV infection by the age of 2 years, and most of the severe patients requiring hospitalization were infants.24,26 However, our findings suggest that the cases of older children whose immunity for RSV has not developed in the post-COVID-19 era may have become more severe. The number of patients with RSV is gradually increasing as of the end of June 2022 (5 patients per week), and we should be cautious about the potential for large out-of-season RSV epidemics for the second consecutive year.

Influenza, hMPV, and M. pneumoniae, which also decreased dramatically after the school closure period (March–May 2020), were rarely seen for more than 2 years until June 2022 in our survey (Fig. 2B, C). Similar observations have been reported from different countries, highlighting the remarkable decrease in these respiratory infections in the post-COVID-19 era.10,11 Nevertheless, in Australia, where no two-season influenza epidemic had been seen since 2020 like Japan, influenza A outbreaks occurred in May and June 2022.27 Similarly, hMPV re-emerged in 2021–2022 in the United States.28 The low detection rate of influenza can be attributed to the increased use of masks and reduced contact with others, which has been extensively practiced in Japan since the COVID-19 pandemic.29 These effective infection control measures implemented in Japan during the survey period may have contributed to the absence of respiratory infectious diseases such as influenza, in contrast to other countries. Moreover, influenza and hMPV have a significantly higher rate of symptomatic infections than other respiratory infectious diseases, including RSV.25 Hence, patients with influenza or hMPV who had even mild symptoms may have refrained from going out to prevent infection spread in the post-COVID-19 era. M. pneumoniae mainly presents in school-age children who can partake in NPIs properly, which may have resulted in the reduced number of patients.30 Additionally, M. pneumoniae epidemics occur in 3- to 7-year cycles, and Hokkaido Prefecture recently had epidemics in 2016 and 2019.15 Thus, the 2021–2022 period may have been when the epidemic was less prevalent in the first place.

In our previous study conducted between July 2019 and February 2021, we reported that the incidence of norovirus gastroenteritis decreased drastically to 27.8% of its former level post-COVID-19.6 However, in this study, which extended its survey period until June 2022, a delayed norovirus gastroenteritis epidemic was identified in April 2021, with a greater number of patients compared with pre-COVID-19 period (Fig. 2E). It was reported that norovirus gastroenteritis also decreased sharply in the post-COVID-19 era in various countries, but most of the survey period was until December 2020.12–14 Similar to our findings, a report from Hong Kong identified the same norovirus epidemic as usual in the 2020/2021 winter season.14 Thus, we should still be vigilant against norovirus in the post-COVID-19 era. In 2022, no noticeable norovirus epidemic was observed until June, but the number of patients is gradually increasing. The epidemic pattern in the post-COVID-19 era is unpredictable and continued careful surveillance is required.

Unlike norovirus gastroenteritis, patients with rotavirus gastroenteritis were rarely seen for more than 2 years, until June 2022 (Fig. 2F). In many countries, the number of patients with rotavirus gastroenteritis also decreased sharply in the post-COVID-19 era.13,14,19,31 However, in Hong Kong, a rotavirus epidemic took place in winter 2020/2021, while multiple NPIs for COVID-19 remained in effect.14 The reason why rotavirus has been strongly suppressed in Japan may therefore not only be the effect of NPIs for COVID-19 but also because of its high vaccine coverage for rotavirus. Rotavirus vaccines produce a herd immunity effect and reduce hospitalization from rotavirus gastroenteritis.32–34 In Japan, rotavirus vaccines have been commercially available since 2011 for voluntary vaccination, and vaccine coverage increased from 30.0% in 2012 to 78.4% in 2019, though full coverage has not been reached.35 In October 2020, the rotavirus vaccine became available for routine vaccination in Japan,36 and the vaccine coverage is expected to reach >95%, which is the coverage level for other routine vaccines.37 This would have decreased the number of young children with severe rotavirus gastroenteritis. In Hong Kong, the rotavirus vaccine has not yet been included in the Hong Kong Childhood Immunisation Programme, and the vaccine coverage has been reported as only 33.3% in 2009–2012.38 This difference of vaccine coverage between Japan and Hong Kong may be related to the difference of rotavirus epidemics in both countries in the post-COVID-19 era. It will be interesting to see if epidemics of rotavirus gastroenteritis can be suppressed continuously in Japan in the future.

There are several limitations to this study. First, we started the HPIDS survey from July 1, 2019, meaning that we only have 8 months of pre-COVID-19 data available. Therefore, it is not possible to compare the post-COVID-19 period with data from several years prior to the pandemic. Second, there were no established protocols for conducting pathogen testing. The decision on conducting testing for each patient was made by the attending physician at each hospital, based on the individual needs of the patient. This could have resulted in a selection bias. Third, for several diseases, such as influenza and norovirus/rotavirus/adenovirus gastroenteritis, we included patients with positive rapid tests. Rapid antigen tests have lower sensitivity and specificity, and may therefore be less reliable for diagnosis.39,40 Fourth, we did not specify rapid antigen detection test kits in the inclusion criteria; therefore, the 18 hospitals may not have used the same products. This could have resulted in some detection bias. Fifth, with the emergence of SARS-CoV-2, the possibility of detecting multiple respiratory pathogens has increased due to the increased use of multiplex PCR testing in hospitalized patients. Additionally, multiplex PCR systems may sometimes detect mixed viral infections, leading to an overcount of cases.41,42

In conclusion, our findings indicate that the trends in common infectious diseases have changed from the immediate post-COVID-19 era toward 2022 in Japan. RSV, which had been reduced markedly in the post-COVID-19 era, re-emerged dramatically, including among older children, in August 2021. Norovirus gastroenteritis does not appear to be sufficiently inactivated by current NPIs for COVID-19 and has spread without significant change from the pre-COVID-19 era. However, influenza, hMPV, M. pneumoniae, and rotavirus gastroenteritis are still strongly suppressed as of June 2022. Nonetheless, in several countries, these infectious diseases have already re-emerged after relaxing COVID-19 restrictions. Therefore, we should be cautious of a possible resurgence of these diseases in Japan after easing infection control measures against COVID-19.


We thank all the pediatricians who participated in this survey. We thank John Daniel from Edanz ( for editing a draft of this article.


1. Centre for Health Protection of the Hong Kong Special Administrative Region Government. CHP closely monitors cluster of pneumonia cases on Mainland. Dec 31, 2019. Available at: Accessed February 9, 2023.
2. Hokkaido government. Data for COVID-19 in Hokkaido Prefecture (in Japanese). Available at: Accessed August 17, 2022.
3. World Health Organization. Transmission of SARS-CoV-2: implications for infection prevention precautions. Available at: Accessed February 9, 2023.
4. Hokkaido government. Efforts to control COVID-19 (in Japanese). Accessed April 11, 2023.
5. Hokkaido Government Board of Education. Information about COVID-19 (in Japanese). Available at: Accessed February 9, 2023.
6. Fukuda Y, Tsugawa T, Nagaoka Y, et al. Surveillance in hospitalized children with infectious diseases in Japan: Pre- and post-coronavirus disease 2019. J Infect Chemother. 2021;27:1639–1647.
7. Kobayashi T, Ayusawa M, Suzuki H, et al. Revision of diagnostic guidelines for Kawasaki disease (6th revised edition). Pediatr Int. 2020;62:1135–1138.
8. Japan Pediatric Society. Pediatric COVID-19-Associated Multisystem Inflammatory Syndrome (MIS-C/PIMS) Clinical Consensus Statement (in Japanese). Available at: Accessed April 10, 2023.
9. Hokkaido Government. Overview of population and number of households in Hokkaido (in Japanese). Available at: Accessed April 10, 2023.
10. Olsen SJ, Azziz-Baumgartner E, Budd AP, et al. Decreased Influenza Activity During the COVID-19 Pandemic — United States, Australia, Chile, and South Africa, 2020. Am J Transplant. 2020;20:3681–3685.
11. Groves HE, Piché-Renaud P, Peci A, et al. The impact of the COVID-19 pandemic on influenza, respiratory syncytial virus, and other seasonal respiratory virus circulation in Canada: A population-based study. Lancet Reg Health Am. 2021;1:100015.
12. Eigner U, Verstraeten T, Weil J. Decrease in norovirus infections in Germany following COVID-19 containment measures. J Infect. 2021;82:276–316.
13. Wang LP, Han JY, Zhou SX, et al.; Chinese Centers for Disease Control and Prevention (CDC) Etiology of Diarrhea Surveillance Study Team. The changing pattern of enteric pathogen infections in China during the COVID-19 pandemic: a nation-wide observational study. Lancet Reg Health West Pac. 2021;16:100268.
14. Chan MC. Return of norovirus and rotavirus activity in winter 2020–21 in city with strict COVID-19 control strategy, Hong Kong, China. Emerg Infect Dis. 2022;28:713–716.
15. Hokkaido Infectious Disease Surveillance Center. Sentinel weekly reporting diseases (in Japanese). Available at: Accessed February 9, 2023.
16. National Institute of Infectious Diseases, Japan. Infectious diseases weekly report (in Japanese). Available at: Accessed February 9, 2023.
17. Agha R, Avner JR. Delayed seasonal RSV surge observed during the COVID-19 pandemic. Pediatrics. 2021;148:e2021052089.
18. Opek MW, Yeshayahu Y, Glatman-Freedman A, et al. Delayed respiratory syncytial virus epidemic in children after relaxation of COVID-19 physical distancing measures, Ashdod, Israel, 2021. Euro Surveill. 2021;26:2100706.
19. Liu P, Xu M, Lu L, et al. The changing pattern of common respiratory and enteric viruses among outpatient children in Shanghai, China: two years of the COVID-19 pandemic. J Med Virol. 2022;94:4696–4703.
20. Foley DA, Phuong LK, Peplinski J, et al. Examining the interseasonal resurgence of respiratory syncytial virus in Western Australia. Arch Dis Child. 2022;107:e7.
21. Paynter S. Humidity and respiratory virus transmission in tropical and temperate settings. Epidemiol Infect. 2015;143:1110–1118.
22. Otomaru H, Sornillo JBT, Kamigaki T, et al. Risk of transmission and viral shedding from the time of infection for respiratory syncytial virus in households. Am J Epidemiol. 2021;190:2536–2543.
23. Moreira LP, Watanabe ASA, Camargo CN, et al. Respiratory syncytial virus evaluation among asymptomatic and symptomatic subjects in a university hospital in Sao Paulo, Brazil, in the period of 2009-2013. Influenza Other Respir Viruses. 2018;12:326–330.
24. Noble M, Khan RA, Walker B, et al. Respiratory syncytial virus-associated hospitalisation in children aged ≤5 years: a scoping review of literature from 2009 to 2021. ERJ Open Res. 2022;8:00593–02021.
25. Galanti M, Birger R, Ud-Dean M, et al. Rates of asymptomatic respiratory virus infection across age groups. Epidemiol Infect. 2019;147:e176.
26. Glezen WP, Taber LH, Frank AL, et al. Risk of primary infection and reinfection with respiratory syncytial virus. Am J Dis Child. 1986;140:543–546.
27. Department of Health and Aged Care, Australian Government. Australian Influenza Surveillance Reports. Available at: Accessed February 9, 2023.
28. Centers for Disease Control and Prevention. The National Respiratory and Enteric Virus Surveillance System (NREVSS): Human Metapneumovirus Surveillance. Available at: Accessed February 9, 2023.
29. Takeuchi H, Kawashima R. Disappearance and re-emergence of influenza during the COVID-19 pandemic: association with infection control measures. Viruses. 2023;15:223.
30. Sauteur PMM, Unger WWJ, Nadal D, et al. Infection with and carriage of Mycoplasma pneumoniae in children. Front Microbiol. 2016;7:329.
31. Burnett E, Parashar UD, Winn A, et al. Trends in rotavirus laboratory detections and Internet search volume before and after rotavirus vaccine introduction and in the context of the COVID-19 pandemic—United States 2000–2021. J Infect Dis. 2022;226:967–974.
32. Burnett E, Parashar UD, Tate JE. Global impact of rotavirus vaccination on diarrhea hospitalizations and deaths among children <5 years old: 2006–2019. J Infect Dis. 2020;222:1731–1739.
33. Ito M, Higashigawa M. Effectiveness of self-financed rotavirus vaccination in Ise City, Japan. Hum Vaccin Immunother. 2021;17:5650–5655.
34. Fujii Y, Noguchi A, Miura S, et al. Effectiveness of rotavirus vaccines against hospitalisations in Japan. BMC Pediatr. 2017;17:156.
35. Tsugawa T, Akane Y, Honjo S, et al. Rotavirus vaccination in Japan: efficacy and safety of vaccines, changes in genotype, and surveillance efforts. J Infect Chemother. 2021;27:940–948.
36. Saitoh A, Okabe N. Changes and remaining challenges for the Japanese immunization program: Closing the vaccine gap. Vaccine. 2021;39:3018–3024.
37. Number of people who received routine vaccination. Ministry of Health, Labour and Welfare, Japan (in Japanese). Available at: Accessed February 9, 2023.
38. Yeung KHT, Lin SL, Clark A, et al. Economic evaluation of the introduction of rotavirus vaccine in Hong Kong. Vaccine. 2021;39:45–58.
39. Gonzalez MD, McElvania E. New developments in rapid diagnostic testing for children. Infect Dis Clin North Am. 2018;32:19–34.
40. Centers for Disease Control and Prevention. Information for Clinicians on Rapid Diagnostic Testing for Influenza. Available at: Accessed February 9, 2023.
41. Antalis E, Oikonomopoulou Z, Kottaridi C, et al. Mixed viral infections of the respiratory tract; an epidemiological study during consecutive winter seasons. J Med Virol. 2018;90:663–670.
42. Lin CY, Hwang D, Chiu NC, et al. Increased detection of viruses in children with respiratory tract infection using PCR. Int J Environ Res Public Health. 2020;17:564.

child; COVID-19; influenza; norovirus gastroenteritis; respiratory syncytial virus

Supplemental Digital Content

Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.