Background Coronavirus disease 2019 (COVID‐19) has drastically changed the recommended activities and environment for patients worldwide. Our aim was to assess the impact of COVID‐19 on pediatric hospitalizations in Kitami, Japan. Methods A retrospective, single‐center study was conducted on hospitalized patients aged 0–14 years at the Japanese Red Cross Kitami Hospital. We compared the incidence of pediatric patients hospitalized in 2020 with those in 2017–2019. Results The number of pediatric hospitalized patients dropped significantly in 2020 compared to that in 2017–2019 (median 43.0 vs 78.5 per month, P < 0.001). The patients were significantly older in 2020 (4.3 vs 3.4 years, P < 0.001). Hospitalization from respiratory (8.5 vs 30.5, P < 0.001) and gastrointestinal infections (3.0 vs 6.0, P = 0.004) significantly decreased. Admission due to respiratory syncytial virus (0.0 vs 4.0, P < 0.001), human metapneumovirus (0.0 vs 1.0, P = 0.005), influenza (0.0 vs 0.0, P = 0.009), adenovirus (0.0 vs 1.0, P = 0.003), and rotavirus infection (0.0 vs 0.0, P = 0.025) also decreased significantly. The <1–5 age groups significantly decreased (<1 year old, 6.5 vs 12.5, P < 0.001; 1–3 years old, 13.0 vs 29.5, P < 0.001; 4–5 years old, 5.5 vs 11.5, P < 0.001). Hospitalization due to foreign body ingestions increased significantly in 2020 (1.0 vs 0.0, P = 0.010). Conclusions The COVID‐19 control measures inadvertently reduced the number of hospitalized pediatric patients, especially younger children with respiratory and gastrointestinal infections.

Periodic fever syndromes are heterogeneous diseases. Familial Mediterranean fever (FMF) is one of the hereditary periodic fever diseases caused by a Mediterranean fever (MEFV) gene abnormality. FMF can be categorized as typical or atypical, based on clinical findings and genetic screening. Atypical FMF has a wide variation of clinical findings and disease-causing mutations of MEFV. Therefore, it is sometimes difficult to diagnose an unknown fever as FMF. To date, a large number of various typical and atypical FMF cases have been reported in Japan. Here, we describe a Japanese boy with heterozygous MEFV p.Ser503Cys exon 5 variant who developed periodic fever. He was treated with colchicine; a complete eradication of his fever and various accompanying symptoms have been subsequently achieved for more than a year. Given that there have been a few reports about patients with this variant, little is known about the genetic and phenotypic role of heterozygous MEFV p.Ser503Cys exon 5 variant. It is therefore imperative to consider atypical FMF as a differential diagnosis when a periodic fever is encountered. Furthermore, we suggest that it is worthwhile to integrate MEFV gene analysis with the potential effects of colchicine treatment in patients with periodic fever.

Human parvovirus B19 (B19) is a single-stranded DNA virus that targets erythroid progenitor cells in the bone marrow. B19 causes erythema infectiosum in children, transient aplastic anemia, pure red cell aplasia, hydrops fetalis, and contributes to other illnesses. An association between B19 infection and hypocomplementemia and rheumatoid arthritis has been reported, but the underlying mechanisms remain unclear. We report the case of a 1-year-old Japanese girl with persistent fever, skin rash, transient edema of the extremities, hypoalbuminemia, and hypocomplementemia associated with B19 infection. We considered Kawasaki disease (KD) and collagen diseases, particularly systemic lupus erythematosus, in our differential diagnosis. B19 infection might be associated with serological features that suggest systemic lupus erythematosus and may present with clinical symptoms seen in KD. Especially during erythema infectiosum epidemics, we must consider B19 infection in the differential diagnosis of KD patients who demonstrate atypical clinical symptoms and unexplained laboratory findings.