The regional respiratory syncytial virus (RSV) outbreak in southern Taiwan in late 2020 followed the surge of RSV cases in the national surveillance data and displayed distinct clinical features. This study investigated RSV epidemiology in the most recent five years and compared the clinical manifestations of this outbreak with non-outbreak period. Medical records of RSV-infected children at the National Cheng Kung University Hospital from January 2016 to December 2020 were retrospectively retrieved from hospital-based electronic medical database. Cases of RSV infection were identified by RSV antigen positive and/or RSV isolated from respiratory specimens. The demographic, clinical presentations, and laboratory data were recorded. The RSV isolates in 2020 was sequenced for phylogenetic analysis. Overall, 442 RSV-infected cases were retrieved and 42.1% (186 cases) clustered in late 2020. The 2020 outbreak started in September, peaked in November, and lasted for 3 months. 2020 RSV-infected children were older (2.3 ± 2.2 years vs. 1.0 ± 1.0 years), more likely to be diagnosed with bronchopneumonia (57.5% vs. 31.6%), but also had a lower hospitalization rate, shorter hospital stay, less oxygen use, and less respiratory distress than those in 2016-2019 (all p value < 0.05). The RSV isolates in 2020 belonged to RSV-A subtype ON1 but were phylogenetically distinct from the ON1 strains prevalent in Taiwan previously. The 2020 RSV outbreak was led by the novel RSV-A subtype ON1 variant with clinical manifestations distinct from previous years. Continuous surveillance of new emerging variants of respiratory viruses in the post-pandemic era is warranted.

Although serious adverse events have remained uncommon, cases of myocarditis induced by messenger RNA (mRNA) COVID-19 vaccines have been reported. Here, we presented a rare but potentially fatal disorder, hemophagocytic lymphohistiocytosis, in a 14-year-old previously healthy adolescent after BNT162b2 mRNA vaccination. The initial evaluation showed splenomegaly, pancytopenia, hyperferritinemia, and hypofibrinogenemia. Further examination revealed positive blood EBV DNA, and other infectious pathogen surveys were all negative. Hemophagocytosis was observed in the bone marrow aspiration and biopsy. HLH was confirmed and intravenous immunoglobulin (IVIG) and methylprednisolone pulse therapy were given. Venoarterial extracorporeal membrane oxygenation (VA-ECMO) was set up for cardiopulmonary support for 3 days due to profound hypotension. The patient was kept on oral prednisolone treatment for 28 days with the following gradual tapering. The hemogram and inflammatory biomarkers gradually returned to normal, and the patient was discharged. The fulminant presentation of HLH in our case could be the net result of both acute immunostimulation after COVID-19 vaccination and EBV infection. Our case suggests that the immune activation after COVID-19 vaccination is likely to interfere with the adequate immune response to certain infectious pathogens, resulting in a hyperinflammatory syndrome.

Protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reinfection relies on immunity generated after primary infection. However, humoral immunity following primary infection with the Omicron variant is not well understood. We prospectively recruited children <19 years with virologically-confirmed SARS-CoV-2 infection at National Cheng Kung University Hospital from February 2022 to September 2022 during the first wave of Omicron BA.2 outbreak in Taiwan. Serum samples were collected one month after acute infection to measure anti-spike protein receptor binding domain antibody levels and surrogate virus neutralizing antibody (NAb) levels against wild type disease and variants. Of the 164 patients enrolled, most were under 5 years (65.2%) with a diagnosis of upper respiratory tract infection. Children under 6 months with maternal coronavirus disease 2019 (COVID-19) vaccination had higher levels of both anti-SARS-CoV-2 spike antibody (119.0 vs 27.4 U/ml, p < 0.05) and anti-wild type NAb (56.9% vs 27.6% inhibition, p = 0.001) than those without. Children aged 5-12 years with prior vaccination had higher anti-spike antibody, anti-wild type, and anti-Omicron BA.2 NAb levels than those without (all p < 0.05). In previously naïve children without maternal or self-vaccination, those 6 months to 2 years had the highest antibody levels. Multivariable linear regression analysis showed age was the only independent factor associated with antibody level. In our study, children aged 6 months to 2 years have the highest antibody responses to SARS-CoV-2 Omicron variant infection. Age and prior vaccination are the main factors influencing the immunogenicity of SARS-CoV-2 infection.