IMPORTANCE Multisystem inflammatory syndrome in children (MIS-C) is associated with recent or current SARS-CoV-2 infection. Information on MIS-C incidence is limited. OBJECTIVE To estimate population-based MIS-C incidence per 1 000 000 person-months and to estimate MIS-C incidence per 1 000 000 SARS-CoV-2 infections in persons younger than 21 years. DESIGN, SETTING, AND PARTICIPANTS This cohort study used enhanced surveillance data to identify persons with MIS-C during April to June 2020, in 7 jurisdictions reporting to both the Centers for Disease Control and Prevention national surveillance and to Overcoming COVID-19, a multicenter MIS-C study. Denominators for population-based estimates were derived from census estimates;denominators for incidence per 1 000 000 SARS-CoV-2 infections were estimated by applying published age-and month-specific multipliers accounting for underdetection of reported COVID-19 case counts.

The reemergence of deadly pandemic influenza virus strains has necessitated the development of improved methods for rapid detection and subtyping of influenza viruses that will enable more strains to be characterized at the molecular level. Representative circulating strains of human influenza viruses from primary clinical specimens were grown in cell culture, purified through polyethylene glycol precipitation, proteolytically digested with an endoproteinase, and analyzed and identified by high-resolution mass spectrometry using unique signature peptides that are characteristic of type A H1N1 and H3N2 and type B influenza viruses. This proteotyping approach enabled circulating strains of type A influenza virus to be typed and subtyped, cocirculating seasonal and pandemic H1N1 viruses to be differentiated, and the lineage of type B viruses to be determined through single-ion detection by high-resolution mass spectrometry. Results were obtained using virus titers comparable to those used in reverse transcription (RT)-PCR assays with clinical specimens grown in cell cultures. The methodology represents a more rapid and direct approach than RT-PCR and can be integrated into existing procedures currently used for the surveillance of emerging pandemic and seasonal influenza viruses.

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