Objective:To obtain a prevalent respiratory syncytial virus (RSV) clinical isolate in Beijing and analyze the genotype and biological characteristics of the strain.Methods:A nasopharyngeal secretion specimen was collected from a child with RSV infection in Beijing in 2023 and used for viral isolation. Viral nucleic acid was amplified using qRT-PCR. The isolated virus was identified by transmission electron microscopy, indirect immunofluorescence assay, and plaque formation assay. A phylogenetic analysis was conducted based on the whole-genome sequencing results. Virus titers were determined, and replication characteristics were analyzed. The efficacy of the isolated strain for in vitro screening of antiviral drugs was validated. Results:A clinical RSV isolate, named hRSV/C-Tan/BJ 202301, was successfully isolated, which could form syncytia in Hep-2 cells. Spherical, filamentous, and irregular virus particles were observed by electron microscopy. Immunofluorescence detection showed green fluorescence in Hep-2 cells, and plaque assay showed round plaques, which were similar to the Long strain in morphology. Genomic sequence analysis showed that it belonged to ON1 genotype. It exhibited similar cell growth kinetics characteristics with the Long strain and could be used for antiviral drug screening in vitro. Conclusions:In this study, one RSV strain is successfully isolated and identified. The biological characteristics and the phylogenetic relationship of this strain reflect the characteristics of the circulating strains in Beijing, which provides experimental material for RSV vaccine development and antiviral drug screening in China.

Objective:To establish and optimize a novel method, focus forming assay (FFA), for quantitation of influenza A virus (FluA) and compare its application performance with traditional plague forming assay (PFA).Methods:The foci chromogenic effects of three peroxidase substrates in immunostaining were compared. The PFA and FFA methods were used to explore FluA incubation times and plaque morphology on 12-well plates, and to determine optimal incubation times and virus adsorption volumes for different FluA subtypes on 96-well plates. The correlation between FFA and PFA was evaluated, and the optimized FFA was applied to the in vitro antiviral efficacy analysis of Favipiravir and neutralization test against different subtypes of FluA. Results:TRUEBLUE substrate was identified as the optimal substrate for foci visualization. Compared with the PFA, the FFA showed improved sensitivity and reduced detection time in FluA titration, and good correlation was shown between the two methods′ results. By replacing the 96-well plate with the 12-well plate for FFA titration of different subtypes of FluA, the detection time was shortened, and the amount of serum samples used could be further reduced by optimizing the virus adsorption volume. The half-maximal effective concentration of favipiravir against influenza viruses assessed by the FFA and PFA methods showed no significant difference, and was consistent with the results obtained from quantitative PCR. Additionally, the focus reduction neutralization test and hemagglutination inhibition assays demonstrated strong correlation in determining antibody titers against FluA in serum neutralization assays.Conclusions:The improved FFA method developed here provides a more efficient experimental tool for FluA titration, antiviral drug screening and broad-spectrum vaccine evaluation.

Objective:To obtain a prevalent respiratory syncytial virus (RSV) clinical isolate in Beijing and analyze the genotype and biological characteristics of the strain.Methods:A nasopharyngeal secretion specimen was collected from a child with RSV infection in Beijing in 2023 and used for viral isolation. Viral nucleic acid was amplified using qRT-PCR. The isolated virus was identified by transmission electron microscopy, indirect immunofluorescence assay, and plaque formation assay. A phylogenetic analysis was conducted based on the whole-genome sequencing results. Virus titers were determined, and replication characteristics were analyzed. The efficacy of the isolated strain for in vitro screening of antiviral drugs was validated. Results:A clinical RSV isolate, named hRSV/C-Tan/BJ 202301, was successfully isolated, which could form syncytia in Hep-2 cells. Spherical, filamentous, and irregular virus particles were observed by electron microscopy. Immunofluorescence detection showed green fluorescence in Hep-2 cells, and plaque assay showed round plaques, which were similar to the Long strain in morphology. Genomic sequence analysis showed that it belonged to ON1 genotype. It exhibited similar cell growth kinetics characteristics with the Long strain and could be used for antiviral drug screening in vitro. Conclusions:In this study, one RSV strain is successfully isolated and identified. The biological characteristics and the phylogenetic relationship of this strain reflect the characteristics of the circulating strains in Beijing, which provides experimental material for RSV vaccine development and antiviral drug screening in China.

Objective:To establish and optimize a novel method, focus forming assay (FFA), for quantitation of influenza A virus (FluA) and compare its application performance with traditional plague forming assay (PFA).Methods:The foci chromogenic effects of three peroxidase substrates in immunostaining were compared. The PFA and FFA methods were used to explore FluA incubation times and plaque morphology on 12-well plates, and to determine optimal incubation times and virus adsorption volumes for different FluA subtypes on 96-well plates. The correlation between FFA and PFA was evaluated, and the optimized FFA was applied to the in vitro antiviral efficacy analysis of Favipiravir and neutralization test against different subtypes of FluA. Results:TRUEBLUE substrate was identified as the optimal substrate for foci visualization. Compared with the PFA, the FFA showed improved sensitivity and reduced detection time in FluA titration, and good correlation was shown between the two methods′ results. By replacing the 96-well plate with the 12-well plate for FFA titration of different subtypes of FluA, the detection time was shortened, and the amount of serum samples used could be further reduced by optimizing the virus adsorption volume. The half-maximal effective concentration of favipiravir against influenza viruses assessed by the FFA and PFA methods showed no significant difference, and was consistent with the results obtained from quantitative PCR. Additionally, the focus reduction neutralization test and hemagglutination inhibition assays demonstrated strong correlation in determining antibody titers against FluA in serum neutralization assays.Conclusions:The improved FFA method developed here provides a more efficient experimental tool for FluA titration, antiviral drug screening and broad-spectrum vaccine evaluation.