Epidemiological and Virological Surveillance of Seasonal Influenza Viruses—China,2020–2021

Introduction:During the coronavirus disease 2019(COVID-19)pandemic,the circulation of seasonal influenza virus declined globally and remained below previous seasonal levels.We analyzed the results of the epidemiology,antigenic,and genetic characteristics,and antiviral susceptibilities of seasonal influenza viruses isolated from the mainland of China during October 5,2020 through September 5,2021,to better assess the risk of influenza during subsequent influenza season in 2021–2022.Methods:Positive rates of influenza virus detection during this period were based on real-time polymerase chain reaction(PCR)detection by the Chinese National Influenza Surveillance Network laboratories,and isolated viruses from influenza positive samples were submitted to the Chinese National Influenza Center.Antigenic analyses for influenza viruses were conducted using the hemagglutination inhibition assay.Next-generation sequencing was used for genetic analyses.Viruses were tested for resistance to antiviral medications using a phenotypic assay and next-generation sequencing.Results:In southern China,the influenza positivity rate was elevated especially after March 2021 and was higher than the same period the previous year with the COVID-19 pandemic.In northern China,influenza positive rate peaked at Week 18 in 2021 and has declined since then.Nearly all isolated viruses were B/Victoria lineage viruses during the study period,and 37.3%of these viruses are antigenically similar to the reference viruses representing the vaccine components for the 2020–2021 and 2021–2022 Northern Hemisphere influenza season.All seasonal influenza viruses were susceptible to neuraminidase inhibitors and endonuclease inhibitors.Conclusions:Influenza activity has gradually increased in the mainland of China in 2021,although the intensity of activity is still lower than before the COVID-19 pandemic.The diversity of circulating influenza types/subtypes decreased,with the vast majority being B/Victoria lineage viruses.The surveillance data from this study suggest that we should strengthen influenza surveillance during the upcoming traditional influenza season.It also provided evidence for vaccine recommendations and prevention and control of influenza and clinical use of antiviral drugs.

Characterization of Influenza Viruses-China, 2019−2020

Introduction:Influenza surveillance is necessary for detection of emerging variants of influenza viruses and determining their epidemiological and clinical significance.Vaccination and antiviral drugs remain the most useful ways to protect against seasonal influenza and its potentially severe consequences.This study describes the epidemiology,antigenic and genetic characteristics,and antiviral susceptibilities of influenza viruses isolated from the mainland of China during the April 1,2019 through October 4,2020.Methods:All viruses analyzed were isolated and submitted by Chinese National Influenza Surveillance Network laboratories.The Chinese National Influenza Center performed antigenic analysis,sequencing,and antiviral resistance testing after propagation of the viruses.Antigenic characterizations were determined by hemagglutinin inhibition assay;next-generation sequencing was used for genetic analyses;phenotypic assay and next-generation sequencing were used for determining antiviral resistance.Results:The influenza positivity rate declined significantly starting in late January 2020 and has remained low.There was no summer influenza peak season in south China.Influenza A(H3N2)and B/Victoria lineage viruses were the dominant subtype/lineage during April 1,2019 through October 4,2020.The majority of influenza viruses were antigenically and genetically similar to reference viruses representing components of vaccines for the 2020–2021 northern hemisphere influenza season.Nearly all seasonal influenza viruses were susceptible to oseltamivir and zanamivir.Conclusions and Implications for Public Health Practice:Since the outbreak of COVID-19,the influenza positivity rate declined with implementation of strong COVID-19 control measures.The majority of circulating viruses are well matched with the current 2020–2021 northern hemisphere influenza vaccine viruses.Circulating seasonal influenza viruses were sensitive to neuraminidase inhibitors and Baloxavir marboxil.This study provided evidence for World Health Organization(WHO)recommendations for vaccine viruses,for prevention and control of influenza,and for clinical use of antiviral medications.

Mutations associated with egg adaptation of influenza A(H1N1)pdm09 virus in laboratory based surveillance in China, 2009–2016

Mutations of influenza virus associated with adaptation occurring during passage in embryonated chicken eggs could result in antigenic change or reduced vaccine effectiveness.In this study,we investigated the mutations of influenza A(H1N1)pdm09 egg isolates from the Chinese National Influenza Surveillance Network between 2009 and 2016.Thirteen mutations were identified in the hemagglutinin(HA)protein from viruses passaged in eggs,in comparing to those in cells.After scanning public database,four mutations,D127E,L191I,D222G/N and Q223R in HA1,which may alter the receptor-binding specificity,were observed frequently.Although the A(H1N1)pdm09 virus has evolved in human for nearly ten years,most egg-cultured viruses acquired one or more further mutations.Using the egg isolates for influenza surveillance requires extra caution because of these selected mutations,and their impacts on antigenicity and receptor-binding property need further evaluation.Currently,most of the influenza vaccines are produced using egg isolates,particularly in China.Thus,there is an urge to promote the establishment of an alternative influenza vaccine production platform.

Different Starting Dominant Strain of Seasonal Influenza in China and Other Neighboring Asian Countries in 2019−2020 Winter Season

Introduction:Seasonal influenza is a prevalent and highly contagious acute respiratory disease that causes a major global disease burden.New strains of influenza viruses also have the potential to cause an influenza pandemic.Due to the high variability and the consequent uncertainty about virus strains,the prevention and control of influenza are faced with many challenges.Surveillance is considered a key strategy to lead to the prevention and control of influenza,and influenza is one of first infectious diseases to be monitored globally.Methods:The Chinese National Influenza Surveillance Network conducts routine influenza surveillance and includes 410 network laboratories and 554 sentinel hospitals based on the requirements of National Influenza Surveillance Guidelines.China and other countries submit epidemiological and virological data to the WHO FluMart database,which can be viewed online.Results:Seasonal influenza H3N2 virus was the dominant subtype in China when entering the 2019–2020 winter influenza season,while neighboring countries such as Japan and Republic of Korea reported the dominance of H1N1pdm09 and Mongolia and Russia reported a higher proportion of type B virus than type A virus.Conclusions and Implications for Public Health Practice:Asian countries entered the 2019–2020 winter influenza season with different type/subtypes of influenza virus dominant in different areas.Influenza surveillance needs to be strengthened to closely monitor changes in the antigenicity and genetic characteristics of emerging viruses.