| Background:Influenza is an acute respiratory infection caused by influenza virus,which is highly infectious and spreads rapidly.According to the Infectious diseases law of the People's Republic of China,influenza is one of the third-class infectious diseases and the first infectious disease to achieve global surveillance.In China,there was a severe seasonal influenza epidemic during the period from the end of 2017 to the early of2018,with a significant increase in the number of influenza patients compared with previous years,leading to a large increase in influenza-related hospitalizations,severe illnesses and deaths.This was the first serious seasonal influenza outbreak in China since the 2009-2010 pandemic.The lack of an effective assessment of the size,timing and severity of an influenza pandemic makes it extremely difficult to take preventive measures against influenza.While severe illness and deaths have declined in countries with influenza vaccination programs,relatively few countries have well-developed vaccination programs for seasonal influenza,and the effect of interventions such as isolation in the population is unclear.On the one hand,this study obtained the main parameters of influenza outbreak by establishing the influenza transmission model,so as to accurately grasp the latest infectious diseases and provide scientific basis for the formulation of prevention and control strategies.On the other hand,the analysis of clinical data of influenza can provide scientific basis for early warning and early treatment of critical illness or death in clinic and reduce economic burden.Method:1.Influenza incidence and fatality data from 2011 to 2018 were extracted from reports of notifiable infectious diseases,which were open and available from the official website of National Health Commission of the People's Republic of China.Joinpoint regression model was used to evaluate the incidence trend.The transmission dynamics model(SEIR)was used to assess the main parameters of an influenza outbreak and to evaluate interventions for influenza epidemics.2.The gene sequences of influenza and influenza vaccine from 2011-2018 were downloaded from the GISAID gene bank,and the phylogenetic tree analysis of influenza etiology was conducted to understand the pathogenic variation of influenza and the matching of influenza vaccine strains.3.Five hospitals were selected from Hangzhou,Shaoxing and Jiaxing regions of Zhejiang province to collect clinical characteristics,laboratory examination,treatment,prognosis and other data of hospitalized patients with influenza from 2011 to 2018,and to conduct comparative analysis of clinical characteristics of influenza cases with different epidemic seasons and different subtypes of influenza.Results:1.The average reported incidence of influenza from January 2011 to February2018 was 19.21 per 100,000,while the reported incidence from September 2017 to February 2018 was as high as 87.29 per 100,000.During September 2017 to February2018,the basic reproduction(R0)was 1.53,and the estimated eventual prevalence without intervention is 9.13%.2.The HA genes of most influenza A(H1N1 and H3N2)strains from September2017 to February 2018 had lower homology to those before August 2017.The HA of the recommended A(H3N2,H1N1)and B(Victoria)stains for vaccines 2017/18 had low matches with the epidemic strains.3.The number of influenza cases was expected to reduce by 65 million at the vaccine coverage rate of 40%.The incidence reduction was linearly related to the influenza vaccine coverage rate.When the quarantine rate of infected population was less than 60%,the quarantine measures had no significant benefit in controlling the incidence of influenza.4.In-hospital fatality was over 5-folds higher during the 2017-2018(P<0.01)in which 19 patients died(6.6%),whereas only 2 mortalities(1.2%)were observed during 2011-2017.Of the 289 hospitalized in 2017-2018,153 were confirmed with influenza B virus,110 with A/H1N1pdm09,and 26 A/H3N2,whereas A/H1N1pdm09 was the predominant cause of hospitalization in previous seasons combined(45%).Fatal cases in 2017-2018 were exclusively associated with either influenza B or A/H1N1pdm09.Our results show that a significant lower proportion of patients aged14 or greater were treated with oseltamivir,during the 2017-2018 epidemic,and the clinical manifestations are more serious.Conclusion:1.The incidence of influenza has increased from January 2011 to February 2018,and there was a significant epidemic peak between September 2017 and February 2018.This was the first serious seasonal influenza outbreak since the 2009-2010 influenza pandemic,and the genetic sequence of the pandemic strain was less matched with that of the vaccine strain.2.Vaccination is a more effective way to prevent influenza than quarantine.To prevent severe influenza epidemics,strategies should be adopted to increase influenza vaccination coverage,which can effectively control the scale of influenza epidemics by reaching 40%.3.In-hospital fatality rate might be significantly higher in the 2017-2018 season in China.Providing antiviral therapy such as oseltamivir within 48 hours of onset may reduce fatality rates. |
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