A Severe Epidemic of Influenza from September 2017 to February 2018 in China after the 2009 Pandemic Influenza

Background Influenza viruses continue to cause epidemics worldwide every year. However, due to the lack of an effective assessment for the severity of influenza epidemics, it was extremely difficult to take preventative measures. Methods To explore the severity of influenza epidemics and subsequent preventative measures, influenza incidence and fatality data were extracted from reports of notifiable infectious diseases from 2011 to 2018. Joinpoint regression models were used for the description of the trends of influenza activities. Phylogenetic trees were employed for analysis of the homology of influenza viruses and vaccine strains. Additionally, susceptible-exposed-infectious-recovered (SEIR) models were used for an assessment of intervention measures against the influenza epidemic. Findings The reported incidence of influenza was 1,913,698 from January-2011 to February-2018, with an average-yearly-reported-incidence-rate of 19·21 per 100,000. However, there had been a substantial nationwide epidemic of influenza after September 2017, when the average yearly reported incidence rate was 87·29 per 100,000 and an annual percentage change of 48·1%. The HA genes of most influenza A (H1N1 and H3N2) viruses from the period of the epidemic had lower homology to those before August 2017. All the HA of the recommended A (H3N2, H1N1) and B (Victoria) viruses for vaccines 2017/2018 had low matches with the epidemic viruses. The epidemics of the subtypes H1, H3 and B appeared to be overlapping, and the epidemic peak for the subtype H1 was sharply high. The average basic reproduction (R0) was 1·53. The vaccination benefit was linearly related to vaccination coverage, while the quarantine measure had only significantly benefited over 60% of the quarantined population. Interpretation The most severe epidemic of influenza in China since 2011 occurred during the period from September-2017 to February-2018. The epidemic severity was related to substantial phylogenetic changes in and overlapping epidemics of the influenza H1N1, H3N2 and B viruses, as well as their low matches with the recommended vaccine strains. The most effective way to prevent severe epidemics of influenza is through the use of vaccinations, using precise recommended strains based on regional differences. Funding: This study was supported by grants from the National Natural Science Foundation of China (81672005, 81001271), the Key Joint Project for Data Center of the National Natural Science Foundation of China (U1611264), the Mega-Project of National Science and Technology for the 12th Five-Year Plan of China (2014ZX10004008, 2013ZX10004901, 2013ZX10004904 and 2011ZX10004-901), and the Fundamental Research Funds for the Central Universities. Declaration of Interest: We declare no competing interests. Ethical Approval: The research ethics board at the First Affiliated Hospital, School of Medicine, Zhejiang University, approved the study.