Surveillance results of influenza in Quzhou City, Zhengjiang Province, 2016-2020
YANG Hui1, YIN Zhi-ying1, ZHONG Jian-yue1, CAO Guo-ping1, YU Zhao2
1. Quzhou Municipal Center for Disease Control and Prevention, Quzhou, Zhejiang 324000, China; 2. Zhejiang Provincial Center for Disease Control and Prevention, Hangzhou, Zhejiang 310000, China
Abstract:Objective To study the epidemic characteristics of influenza in Quzhou City from 2016 to 2020, to analyze the relationship between influenza-like illness (ILI) cases and positive rate of influenza virus detection, and to provide a scientific basis for effective influenza prevention and control. Methods According to the national influenza surveillance program, we statistically analyzed the data about ILI cases reported by a national sentinel hospital and pathogen surveillance results from Quzhou Municipal Center for Disease Control and Prevention in Quzhou City from 2016 to 2020. Results A total of 41,995 ILI cases were reported in Quzhou City from 2016 to 2020, accounting for 4.41% (ILI%) of the total number of medical visits. The ILI% in 2016-2020 showed an upward trend (χ2trend=84.369, P<0.001). The ILI% in 2020 was significantly lower than the average ILI% in 2016-2019 (4.06% vs. 4.47%, χ2=33.657, P<0.001). Most ILI cases reported were from the 0- age group and the 0-14 age group, accounting for 45.98% (n=19,308) and 63.38% (n=26,617), respectively. A total of 5,589 ILI samples were collected, with a total positive rate of 17.07% (954/5,589). The positive rates in males and female were 18.12% (463/2,555) and 16.18% (491/3,034), respectively, and no statistically significant difference was found in the positive rate between the genders (χ2=3.680, P=0.055). The positive rates of samples in 2016-2020 were 14.95% (164/1,097), 20.02% (211/1,054), 14.11% (152/1,077), 33.36% (373/1,118) and 4.34% (54/1,243), respectively, showing an overall decreasing trend (χ2=8.656, P=0.003). The epidemic peak occurred in winter and spring of each year from 2016 to 2019, mainly from November to February of next year, and there was an epidemic peak in summer of 2018. The dominant strain was subtype A (H3N2), accounting for 42.14% (402/954). The positive rate of influenza virus was significantly different in different age groups (χ2=104.981, P<0.001). The numbers of ILI cases in the age groups 0-4 years, 5-14 years and ≥15 years were positively correlated with the positive rate of virus nucleic acid test, and the correlation coefficients were statistically significant (Spearman r=0.716, P=0.009; Spearman r=0.905, P<0.001; Spearman r=0.839, P=0.001). Conclusion The influenza A virus was the dominant species in Quzhou City from 2016 to 2020, with a high incidence in winter and spring, and occasionally in summer. Most ILI cases were children under 14 years old. It is suggested that influenza prevention and control should be strengthened among children below 14 years old. The number of ILI cases was positively correlated with the positive rate of virus nucleic acid test.