Epidemiological Characteristics And Effect Evaluation Of Varicella Before And After Adjustment Of Immunization Strategy In Qingdao

ObjectiveVaricella is an acute infectious disease caused by Varicella-Zoster virus,characterized by systemic papules,blisters and scabs.VZV mainly transmit through respiratory droplets,direct or indirect contact,and can cause outbreaks in crowded areas within a short period.The inoculation of Live Attenuated Varicella Vaccine is regarded as the most effective way to prevent varicella.VarV vaccination can not only reduce the incidence and mortality of varicella,but also reduce the severe clinical symptoms in patients with varicella.To investigate the epidemiological characteristics of varicella before and after the adjustment of VarV immunization strategy in Qingdao from 2007 to 2020,and to evaluate the effect of immunization strategy on the incidence of varicella by using Interrupted Time Series analysis which can provide scientific basis for prevention and control of varicella epidemic and formulation of immunization strategy.MethodDescriptive epidemiological methods were used to analyse the data on incidence of varicella in Qingdao during the implementation of voluntary vaccination period（January2007 to June 2013）,the free 1-dose（July 2013 to June 2016）and the free 2-dose（July 2016to December 2020）immunization strategies periods with VarV.Chi-square test was used for comparison of multiple group incidence or ratios.The Cochran-Armitage test was used for the trend of birth cohorts incidence.Segmented linear regression models of interrupted time series analysis were constructed by R Studio Version 1.4.1103 to evaluate short-term and long-term effect of VarV vaccination immunization strategies on varicella incidence among the whole population,0 year,1-6 years,and 6 years and older,respectively.ResultsA total of 39,673 varicella cases were reported in Qingdao,with the incidence of32.54/100,000 from 2007 to 2020.The average incidence of varicella in three periods were33.16/100,000（16,111 cases）,31.18/100,000（8,368 cases）and 32.68/100,000（15,194cases）,respectively.Varicella mainly occured in people under 19 years old in Qingdao,with27,590 reported cases,accounting for 69.54%of the total cases in the whole population.The cases of varicella among people under 14 years old accounted for 63.16%,48.06%and40.52%in three periods,showing a decreasing trend.The varicella incidence of male in three periods were 37.76/100,000,32.88/100,000and 34.77/100,000 which were higher than the incidence of female 28.53/100,000（χ²=313.023,P（27）0.05）,29.44/100,000（χ²=25.548,P（27）0.05）and 30.50/100,000（χ²=64.658,P（27）0.05）,respectively.The difference was statistically significant.By 2020,the result of birth cohort analysis showed that the incidence of varicella in six cohorts of 2010 and of 2012 to 2016 were 65.54/100,000,51.84/100,000,59.10/100,000,41.65/100,000,47.64/100,000 and 24.04/100,000 which were the lowest at age 2,showing a decreasing trend,respectivly.Subsequently,the incidence of varicella began to increase in all cohorts.The varicella incidence of 2013 and 2016 cohorts were 75.54/100,000 and46.34/100,000 which were the highest at the age of 3 years.The varicella incidence of the2015 birth cohort was the highest at the age of 4,with an incidence of 77.70/100,000.The varicella incidence of three birth cohorts in 2010,2012 and 2014 were 164.28/100,000,111.69/100,000 and 68.47/100,000 which were the highest at the age of 5.The protection of 1-dose VarV lasted for 1～3 years.The varicella incidence of the five birth cohorts from2008 to 2012 decreased immediately in the year when the free 2-dose VarV immunization strategy was implemented and the incidence of each cohort ranged from 71.70/100,000 to40.15/100,000.Besides,the varicella incidence of cohort 2010 and cohort 2011 decreased to the lowest in 1 year and 2 years after the free 2-dose VarV immunization strategy implementation,the incidence were 74.71/100,000 and 108.46/100,000,and then began to rise which the protection of VarV lasted for 1～2 years.The results of the interrupted time series analysis showed that the trend of monthly varicella incidence in the whole population of Qingdao was escalating during the self-paying period significantly（β₁=0.0058,P（27）0.05）.The incidence decreased immediately after the free 1-dose VarV vaccination,with a significant short-term intervention effect（β₂=-0.2729,P（27）0.05）;the incidence trend change declined by 0.0105,with no significant effect of long-term intervention（P=0.0645）.The incidence dropped immediately after the free 2-dose VarV vaccination,with no significant effect of short-term intervention（β₄=-0.1049,P=0.4339）;the incidence trend change added by 0.0235,with a significant long-term intervention effect（P（27）0.05）.The monthly varicella incidence of 0-year-old infants shows a decreasing trend during the self-paying period,with no statistical significance（β₁=0.0087,P=0.6470）.After the free 1-dose VarV vaccination,the incidence increased briefly with a significant short-term intervention effect（β₂=5.3331,P（27）0.05）;the trend parameter of incidence for long-term decreased by 0.0229 with no statistical significance（P=0.5840）.After the free 2-dose VarV vaccination,there was a transient increase in incidence with a significant short-term intervention effect（β₄=5.0481,P（27）0.05）;the trend parameter of incidence for long-term decreased by 0.0482 with no statistical significance（P=0.2560）.The monthly varicella incidence of people aged 1-6 years old in Qingdao showed an increasing trend during the self-paying period,there was a statistical significance（β₁=0.0768,P（27）0.05）.The incidence decreased immediately after the free 1-dose VarV vaccination,with a significant short-term intervention effect（β₂=-1.6368,P（27）0.05）;the trend parameter of incidence for long-term decreased by 0.1351,with a statistical significance（P（27）0.05）.The incidence increased briefly after the free 2-dose VarV vaccination,with a significant short-term intervention effect（β₄=1.1889,P（27）0.05）;the trend parameter of incidence for long-term decreased by 0.0553,with a statistical significance（P（27）0.05）.The monthly varicella incidence of people aged over 7 years in Qingdao showed an increasing trend during the self-paying period,there was no statistical significance（β₁=0.0042,P=0.0649）.The incidence decreased immediately after the free 1-dose VarV vaccination,with no significant short-term intervention effect（β₂=-0.1815,P=0.1254）;the trend parameter of incidence for long-term decreased by 0.0041,with no statistical significance（P=0.4072）.The incidence decreased immediately after the free 2-dose VarV vaccination,with a non-significant short-term intervention effect（β₄=-1.1899,P=0.1064）;the trend parameter of incidence for long-term added by 0.0257,with a statistical significance（P（27）0.05）.Conclusion（1）VarV immunization strategy can reduce the varicella incidence of target people effectively in Qingdao.（2）The protection of the 1-dose VarV lasted for 1～3 years,and protection of the second dose of VarV lasted for 1～2 years after vaccination.（3）The effect of the free 2-dose VarV immunization strategy was better than the free 1-dose VarV immunization strategy.（4）The short-term and long-term effects of the free 1-dose and 2-dose VarV immunization strategies had the best protection for people aged 1-6 years old.