A Case-Control Study On Varicella Vaccine Effectiveness

BackgroundVaricella is a kind of childhood infection which the main feature is fever and itching systemic dissemination of herpes and it's primary infection caused by the varicella zoster virus (varicella-zoster virus, VZV). The VZV infection in China is as high as 70% to 80% and there are more outbreaks at school. At present, the most effective means controlling VZV transmission is varicella vaccine. The United States has carried out routine immunization since 1996and the varicella morbidity, hospitalization and death rates has decreased significantly. China hasn't incorporated varicella vaccine into national immunization programs yet and only recommends children of 12 months to 12 years old to voluntary vaccinated once. The immunogenicity and safety of varicella vaccine has been assessed through clinical trials before being approved for marketing, but the study of the protective effect of vaccine post-marketing is comparative rare. This study used the methods of research design of epidemiology, observed and evaluated the protective effect and persistence of varicella vaccine, breakthrough cases, in order to fully evaluate the protective effect of varicella vaccine post-marketing, and to provide policy implications for the national immunization strategies.ObjectiveTo evaluate the protective effects of varicella vaccine, discuss the attenuation of varicella vaccine effectiveness over time, and compare the manifestations between breakthrough cases and non-vaccinated cases.MethodsBased on the data from the kindergarten children and primary school students in five municipal counties (districts) of the Tai'an City, Shandong Province, this paper uses 1:4 matched case-control study design, to collect information on the general situation of objects, varicella vaccination, disease exposure history and morbidity situation. The distribution of varicella cases and vaccine coverage is described, along with analyzing the protective effect and persistence of varicella vaccine in different areas and different exposure intensity andcomparing the manifestations between breakthrough cases and the unvaccinated cases.Results1.580 research objects,124 case group and 456 control group, which are from five counties(districts) of the 22 kindergartens and primary schools in Tai'an City, are intensively studied. The results show that there is no statistically difference between case and control groups in gender, age, ethnicity and history of chronic diseases aspects. Case groups are among the age of 2-12 years, but the majority is of 5-7 years oldand mostly morbidity in winter and spring.2. The overall vaccine efficacy of varicella in the kindergartens and primary schools in Tai'an City was 84%. Analysis indicates that the vaccine efficacy of varicella were 80%,88% in low exposure and high levels respectively in terms of the exposure level. In terms of different vaccination coverage, the vaccine efficacy of varicella was 84%,85% in low vaccination coverage and high vaccine coverage. Considering the level of exposure and analysis of vaccine coverage, the vaccine efficacy of varicella in low vaccination coverage levels with lower exposurewas 80%, that in low levels of vaccine coverage under the high-exposure group was 88%, that in high vaccine coverage group with Low-level exposure was 80%, and that in high vaccine coverage of high exposure level group the vaccine efficacy of varicella was 86%.3. Concerning about the preschool children and primary school students among research objects received varicella vaccinein terms of the different time intervals after vaccination in the survey time, after the vaccination of 1,2,3,4,5, and 6 years, the vaccine efficacy of varicella were 100%,91%,87%,76%,76%, and 51% respectively.The trend of gradual decreasing of vaccine efficacy over time was statistically significant (Chi-square test, P=0.0281<0.05). 4. Comparing disease severity of breakthrough cases group with the unvaccinated cases group indicates that the two groups are statistically different in 4 aspects:the number of rash, rash duration, whether fever and the duration of fever. In the unvaccinated cases group, the number of rash, rash duration, rate of fever and duration of fever were higher than that of the breakthrough cases group; but no statistically difference between two groups in skin lesion characteristics, bed rest time, the delay time for school, whether it lead to complications and whether need hospital treatment aspects.ConclusionThe varicella vaccine has high protective effects, but the protective effects decrease over time; Varicella vaccine may also reduce the clinical symptoms after varicella infection to some extent. The state or the high outbreak provinces could consider incorporate the varicella vaccine into immunization programs, implement 2 dose of varicella vaccine of children immunization strategy for better vaccine performance and establish the effective immunity barrier for further controlling the varicella outbreaks, epidemic, and reducing the public health risk and disease burden of varicella.