Changes in epidemiological characteristics of varicella and breakthrough cases: Insights from Surveillance Data in Ningbo Spanning 2010 to 2023 (Preprint)
Authors: Pan Xingqiang 
Published: January 2025

BACKGROUND

Varicella is a prevalent respiratory infectious disease. It is often inadequately managed in many regions, leading to an underestimation of its true incidence rate. The varicella vaccine remains the most effective measure for controlling the disease, and its widespread use has significantly reduced incidence rates. Additionally, non-pharmacological interventions during the COVID-19 pandemic have notably altered the epidemiological patterns of varicella. Continuous monitoring is therefore essential to understand these evolving patterns, aiding in the adjustment of immunization strategies and the development of prevention and control measures.

OBJECTIVE

This study aims to monitor the epidemiological characteristics of varicella and the changes in breakthrough cases to inform adjustments in immunization strategies and enhance prevention efforts.

METHODS

From 2010 to 2011, varicella incidence was monitored using active surveillance methods, and the immune history of patients was investigated. From 2012 to 2023, passive surveillance methods were employed. Data on varicella patients were obtained from the Chinese Center for Disease Prevention and Control's information system. The study period was divided into four intervals: 2010-2011, 2012-2013, 2014-2019, and 2020-2023, to analyze epidemiological characteristics and trends. A birth cohort of children born at surveillance sites from 2009 to 2013 was established to examine the epidemiological characteristics and trends of breakthrough cases.

RESULTS

Between 2010 and 2023, 70,163 varicella cases were reported in Ningbo. Seasonal distribution indicated two incidence troughs before 2020, and only one from 2020 to 2023. The predominant age of onset was 10-14 years, accounting for 23.93% of cases. From 2010 to 2013, the highest incidence was among children aged 5-9; from 2014 to 2019, it shifted to those aged 10-14; and from 2020 to 2023, it was primarily among individuals aged 15-19. Following the introduction of a second vaccine dose (2014-2019), incidence among children under 10 decreased, notably by 59.54% in those aged 1-4. Conversely, incidence among individuals aged 10 and older increased, particularly by 123.78% in the 15-19 age group, with a significant upward trend (P trend <0.001). From 2020 to 2023, although incidence rates increased across age groups 15 and older, the rise was modest. The average annual incidence rate of breakthrough cases after one vaccine dose was 83.40/100,000 (range: 51.21-119.50/100,000), significantly higher than the 24.80/100,000 (range: 17.67-32.90/100,000) observed after two doses. However, the incidence of breakthrough cases after the first dose declined following the implementation of the two-dose program. The median time from vaccination to breakthrough case occurrence was 27 months (range: 17.5-48 months).

CONCLUSIONS

The epidemiological characteristics of varicella have changed significantly from 2010 to 2023, underscoring the necessity for continuous monitoring to effectively adjust immunization strategies and address emerging trends in breakthrough cases.