The changing epidemiology of varicella and herpes zoster in Hong Kong before universal varicella vaccination in 2014

Abstract

In Hong Kong, universal varicella vaccination started in July 2014. Before this, children could receive varicella vaccine via the private market. We analysed the epidemiology of varicella and zoster before universal vaccination. We estimated varicella vaccination coverage through surveys in preschool children. We estimated the burden of varicella and zoster with varicella notifications from 1999/00 to 2013/14, Accident and Emergency Department (A&E) attendance and inpatient admissions to public hospitals from 2004/05 to 2013/14. We fitted a catalytic model to serological data on antibodies against varicella-zoster virus to estimate the force of infection. We found that varicella vaccination coverage gradually increased to about 50% before programme inception. In children younger than 5 years, the annual rate of varicella notifications, varicella admission and zoster A&E attendance generally declined. The annual notification, A&E attendance and hospitalisation rate of varicella and zoster generally increased for individuals between 10 and 59 years old. Varicella serology indicated an age shift during the study period towards a higher proportion of infections in slightly older individuals, but the change was most notable before vaccine licensure. In conclusion, we observed a shift in the burden of varicella to slightly older age groups with a corresponding increase in incidence but it cannot necessarily be attributed to private market vaccine coverage alone. Increasing varicella vaccination uptake in the private market might affect varicella transmission and epidemiology, but not to the level of interrupting transmission.

Keywords: Chickenpox; surveillance; vaccines; varicella zoster; zoster (shingles).

Fig. 1.

Varicella vaccination in preschool children in Hong Kong. (a) Proportion of pre-school children in Hong Kong receiving varicella vaccine by birth cohort, 1995–2011 and (b) Interquartile range of age at receipt of varicella vaccination (months) for preschool children born from 2009 to 2011 by birth cohort and a dose of vaccine calculated from the survey in 2015. Note: Uptake on the second dose of vaccine was only recorded for children born in 2009–2011.

Fig. 2.

Varicella notification, varicella and zoster A&E attendance and hospitalisation in Hong Kong. (a) The average annual rate of varicella and zoster in Hong Kong during the study period, (b) relative frequency distribution for cases of all ages by month throughout the study period and (c) rate of varicella and zoster by epidemiological year. Notification is available only for varicella. Epidemiological year was defined as 12 months from September to August.

Fig. 3.

Coefficients (trends) of the Poisson regression on the annual rate of varicella notification, varicella and zoster A&E attendance and hospitalsations in Hong Kong. Notification is available only for varicella. A coefficient of zero indicates no change.

Fig. 4.

Varicella serology and estimated transmission parameters in Hong Kong in 1995, 2000, 2005 and 2010. (a) Proportion seropositive against varicella antibody by ELISA test (points with error bars representing 95% CI) and model fitting (line charts with shaded errors bands representing 95% CI), (b) average age of infection [AvAge], annual average force of infection [FOI] and basic reproduction number [R] and (c) reporting ratio between varicella notification and number of infections estimated. Age-specific data on varicella notification are only available for analysis for the year 1999 and onwards.