Varicella Vaccination Two-Dose Recommendations: An Advisory Committee Statement (ACS) National Advisory Committee on Immunization (NACI)†

Population immunity to varicella in Canada: A Canadian Immunization Research Network (CIRN) study

INTRODUCTION

The incidence of varicella in Canada has decreased by almost 99% since vaccination was introduced. However, variation in the timing and eligibility of vaccination programs across the country has resulted in some cohorts being under-vaccinated and therefore potentially susceptible to infection.

METHODS

We used nationally representative specimens from the Biobank of Statistics Canada's Canadian Health Measures Survey (CHMS) as well as residual specimens from Ontario collected between 2009-2014 to estimate population immunity across age-groups and geography, and identify any groups at increased risk of varicella infection.

RESULTS

The weighted proportion of specimens with antibody levels above the threshold of protection was 93.6% (95% CI: 92.4, 95.0). Protection was lowest among those aged 3-5 years (54.3%; 95% CI: 47.3, 61.4), but increased with age. Individuals born outside Canada had more than twice the odds of varicella susceptibility than those born in Canada (aOR: 2.7; 95% CI: 1.4, 5.0; p = 0.004). There were no differences by sex or geography within Canada, and there were no statistically significant differences when Ontario CHMS sera were compared to Ontario residual sera, apart from in participants aged 12-19 year age-group, for whom the CHMS estimate (91.2%; 95% CI: 86.7, 95.7) was significantly higher (p = 0.03) than that from residual specimens (85.9%, 95% CI: 81.1, 90.8).

DISCUSSION

Varicella immunity in Canada is changing. Children appear to have low population immunity, placing them at greater risk of infection and at increased risk of severe disease as they age. Our results underscore the importance of performing periodic serosurveys to monitor further population immunity changes as the proportion of vaccine-eligible birth-cohorts increases, and to continually assess the risk of outbreaks.

Explore the optimal timing for administering the second dose of the varicella vaccine in China

The prevalence of varicella in China has been increasing annually, with a relatively high incidence rate of breakthrough cases. Administering two doses of the varicella vaccine (Varv) proves to be the most effective measure. The objective of this study is to assess the immunogenicity of two doses of the Varv at varying intervals and explore the optimal timing for administering the second dose of the Varv. Utilizing a prospective cohort study design, the quantification of varicella immunoglobulin G (IgG) antibodies' geometric mean concentrations (GMC) is conducted through glycoprotein-based enzyme-linked immunosorbent assay (gpELISA). A total of 903 infants were included in the per-protocol population. After completing the first dose of the Varv, the GMC of antibody after 1 month (Group A) was 463.8 (447.6-480.1) mIU/mL. There was a statistically significant difference in GMC and seroconversion rates among the groups (B/C/D) that received the second dose of the Varv (p < 0.05). Multiple comparisons revealed that the group with a 3-year interval between the two vaccine doses had a higher GMC of 665.2 (622.6-707.8) mIU/mL compared to the group with a 1-year interval of 611.1 (577.1-645.3) mIU/mL and the group with a 5-year interval of 564.7 (540.1-589.4) mIU/mL. To effectively prevent and control the varicella epidemic in Jiangsu Province, two dose Varv vaccination is recommended, the optimal time point for the second dose Varv is 3 years after the first vaccination.

School, child care and camp exclusion policies for chickenpox: A rational approach

Widespread varicella vaccination in Canada since 2007 has changed the epidemiology of chickenpox, with a significant decline in varicella-related hospitalizations. However, there will always be a varicella-susceptible population because of immune-compromising treatments or conditions and/or parental vaccine refusal. This document updates and replaces a 1999 statement by the Canadian Paediatric Society. The CPS position on school and child care exclusion policies has not changed since the original statement, but because there have been two published reports of varicella outbreaks associated with summer camp attendance since 1999, this revision provides additional recommendations to mitigate risk for varicella transmission at summer camps.

A spot of bother: Why varicella vaccine programs matter

Objective

To evaluate Ontario's provincial varicella vaccination program through analysis of aggregate varicella cases in order to determine whether there has been a decrease in reportable disease burden; and to assess varicella vaccine adverse events following immunization (AEFIs).

Methods

Aggregate varicella cases (1993-2013) were extracted from the reportable disease databases. Pre-program (1993-2004) and post-program (2007-2013) periods were chosen according to implementation of the publicly funded vaccination program. AEFIs following administration of varicella vaccines (2010-2013) were also extracted. Reporting rates were calculated using net doses distributed as the denominator. Serious AEFIs were defined using World Health Organization standards.

Results

The incidence of aggregate varicella reports decreased significantly over the study period (from 311.4 to 22.2 cases per 100,000 population in 1993 and 2013, respectively). Incidence also decreased significantly in all age groups between the pre- and the post-program periods with a shift in age distribution towards older individuals in the post-program period. A total of 162 AEFIs following varicella vaccine were reported between 2010 and 2013 for an annualized reporting rate of 14.6 per 100,000 doses distributed. The most common events were rash (37.3%), including eight reports of varicella-like rash (0.7 per 100,000 doses distributed). Ten serious events were reported (0.9 per 100,000 doses distributed), and all vaccine recipients recovered.

Conclusion

Significant reductions in varicella disease incidence and low AEFI reporting rates were observed with the introduction of the publicly funded varicella vaccine program in Ontario. Continued surveillance is indicated to further assess trends in varicella disease and vaccine safety.

Rapid and accurate determination of the potency of varicella vaccine by quantitative polymerase chain reaction

The potency of varicella vaccines is currently determined by a plaque assay technique, which usually takes seven days and is laborious and has considerable inter- and intra-assay variability. Here, we report a new potency assay for varicella vaccine based on quantitative polymerase chain reaction in conjunction with a much more efficient virus infection step. Potency results can be obtained within 24h of infection and demonstrates acceptable accuracy and reproducibility when compared with the plaque assay, which relies on manual counting of plaques formed one week after viral infection. Using multiple vaccine lots from 7 manufacturers, we found no significant difference in infectivity determined between the new assay and plaque assay. The optimized conditions for viral infection and polymerase chain reaction are of significant value for the potency determination of the vaccine due to its rapidity, accuracy and the high throughput capacity of the assay.