VZV Foundation - A Decade Of Leadership In The Fight Against VZV Infections

	Oral Presentation Abstracts: 57 [57] MODELLING THE IMPACT OF VARICELLA VACCINATION ON THE EPIDEMIOLOGY OF VARICELLA AND ZOSTER *M. Brisson (1,2), W.J. Edmunds (1,2), N.J. Gay (1), B Law (3), G DeSerres (4) (1) PHLS Communicable Disease Surveillance Centre, London, UK; (2) City University, London, UK; (3) Department of Paediatrics and Child Health, University of Manitoba, Winnipeg Public health Research Unit, Winnipeg, Manitoba, Canada; (4) CHUL Research Centre, Faculty of Medicine, Laval University, Quebec, Quebec, Canada The objective of this study was to develop and apply a dynamic mathematical model of VZV transmission to predict the effect of different vaccination strategies on the age-specific incidence and outcome of infection. To do so a deterministic realistic age-structured model (RAS) was used which takes account of the increased potential for transmission within school aged groups. All model parameter estimates and intervals were derived from a literature review and from field data from Canada. Vaccine efficacy parameters were validated against observed breakthrough data. Various vaccine efficacy scenarios, vaccine coverages and vaccination strategies were investigated and a sensitivity analysis of varicella incidence predictions to important parameters was performed. The model predicts that the overall (natural and breakthrough) incidence and morbidity of varicella would likely be reduced by mass vaccination of 12-month-old children. However, the level of effectiveness of immunisation depends highly on the type of vaccination strategy, the efficacy of the vaccine and the coverage. Vaccination of children of 12 months with a catch up program in the first year for 1 to 11 year olds seems to be the most efficacious vaccine strategy in the short term. Furthermore, the better the vaccine the more mass vaccination reduces varicella. However, lower vaccine efficacy reduces the shift in the average age at infection (of natural infection) by allowing a certain number of cases to occur every year. Thus, for vaccine coverages under 80-90%, lower efficacy vaccines can be more effective in reducing morbidity. A short to medium term increase in zoster may occur after vaccination, particularly if a catch-up programme is implemented. Vaccination at 12 months with a catch-up in the first year of 1 to 11 year olds seems to be the most effective strategy to reduce both varicella incidence and morbidity (in the short and long term), though with the possible detrimental effect of increasing the incidence of zoster. Using a vaccine with lower efficacy could further reduced morbidity. Corresponding Author: M. Brisson, PHLS Communicable Disease Surveillance Centre, 61 Colindale Avenue, London NW9 5EQ, United Kingdom

[57]

MODELLING THE IMPACT OF VARICELLA VACCINATION ON THE EPIDEMIOLOGY OF VARICELLA AND ZOSTER

*M. Brisson (1,2), W.J. Edmunds (1,2), N.J. Gay (1), B Law (3), G DeSerres (4)
(1) PHLS Communicable Disease Surveillance Centre, London, UK; (2) City University, London, UK; (3) Department of Paediatrics and Child Health, University of Manitoba, Winnipeg Public health Research Unit, Winnipeg, Manitoba, Canada; (4) CHUL Research Centre, Faculty of Medicine, Laval University, Quebec, Quebec, Canada

The objective of this study was to develop and apply a dynamic mathematical model of VZV transmission to predict the effect of different vaccination strategies on the age-specific incidence and outcome of infection. To do so a deterministic realistic age-structured model (RAS) was used which takes account of the increased potential for transmission within school aged groups. All model parameter estimates and intervals were derived from a literature review and from field data from Canada. Vaccine efficacy parameters were validated against observed breakthrough data. Various vaccine efficacy scenarios, vaccine coverages and vaccination strategies were investigated and a sensitivity analysis of varicella incidence predictions to important parameters was performed. The model predicts that the overall (natural and breakthrough) incidence and morbidity of varicella would likely be reduced by mass vaccination of 12-month-old children. However, the level of effectiveness of immunisation depends highly on the type of vaccination strategy, the efficacy of the vaccine and the coverage. Vaccination of children of 12 months with a catch up program in the first year for 1 to 11 year olds seems to be the most efficacious vaccine strategy in the short term. Furthermore, the better the vaccine the more mass vaccination reduces varicella. However, lower vaccine efficacy reduces the shift in the average age at infection (of natural infection) by allowing a certain number of cases to occur every year. Thus, for vaccine coverages under 80-90%, lower efficacy vaccines can be more effective in reducing morbidity. A short to medium term increase in zoster may occur after vaccination, particularly if a catch-up programme is implemented. Vaccination at 12 months with a catch-up in the first year of 1 to 11 year olds seems to be the most effective strategy to reduce both varicella incidence and morbidity (in the short and long term), though with the possible detrimental effect of increasing the incidence of zoster. Using a vaccine with lower efficacy could further reduced morbidity.

Corresponding Author: M. Brisson, PHLS Communicable Disease Surveillance Centre, 61 Colindale Avenue, London NW9 5EQ, United Kingdom