Estimating Age-Specific Immunity and Force of Infection of Varicella Zoster Virus in Norway Using Mixture Models.
This study applies mixture modelling to examine age-specific immunity to varicella zoster virus (VZV) infection in Norway based on the first large-scale serological study in the general population. We estimated the seropositive proportions at different ages and calculated the underlying force of infection by using a sample of 2103 residual sera obtained from patients seeking primary and hospital care. A rapid increase in the VZV-associated immunity is observed in the first years of life with 63% of children being immune by age 5. The increase in the immunity levels slows down thereafter, with a large proportion of adults still susceptible by age 20 (around 14.5%), thus at risk of serious sequelae of varicella infection. The corresponding force of infection peaks during the preschool period, subsequently declines to a minimum between ages 10 and 20 years, and afterwards moderately increases to reach a plateau lasting throughout the childbearing period. In comparison with the traditional cut-off approach, mixture modelling used the whole data without producing any inconclusive cases, led to an unbiased classification of individuals between susceptible and immune, and provided a smoother immune profile by age. These findings represent an important step towards any decision about the introduction of varicella vaccination in Norway, as they are a primary input for mathematical transmission models aimed at evaluating potential vaccination scenarios.
Carlo F. Dondena Centre for Research on Social Dynamics and Public Policy, Bocconi University, Milan, Italy.
Emanuele
http://orcid.org/0000-0001-7352-8525
E
Del Fava
Department of Vaccines, Norwegian Institute of Public Health, Oslo, Norway.
Grazina
G
Rimseliene
Department of Vaccines, Norwegian Institute of Public Health, Oslo, Norway.
Elmira
E
Flem
Oslo Centre for Biostatistics and Epidemiology, University of Oslo, Oslo, Norway.|Department of Infectious Disease Epidemiology, Norwegian Institute of Public Health, Oslo, Norway.
Birgitte
B
Freiesleben de Blasio
Department of Mathematics, University of Rome Tor Vergata, Rome, Italy.
Gianpaolo
G
Scalia Tomba
Department of Economics and Management, University of Pisa, Pisa, Italy.
Piero
P
Manfredi
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