Model structure and agent-characteristics

Statecharts related to within-host dynamics of the model are shown in Fig. 1A. A disease statechart where agents are in protected, susceptible or disease states determined an agent’s probability of contracting chickenpox or shingles. The chickenpox and shingles vaccination schedules’ statecharts represented which vaccination each agent received. The chickenpox vaccination schedule was modeled based on the Alberta VZV vaccination policy (Government of Alberta, 2015). Agents become due for chickenpox vaccines at the ages of 12 months for the first dose and at 4–6 years—for the second dose (Fig. 1B). A representation of one-dose shingles vaccination given to individuals 50 years or older is depicted in the statechart shown in Fig. 1C; however, currently shingles is not part of the publicly funded schedule in Alberta and therefore was not included as part of this analysis. An agent receives a vaccine dose with a probability based on its vaccine attitude as described below. If an agent receives the second dose but has not yet received the first, it may also receive a catch-up of the first dose with a certain probability. These probabilities are specified by parameters, as shown in Table 1. A demographic statechart represented Alberta births, ageing and mortality characteristics (Fig. 1D).

(A) Disease and protection. (B) Chickenpox vaccination schedule. (C) Shingles vaccination schedule. (D) Demographics.

Notes:

An agent’s chance of being infected with chickenpox was dependent on whether they came into contact with someone with infection and the risk of transmission on such a contact. In comparison, an agent’s likelihood of getting shingles was determined by their individual waning immunity timer (i.e., after chickenpox infection, a countdown on “Immunity Waning Time” initiates; when it completes, the agents become susceptible to shingles) as well as the number and degree of boosting they received. The immunity waning time was represented by a formula derived from Ogunjimi et al. (2015) and further by calibration (Fig. S1; Table 2). This equation inherits assumptions whereby force of reactivation is represented by a gamma distribution, initial CMI is represented by a normal distribution and the waning of immunity rate for shingles is a fixed rate that can be altered using a coefficient—waning of immunity coefficient (WoI) (Ogunjimi et al., 2015). Furthermore, to account for the small but sharp increase in childhood shingles cases as observed in Russell et al. (2014), we included a small proportion of the population between the ages of 0 and 19 (5%) who had a short waning of immunity timer, allowing the model to account for individuals who may have a weaker force of boosting, lower initial CMI (e.g., immunocompromised) or a very quick waning of the immunity rate.

Notes:

An agent’s chance of getting shingles was determined by the number of years they were protected through boosting. The model assumed progressive boosting, as postulated by Guzzetta et al. (2016) and therefore the number of years of boosting protection was calculated by multiplying the number of times an agent comes into significant contact with the VZV by the duration in years of each boosting event. The number of boosting events was determined through the distance-based contact network and the duration of each boost was equal to the number of years of added protection from shingles derived through each boost. Altering the quantitative values of DoB and WoI of shingles resulted in significant changes in the incidence of shingles in a population.