Estimating the basic reproductive number of varicella in South Korea incorporating social contact patterns and seroprevalence

Abstract

Varicella, which is caused by the varicella zoster virus (VZV), is a common infectious disease affecting children. Varicella vaccines have been used for decades; however, vaccination policies vary across countries because of differences in VZV epidemiology. The basic reproductive number $R_0$_a transmissibility measure parameter, also differs from country to country. In this study $R_0$ for varicella was estimated in South Korea using the contact rate matrix derived from averaged POLYMOD contact data, the Korean population, and proportionality factor fitted to the Korean VZV seroprevalence $R_0$ for varicella in South Korea was estimated to be 5.67 (95% CI: 5.33, 6.33). Therefore, to reach the herd immunity threshold, the critical vaccine coverage should be greater than 82.4% with a perfect vaccine, or the primary vaccine failure proportion should be less than 17.6% with 100% coverage. Because of the relatively low seroconversion rate and rapidly waning immunity after one-dose vaccination in South Korea, the herd immunity threshold is difficult to attain with only a one-dose vaccine. Two doses of vaccination may be necessary to effectively interrupt varicella transmission and maintain herd immunity in South Korea. The study results can help guide the decision-making on an effective varicella vaccination policy in South Korea.

Keywords: Korea; Varicella; basic reproductive number; contact patterns; herd immunity threshold.

Figure 1.

Who-Acquired-Infection-From-Whom matrices (a) and age-specific VZV seroprevalence data with model prediction and corresponding age-specific force of infection (b)

Figure 2.

Changes in critical vaccine coverage according to changes in varicella primary vaccine failure for the estimated basic reproductive number (${\mathrm{R}}_0$) of the base case (a) and those of all sensitivity analyses (b)