Contact-adjusted Immunity Levels against SARS-CoV-2 in Korea and Prospects for Achieving Herd Immunity

Abstract

The proportion of population vaccinated cannot be directly translated into the herd immunity. We have to account for the age-stratified contact patterns to calculate the population immunity level, since not every individual gathers evenly. Here, we calculated the contact-adjusted population immunity against severe acute respiratory syndrome coronavirus 2 in South Korea using age-specific incidence and vaccine uptake rate. We further explored options to achieve the theoretical herd immunity with age-varying immunity scenarios. As of June 21, 2021, when a quarter of the population received at least one dose of a coronavirus disease 2019 (COVID-19) vaccine, the contact-adjusted immunity level was 12.5% under the social distancing level 1. When 80% of individuals aged 10 years and over gained immunity, we could achieve a 58.2% contact-adjusted immunity level. The pros and cons of vaccinating children should be weighed since the risks of COVID-19 for the young are less than the elderly, and the long-term safety of vaccines is still obscure.

Keywords: COVID-19; Herd Immunity; Republic of Korea; SARS-CoV-2.

Fig. 1. Age-specific contact patterns in South Korea under the different social distancing measures.

Fig. 2. Contact-adjusted immunity level against COVID-19 (A) based on the reported age-specific incidence and vaccine uptake rates under the various social distancing measures, and (B) scenario modelling through raising immunity of each age group. Plain immunity was calculated by overall proportion of the population immune.

COVID-19 = coronavirus disease 2019. ^*In sensitivity analysis, the lowest value is drawn if there are 1.2 times contacts for age groups under 20 (< 20 years) and 0.8 times contacts for age groups 20 years and over (≥ 20 years); and the highest is drawn if there are 0.8 times contacts for < 20 years groups and 1.2 times contacts for ≥ 20 years groups.