Projecting the transition of COVID-19 burden towards the young population while vaccines are rolled out: a modelling study

Abstract

SARS-CoV-2 infection causes most cases of severe illness and fatality in older age groups. In China, over 85% of individuals aged ⩾12 years have been vaccinated against COVID-19 (albeit with vaccines developed against historical lineages), while children aged 0–11 years are currently not eligible for vaccination (as of September 2021). The aim of this work is to assess whether the importation of Delta variant infections will shift the COVID-19 burden from adults to children. We developed an age-structured susceptible-infectious-removed model of SARS-CoV-2 transmission dynamics to simulate the epidemics triggered by the importation of Delta variant infections and project the age-specific incidence of SARS-CoV-2 infections, cases, hospitalisations, intensive care unit (ICU) admissions, and deaths. In the context of the vaccination programme targeting individuals aged ≥12 years (as of September 2021), and in the absence of non-pharmaceutical interventions, the importation of Delta variant infections could lead to widespread transmission and substantial disease burden in mainland China, even with vaccination coverage as high as 97% across the currently eligible age groups. The symptomatic SARS-CoV-2 infections and hospitalisation are projected to shift towards children and young adolescents, with 13% of symptomatic infections and 30% of hospitalisations occurring in those aged 0–11 years. Extending the vaccination roll-out to include children aged 3–11 years is estimated to dramatically decrease the burden of symptomatic infections and hospitalisations within this age group (54% and 81%, respectively), but would have a low impact on protecting infants (aged 0–2 years). Our findings highlight the need to strengthen vaccination efforts by simultaneously extending the target population and elevating vaccine effectiveness. Author summary Older age groups have the highest risk of severe illness and fatality from SARS-CoV-2 infection. While the vaccination coverage in China is highly skewed towards older age groups. No study has quantified to what extent the spread of Delta variant infections and lack of vaccination in younger age groups will shift the COVID-19 burden towards younger age groups and how this will affect the return to normal. To this end, we developed an age-structured transmission model to mimic the epidemics triggered by imported Delta variant infections in China. In the context of the vaccination programme targeting individuals aged ≥12 years as of September 2021, and in the absence of non-pharmaceutical interventions, the importation of Delta variant infections would lead to substantial disease burden, which will shift towards the 0–11-year age group. Extending the vaccination to children aged 3–11 years, for whom the currently available vaccines have been licensed, is projected to dramatically decrease symptomatic infections and hospitalisations within this age group but would have a low impact in protecting infants aged 0–2 years. This study highlights the value of extending vaccination to children aged 3–11 years and protecting infants 0–2 years by vaccinating their contacts.