Studying the mixed transmission in a community with age heterogeneity: COVID-19 as a case study

TL;DR

This study models COVID-19 transmission considering age differences and vaccination status, revealing how mixed contact rates influence epidemic waves and emphasizing the need for vaccinating younger populations.

Contribution

It introduces an age-stratified model fitted to Ontario data, highlighting the impact of mixed contact rates on epidemic dynamics and vaccination strategies.

Findings

Relaxed between-group contact rates can trigger epidemic waves.

Increasing mixed contact in older groups amplifies incidence in both groups.

Vaccinating younger individuals is crucial for controlling spread.

Abstract

COVID-19 has been prevalent worldwide for about 2 years now and has brought unprecedented challenges to our society. Before vaccines were available, the main disease intervention strategies were non-pharmaceutical. Starting December 2020, in Ontario, Canada, vaccines were approved for administering to vulnerable individuals and gradually expanded to all individuals above the age of 12. As the vaccine coverage reached a satisfactory level among the eligible population, normal social activities resumed and schools reopened starting September 2021. However, when schools reopen for in-person learning, children under the age of 12 are unvaccinated and are at higher risks of contracting the virus. We propose an age-stratified model based on the age and vaccine eligibility of the individuals. We fit our model to the data in Ontario, Canada and obtain a good fitting result. The results show that a relaxed between-group contact rate may trigger future epidemic waves more easily than an increased within-group contact rate. An increasing mixed contact rate of the older group quickly amplifies the daily incidence numbers for both groups whereas an increasing mixed contact rate of the younger group mainly leads to future waves in the younger group alone. The results indicate the importance of accelerating vaccine rollout for younger individuals in mitigating disease spread.