Reliance on Vaccine-Only Pandemic Mitigation Strategies is Compromised by Highly Transmissible COVID-19 Variants: A Mathematical Modelling Study

Abstract

Background: New SARS-CoV-2 variants, such as B1.617.2 (Delta), are appearing that have higher transmissibility than the original Wuhan strain. Throughout 2020, the only responses available to mitigate SARS-CoV-2 transmission were non-pharmaceutical interventions such as social distancing, community lockdowns and quarantine measures. COVID-19 vaccines are now available and being administered, yet in countries with high vaccination rates, such as the United Kingdom and United States, a resurgence in COVID-19 cases has been observed and the Delta variant implicated. Of concern is the possibly lower efficacy of the ChAdOx1-S (AstraZeneca) vaccine against the Delta variant. We analysed alternative vaccination strategies to determine those most likely to mitigate future COVID-19 Delta outbreaks.Methods: We applied detailed agent-based community models to evaluate the impact of SARS-Cov-2 vaccination under a range of coverage levels, vaccine efficacies, and ages of those vaccinated. This was conducted with and without community lockdowns activated in response to significant outbreaks of highly transmissible SARS-CoV-2 variants. Critically, we determined whether vaccination in adolescents is required. We evaluated two vaccine types with different effectiveness potential against transmission.Findings: High vaccine efficacy and extremely high vaccination coverage of 90% of adults and adolescents was shown to be required to mitigate highly transmissible variants such as Delta without activation of strong lockdown measures, in contrast to the Alpha variant. We further demonstrated that inclusion of adolescent vaccination and use of mRNA vaccines (BNT162b2 Pfizer) to boost immunity levels among those previously vaccinated with the ChAdOx1-S (AstraZeneca) vaccine was effective in significantly reducing the scale of future COVID-19 outbreaks. Results suggest that greater than 70% vaccine coverage in those 12 years and older, together with a vaccine boosting regimen, would be sufficient to halt a rapidly growing Delta outbreak if coupled with early activation of moderate lockdown measures, which permit schools to remain open.Interpretation: Increasing the number of immune persons in the community by including adolescents in a vaccination program, and by applying “third dose” boosting vaccinations, may prevent B1.617.2 Delta outbreaks from rapidly growing without the need for strict lockdown measures. Under achievable 80% vaccination coverage moderate social distancing measures may be sufficient to prevent future large scale outbreaks. Compared to strict lockdown, such limited social distancing measures would substantially increase workplace numbers and allow schools to remain open, with attendant economic, mental and physical wellbeing, and educational benefits.Funding: The Department of Health, Western Australia (Future Health Research andInnovation Fund) and the Department of Health, Queensland, Australia.Declaration of Interest: GM reports research funding from the Department of Health, Queensland outside the submitted work. JC and DW declare no competing interests.