Impact of non-pharmacological interventions on COVID-19 boosting vaccine prioritization and vaccine-induced herd immunity: a population-stratified modelling study
Authors: Li Zhiyao , Wang JiaLe , Yang Boran , Li Wenjing , Xu Jian-Guo , Wang Tong 
Published: October 2021

Background

While the COVID-19 pandemic seemed far from the end, the booster vaccine project was proposed to further reduce the transmission risk and infections. However, handful studies have focused on questions that with limited vaccine capacity ether boosting high-risk workers first or prioritizing susceptible normal individuals is optimal, and vaccinating how many people can lead us to the goal of herd immunity. In this study we aimed to explore the conclusions of such two problems with consideration of non-pharmacological interventions including mandatory quarantine for international entrants, keeping social distance and wearing masks.

Methods

By implementing the corresponding proportion of individuals who remain infectious after four lengths of quarantine strategies to the novel population-stratified model, we quantified the impact of such measures on optimizing vaccine prioritization between high-risk workers and normal populations. Furthermore, by setting the hypothetical COVID-19 transmission severity (reproduction number, R0) to the level of the most contagious COVID-19 variant (B.1.617.2, delta variant, R0 = 5.0), we separately estimated the threshold vaccine coverage of five countries (China, United States, India, South Africa and Brazil) to reach herd immunity, with and without the consideration of interventions including wearings masks and keeping social distance. At last, the sensitive analysis of essential parameter settings was performed to examine the robustness of conclusions.

Results

For Chinese scenarios considered with moderate hypothetical transmission rate (R0 = 1.15–1.8), prioritizing high-risk workers the booster dose reached lower cumulative infections and deaths if at least 7-days of quarantine for international travelers is maintained, and the required screening time to remain such vaccinating strategy as optimal increased from 7-days to 21-days with the transmission severity. Although simply maintaining at least 7-days quarantine can lead to over 69.12% reduction in total infections, the improvement of longer quarantine strategies was becoming minimum and the least one was 2.28% between the 21 and the 28-days of quarantine. Besides, without the vaccination program, the impact of such measures on transmission control dropped significantly when R0 exceeded 1.5 and reached its minimal level when R0 equal to 2.5. On the other hand, when we combat the delta variant, the threshold vaccine coverage of total population to reach herd immunity lay within 74%–89% (corresponding to the vaccine efficiency from 70% to 50%), and such range decreased to 71%–84% if interventions including wearing mask and keeping social distance were implemented. Furthermore, Results of other countries with 85% vaccine efficiency were estimated at 79%, 91%, 94% and 96% for South Africa, Brazil, India and United States respectively.

Conclusions

Non-pharmacological interventions can substantially affect booster vaccination prioritization and the threshold condition to reach herd immunity. To combat the delta variant, restrictions need to be integrated with mass vaccination so that can reduce the transmission to the minimum level, and the 21-days might be the suggested maximum quarantine duration according to the cost-effectiveness. Besides, by implementing interventions, the requirement to reach herd immunity can be lower in all countries. Lastly, the following surveillance after vaccination can help ensure the real-time proportion of vaccinated individuals with sufficient protection.