# Ring vaccination and vector control as control strategies for potential yellow fever outbreak in an Asian city

## TL;DR

The study models yellow fever outbreak control in Singapore, finding that vector control and timely ring vaccination are key to reducing infections and deaths.

## Contribution

The novel contribution is evaluating the combined effectiveness of vector control and ring vaccination under varying conditions in a spatial model.

## Key findings

- Vector control significantly reduces exposed cases by 89% when lowering mosquito-to-human ratios.
- Vaccination coverage of 90% reduces cumulative cases by 88% with a 7-day rollout delay.
- Rapid response and stockpiled vaccines are critical for containment when yellow fever is not yet endemic.

## Abstract

Yellow Fever (YF) importation remains an active risk to Southeast Asia. This study aims to determine the effectiveness of vector control and ring vaccination as containment strategies.

We modelled a YF outbreak in Singapore over 1 year using a metapopulation vector-host spatial model to explore the impact of a potential epidemic and intervention effectiveness. 30 different scenarios were examined by varying the vector to human ratio m ([1, 3, 6]), vaccination coverage ([10 %, 50 %, 90 %]) and delay in vaccine rollout ([7, 14, 30 days]), including three non-vaccination scenarios with the vector-to-human ratio m ([1, 3, 6]).

Vector control has a significant protective effect with an 89 % reduction in the cumulative number of exposed cases at Day 365 when lowering m from 6 to 1 in the baseline scenario without ring vaccination. Vaccination coverage levels of 90 %, 50 %, and 10 % reduce the cumulative number of exposed cases by 88 %, 56 %, and 12 %, respectively, compared to baseline, when fixing m = 3 and a 7-day rollout delay. A greater number of severe infections and deaths can be mitigated by decreasing the ratio m compared to ring vaccination strategies. The marginal gains in averting the number of infections and deaths are most significant when m is decreased, followed by increased vaccination coverage and reduced intervention delay as R0 is proportional to m. This highlights the central role of vector control. Our findings suggested that ring vaccination is effective under lower mosquito-to-human ratios up to 1-week post-detection, with vaccination coverage of at least 50 %. Under these settings, vaccine doses equal to 25 % of the total population are needed to contain the initial outbreak, allowing time to monitor its progress and restock the supply. After that, further interventions where YF has not yet been declared endemic.

Our findings suggested that ring vaccination is effective under lower mosquito-to-human ratios up to 1-week post-detection, with vaccination coverage of at least 50 %. After that, further interventions are required to bring the effective reproduction number Reff under 1, highlighting the need for rapid response and containment, preparation in the stockpiling of vaccines, and continual suppression of mosquito vector populations when faced with the risk of YF importation and outbreak.