Risk of yellow fever virus transmission in the Asia-Pacific region

Yellow fever (YF), caused by Yellow Fever Virus (YFV), is endemic to Africa and South America but absent from the Asia-Pacific region. The reasons for this absence are not fully understood, but may include factors such as transmission barriers related to mosquito-virus compatibility, limited human viraemia, absence of a sylvatic cycle, and competition with other flaviviruses. YF presents with fever, headache, muscle aches, and vomiting, and can have a high case fatality rate. Historically, YF spread through slave trade and global commerce. Successful vector control and vaccine development have reduced the burden, but challenges remain due to insecticide resistance and vaccine supply issues. The mosquito vectors, *Aedes aegypti* and *Aedes albopictus*, are widely distributed globally. Increased travel between Asia and Africa raises concerns about YFV introduction into Asia. The 2015-2016 YF outbreak in Angola and subsequent YFV cases in returning Chinese travelers highlighted the risk. This study aimed to assess the vector competence of Asian-Pacific mosquitoes for YFV to evaluate the potential for YF emergence in the region. Previous studies on vector competence in the Asia-Pacific region are limited, making this research crucial.

Several studies have investigated YFV vector competence in Africa, South America, and the Caribbean. However, limited data exist for the Asia-Pacific region. Some studies have shown that certain Asian *Aedes aegypti* populations are capable of transmitting YFV under laboratory conditions. For example, *Ae. aegypti* from Laos (Bolikhamsai province) transmitted YFV after exposure to the YFV S-79 strain, and mosquitoes from Cambodia and Vietnam were susceptible to the American genotype 1. These studies, however, did not provide a comprehensive assessment of vector competence across diverse Asian-Pacific populations. The absence of a comprehensive assessment highlights the need for a broader investigation of YFV vector competence in the Asia-Pacific region to accurately assess the potential risk of YFV transmission.

The study used experimental infections of field-collected *Aedes aegypti* and *Aedes albopictus* mosquitoes from various locations in the Asia-Pacific region. Twelve *Ae. aegypti* and six *Ae. albopictus* populations were included (detailed information provided in Table 2 and Figure 6). Mosquitoes were reared in controlled laboratory conditions. Ten-day-old female adult mosquitoes were infected with a West-African genotype of YFV (strain S-79) via an infectious blood meal with a titer of 10⁷ ffu/mL. Infection, dissemination, and transmission rates were assessed at 14 and 21 days post-infection (dpi) by analyzing viral loads in the mosquito body, head, and saliva using plaque-forming assays (FFA) on C6/36 cells. Statistical comparisons were made using Fisher's exact test (for infection, dissemination, and transmission rates) and Kruskal-Wallis tests (for viral loads). Additionally, the study included *Ae. aegypti* populations from Africa (Cameroon and Congo) to provide a comparative analysis. Viral loads were estimated using a linear regression model to identify factors influencing differences in viral loads between regions. The risk of *Ae. aegypti*-mediated YFV transmission was determined by combining transmission efficiencies from the laboratory experiments with probabilities of vector occurrence from Kraemer et al. (2015).

The results showed high vector competence of *Ae. aegypti* for YFV in the Asia-Pacific region. At 14 dpi, infection rates (IR) varied significantly across *Ae. aegypti* populations, ranging from 41.7% to 95.8%. Dissemination rates (DR) also varied, ranging from 42.8% to 86.9%. Transmission rates (TR) at 14 dpi were observed in five populations, ranging from 12.5% to 45%. At 21 dpi, the IR reached 100% in four populations, and the DR and TR increased significantly across all populations. *Aedes albopictus* showed significantly lower competence than *Ae. aegypti*, with lower IR, DR, and TR. Among *Ae. aegypti* populations capable of transmission at 14 dpi, the number of viral particles in saliva ranged from 10¹·⁶ to 10³. At 21 dpi, all 12 *Ae. aegypti* populations transmitted, with saliva viral particles ranging from 5 to 23,000. Compared to African *Ae. aegypti*, Asian and Pacific *Ae. aegypti* showed significantly lower viral loads in the head but similar loads in the body and saliva. Regions with high mosquito occurrence and high transmission efficiency (Thailand, Vietnam, and Taiwan) presented the highest YFV transmission risk. The analysis suggests that the salivary glands may represent a more significant barrier than the midgut to YFV transmission.

The findings demonstrate that *Ae. aegypti* mosquitoes in the Asia-Pacific region are highly competent vectors for YFV. The high transmission rates observed, particularly in *Ae. aegypti* populations from Thailand, Vietnam, and Taiwan, coupled with the widespread distribution of this mosquito species, suggest a significant risk of YF emergence in the region. The lower competence of *Ae. albopictus* compared to *Ae. aegypti* implies a potentially less significant role for this species in YF transmission. The differences in viral loads between Asia-Pacific and African mosquitoes, specifically the lower head viral load in the former, may suggest the involvement of mosquito immune responses in limiting viral replication and transmission. While this study provides strong evidence of high vector competence, further research is needed to understand the complete picture of YFV transmission dynamics in the field, including the impact of environmental factors and differences in viral titers between laboratory experiments and actual patient viremia.

This study provides critical evidence for the high competence of *Aedes aegypti* mosquitoes in the Asia-Pacific region to transmit YFV. The significant risk of YF emergence in this region is highlighted by the high transmission rates observed in several locations. Further research should focus on field studies to validate these findings, investigate the role of mosquito immune responses, and explore the potential interactions between YFV and other endemic flaviviruses. Public health strategies should prioritize early detection of imported cases, mandatory vaccination for travelers, securing vaccine stockpiles, and exploring novel vector control methods to mitigate the risk of YF outbreaks.

The study was primarily conducted under laboratory conditions, and the findings may not fully reflect real-world transmission dynamics. The use of a single YFV strain might limit the generalizability of the results to other YFV strains. Viral loads in mosquito saliva obtained via the forced salivation method might not precisely represent the physiological dose delivered during a natural bite. The study focused solely on *Ae. aegypti* and *Ae. albopictus*, and other potential vector species were not considered. The risk assessment was based on data from Kraemer et al. (2015), which may have limitations regarding up-to-date mosquito distribution estimates.