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. 2021 Apr 14;28(3):taab015.
doi: 10.1093/jtm/taab015.

Yellow fever in Asia-a risk analysis

Bethan Cracknell Daniels  1 , Katy Gaythorpe  1 , Natsuko Imai  1 , Ilaria Dorigatti  1
Affiliations

Yellow fever in Asia-a risk analysis

Bethan Cracknell Daniels et al. J Travel Med. .

Abstract

Background: There is concern about the risk of yellow fever (YF) establishment in Asia, owing to rising numbers of urban outbreaks in endemic countries and globalisation. Following an outbreak in Angola in 2016, YF cases were introduced into China. Prior to this, YF had never been recorded in Asia, despite climatic suitability and the presence of mosquitoes. An outbreak in Asia could result in widespread fatalities and huge economic impact. Therefore, quantifying the potential risk of YF outbreaks in Asia is a public health priority.

Methods: Using international flight data and YF incidence estimates from 2016, we quantified the risk of YF introduction via air travel into Asia. In locations with evidence of a competent mosquito population, the potential for autochthonous YF transmission was estimated using a temperature-dependent model of the reproduction number and a branching process model assuming a negative binomial distribution.

Results: In total, 25 cities across Asia were estimated to be at risk of receiving at least one YF viraemic traveller during 2016. At their average temperatures, we estimated the probability of autochthonous transmission to be <50% in all cities, which was primarily due to the limited number of estimated introductions that year.

Conclusion: Despite the rise in air travel, we found low support for travel patterns between YF endemic countries and Asia resulting in autochthonous transmission during 2016. This supports the historic absence of YF in Asia and suggests it could be due to a limited number of introductions in previous years. Future increases in travel volumes or YF incidence can increase the number of introductions and the risk of autochthonous transmission. Given the high proportion of asymptomatic or mild infections and the challenges of YF surveillance, our model can be used to estimate the introduction and outbreak risk and can provide useful information to surveillance systems.

Keywords: Aedes; Arbovirus; Flavivirus; Modelling; Outbreak; Surveillance; Travel.

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Figures

Figure 1
Figure 1
Mean (point) and 95% CIs (bars) of the total predicted number of introductions of YF into (A) airports in South and East Asia and (B) airports in West Asia and the Middle East. Introductions presented are aggregated from all endemic countries. Only airports with an upper 95% confidence limit greater than one introduction are shown. CAN: Guangzhou Baiyun International Airport, PEK: Beijing Capital International Airport, PVG: Shanghai Pudong International Airport, HKG: Hong Kong International Airport, AMD: Sardar Vallabhbhai Patel International Airport; BOM: Chhatrapati Shivaji International Airport, DEL: Indira Gandhi International Airport, NRT: Narita International Airport, KUL: Kuala Lumpur International Airport, MNL: Ninoy Aquino International Airport, SIN: Singapore Changi Airport, ICN: Incheon International Airport, BKK: Suvarnabhumi Airport, BAH: Bahrain International Airport, KWI: Kuwait International Airport, BEY: Beirut–Rafic Hariri International Airport, MCT: Muscat International Airport, DOH: Hamad International Airport, DMM: King Fahd International Airport, JED: King Abdulaziz International Airport, MED: Prince Mohammad bin Abdulaziz International Airport, RUH: King Khalid International Airport, IST: Istanbul Airport, AUH: Abu Dhabi International Airport, DXB: Dubai International Airport
Figure 2
Figure 2
Mean (point) and 95% CIs (bars) of the predicted probabilities of autochthonous transmission in Asian cities, assuming transmission by (A) A. aegypti and (B) A. albopictus, given the independent introduction of at least one infectious individual. Probabilities are estimated at the average, minimum and maximum temperature for each location. Probabilities in red indicate an upper 95% probability of autochthonous transmission exceeding 0.5, denoted by the dashed red line
Figure 3
Figure 3
Probability of autochthonous transmission across increasing levels of population immunity at (A) the average temperature in Bangkok, Hong Kong and Mumbai assuming transmission by A. aegypti and (B) the maximum temperature in Beijing, Beirut and Guangzhou assuming transmission by A. albopictus. Also presented are probabilities at different values of the dispersion parameter k, where purple = 1, blue = 0.5, green = 0.1(baseline value) and red = 0.01. Thick lines are the mean and shaded areas are the 95% CIs. Horizontal dashed red line represents the point at which the probability of autochthonous transmission is <0.5. Vertical dashed line denotes 80% population immunity recommended by the WHO to stop local transmission in endemic countries
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