圖、文／無國界醫生（MSF Taiwan）
黃熱病疫情自 2015 年 12 月以來在安哥拉肆虐，令人擔心會否蔓延至其他非洲國家或亞洲地區。疫苗的供應有限更是一項重大挑戰。無國界醫生（Doctors Without Borders/Médecins Sans Frontières，簡稱 MSF）流行病學專家范赫爾普（Michel Van Herp）講述最新進展。
黃熱病有何特徵？
黃熱病由病毒引起，被視為出血熱的一種。病毒主要由斑蚊傳播。在短暫的潛伏期後，受感染者可能會出現和感冒、瘧疾等大部分病毒一樣的輕微症狀，甚至完全沒有症狀，沒有任何事發生，約 80% 個案都屬於這一類。但有些人可能會進入第二期，肝臟受到影響，也因此稱之為「黃熱病」。在這段時期，一旦其他器官亦受到感染，便可能出現出血現象，死亡率可以高達 25% 至30%。
從醫學角度看，有什麼方法可以對抗黃熱病？
黃熱病在19世紀末和20世紀初在全球多個地區肆虐，通過研究已發展出一種十分有效的疫苗。有系統的疫苗接種計劃的進行顯著減少了疫情爆發。但在20世紀末，某些非洲國家不再把為人們有系統地接種疫苗視為優先工作，導致新疫情爆發，例如2000年在幾內亞的疫情。由於市面上疫苗出現短缺，一個國際統籌小組成立，負責管理每年600萬劑疫苗，作為策略性庫存，以應對疫情爆發。不過，黃熱病目前沒有專門的治療方法。治療只是醫治病人的症狀，透過控制症狀來幫助病人戰勝病魔。在第二階段，當病毒消失，病人不再具傳染性，最重要的便是防止肝臟壞死。
今年年初在安哥拉爆發的疫情有什麼不尋常之處？
近年，過往幾次疫情都是在樹林地區或水源附近爆發。但在安哥拉，疫情是在首都魯安達（Luanda）出現，當地已數十年沒有爆發過黃熱病，也沒有傳播病毒的蚊子。疾病可能是由在樹林裡染病的人，或者帶著帶有病毒的蚊卵的人傳開。一開始時只有少數人和蚊子受到感染，但疫情後來蔓延至一個很熱門的市集，全市不同地區的人都會來這個市集，因而把疾病傳開。當發現這個情況，並開始進行應對工作之前，受感染人數已上升，受感染的蚊子也跟著增加。然後疫情蔓延至其他省份，並透過旅遊人士傳到中國、肯亞和之後的剛果民主共和國。
疫情傳到安哥拉以外地區，會帶來什麼風險？
這難以預料。這涉及很多因素：旅遊人士是否在疾病的傳染期？那裡有否蚊子？例如在北京，受感染的旅遊人士是在冬季到達，因此沒有蚊子。第三個因素是：叮咬人類的蚊子本身必須已感染了病毒，但這並不一定發生。最後，透過受感染蚊子的卵傳播的情況亦不一定發生。
在爆發疫情之前，疾病會慢慢發展，尤其因為這是一種懶惰的蚊子，它們不會到處亂飛，因此主要是人們在國內的遷移促成疾病傳播。在這段個案逐步增加的期間，必須採取行動，就像我們目前在剛果正在做的一樣，結合針對性疫苗接種計劃和病媒控制工作，也就是杜絕蚊患和牠們的幼蟲。一旦確認個案，我們便要找出病人所在的地區，進行疫苗接種，派出隊伍在方圓150公里以內的範圍落實病媒控制措施。
在其他非洲國家或亞洲等從未受過黃熱病影響、但有斑蚊存在的地方，有機會爆發大規模疫情嗎？
無論如何，都要考慮到這個風險。作為預防措施，我們必須考慮到最差的情況，以便找出抗疫的不足或障礙，特別是疫苗生產方面。
熱病疫苗在大部分情況下，都不需要大規模生產，而且生產過程困難複雜。目前，600萬劑疫苗存貨已在安哥拉用完，但我們已透過捐贈，或把本來打算用在更廣泛疫苗接種項目中的黃熱病疫苗抽出來先用，找到市面上倘存的疫苗來填補庫存。另一個途徑是研究使用分散劑量：有針對其中一種疫苗的研究顯示，即使把疫苗分為5劑使用，因疫苗極為有效，故免疫力仍然存在。但目前未有就此作進一步研究，例如評估免疫力可以維持多長的時間。
疾病的爆炸性蔓延，只會因為人們的忽略而出現。只要當有個案出現後，持續保持警惕、監測和回應的工作，這情況完全可以避免。
Yellow fever: "Everyone needs to remain vigilant and responsive to avoid an explosion"
An outbreak of yellow fever has been ravaging Angola since December 2015, raising fears that the disease will spread to other African countries or Asia. The limited stocks of vaccines constitute a particular challenge. Michel Van Herp, an epidemiologist with MSF, gave us an update on the situation.
What are the characteristics of yellow fever?
It is a disease caused by a virus, considered to be a haemorrhagic fever. The virus is transmitted by the Aedes mosquito. After a short incubation period, the infected person may develop slight symptoms common to the majority of viruses (flu, malaria etc.), or even no symptoms at all, and nothing more may occur, which is what happens in about 80% of cases. But there may be a second phase, known as yellow because the liver is affected. This phase may also become haemorrhagic if other organs are affected. In this second phase, the mortality rate can be high: around 25% or 30%.
From a medical point of view, which tools are available to fight the disease?
Yellow fever wreaked havoc at the end of the 19th century and at the beginning of the 20th century in many parts of the world. Research led to the development of a highly effective vaccine. Systematic vaccination campaigns were launched and considerably reduced the epidemics. But at the end of the 20th century, systematic vaccination of the population became less of a priority in certain African countries and this led to new epidemics, such as the one that struck Guinea in 2000. In view of the shortage of vaccines on the market, an international coordination group (ICG) was set up to manage a strategic stock of six million doses per year, so as to be able to respond to an epidemic. However, there is no specific treatment available. The treatment is symptomatic: the patient is helped to overcome the disease by attacking the symptoms. During the second phase, when the virus disappears and the patient is no longer contagious, the most important thing is to prevent the liver from becoming necrotic.
What is unusual about the outbreak in Angola at the beginning of the year?
In recent times, epidemics have developed in forested areas or around water points. In Angola, it developed in the capital Luanda, which had been spared for decades and where there were no mosquitoes carrying the virus. The disease was probably brought there by a person who contracted it in the forest, or who brought in the eggs of mosquitoes carrying the virus. So it started with a limited number of infected humans and mosquitoes. But the epidemic affected a very popular market, visited by people from across the entire city who then spread the virus. By the time this was discovered and a response could be put in place, the number of cases in humans had risen and therefore the number of infected mosquitoes rose too. The epidemic then spread to the provinces and, via travellers, spread to countries like China, Kenya, and at a later stage, DR Congo.
What is the risk of the outbreak spreading outside Angola?
That is hard to predict. Multiple factors are involved: is the traveller in a contagious phase of the disease? Are there any mosquitoes in the area? In Beijing, for example, the infected travellers arrived during winter, when there weren't any. Third factor: the mosquito that bites the person must itself fall sick, which does not happen systematically. Finally, transmission via the eggs of infected mosquitoes does not happen systematically either.
Before an outbreak becomes explosive, it develops slowly, especially as this is a lazy mosquito that does not move around a great deal. So it is the movement of people that spreads the disease within a country. Action must be taken during this gradual build-up of cases, as we are doing now in the Congo by combining targeted vaccination campaigns and vector control activities, i.e. against the mosquito and its larvae. Once a case has been confirmed, we must identify the district where the victim has come from, carry out vaccination and send teams to implement vector control measures within a radius of 150 metres.
Is there a risk of massive outbreaks in other African countries or in Asia, which has never been affected by yellow fever but where the Aedes mosquito is present?
In any case, it is important to consider this risk. As a precaution, we must consider a worst-case scenario to find any weaknesses or obstacles, particularly in terms of vaccine production.
The yellow fever vaccine is a vaccine that most of the time does not require large-scale production. Moreover, it is very tricky to manufacture. The reserve stock of six million doses was used in Angola. But we replenished the stock with doses we found still on the market, through donations and by drawing on vaccines intended for broader vaccination programmes. Another avenue to explore is the use of divided doses: a study carried out on one type of vaccine shows that it is so effective that if the dose is divided by five, it will still provide immunity. Further studies, particularly to evaluate how long that immunity lasts, have yet to be carried out, however.
An explosive spread of the disease can only come about through negligence. It is perfectly avoidable by a continuing effort of vigilance, surveillance and responsiveness wherever cases arise.
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