Chinese people are generally not familiar with yellow fever . This is not surprising, because this disease mainly breaks out and is popular in Africa, America and Europe, and has not been affected in mainland China's history. However, looking around the world, yellow fever has always been a serious infectious disease that cannot be ignored.
Vaccination is by far the most effective way to prevent yellow fever. As the most successful live attenuated vaccine in history, the yellow fever vaccine is both safe and efficient. Its developer Max Theiler won the 1951 Nobel Prize in Physiology or Medicine. He is the first and currently only vaccine researcher to win the Nobel Prize for and . Let’s start with the discovery of yellow fever virus and learn the story behind the development of yellow fever vaccine.
The conqueror of the yellow plague
Yellow fever, as the name suggests, "yellow" refers to jaundice and "hot" refers to fever. It is a highly fatal viral infectious disease that can cause damage to the liver, kidneys and myocardium, and cause bleeding. and shocks . The disease originated in Africa and spread to Central and South America through the notorious slave trade in the 17th century. In the next two centuries, large-scale epidemics broke out in the Americas, Africa, and Europe. Because yellow fever can not only infect humans but can also be transmitted among non-human primates, it cannot be eradicated like smallpox . The only reliable method of prevention is to vaccinate susceptible populations.
However, without finding the pathogen, vaccination is a distant goal. In 1898, the United States and Spain were at war in Cuba . The wartime epidemic of yellow fever killed countless soldiers, so the U.S. Army established a Yellow Fever Committee headed by pathologist Walter Reed. . Prior to this, Cuban doctor Carlos Finlay (Carlos Finlay) proposed that yellow fever is transmitted through mosquitoes based on the incidence environment and epidemic characteristics, and mosquitoes carry the "germs" of yellow fever. But because he didn't know at the time that not all mosquitoes were vectors, the experiment failed. Inspired by Finlay, Reed designed a human experiment in which mosquitoes were continuously bred to bite volunteers. After round after round of trials, in 1901 Reed and the committee confirmed that yellow fever could be transmitted by direct injection of blood or Aedes aegypti and that the pathogen was a filterable virus. At this point, yellow fever virus became the first virus discovered to be able to infect humans. This discovery triggered a mosquito eradication campaign, and epidemics in various places were quickly brought under control. But for this result, many volunteers also sacrificed their precious lives. For example, committee member Jesse William Lazear (Jesse William Lazear), in order to study this disease, allowed a mosquito to bite him, and finally died of yellow fever infection.
Yellow fever virus is transmitted mainly through the bite of the Aedes aegypti mosquito.
Until 1927, yellow fever swept coastal areas of Africa, from Alexander Mahaffy, a member of the West African Yellow Fever Commission established by the Rockefeller Foundation from Yellow fever virus, also known as the Asibi strain, was isolated from the blood of a 28-year-old patient in Ghana named Asibi, and the culprit of yellow fever was finally "caught". During the same period, the French branch of the Pasteur Institute in Dakar isolated yellow fever virus from a Syrian patient and named it the French strain. It can be said that the isolation of the Asibi virus strain and the French virus strain is the key to developing a vaccine. From the perspective of scientific progress, it is precisely because so many scientists have devoted themselves to research that humans have been able to discover the "culprit" of yellow fever. On December 10, 1951, South African virologist Max Taylor delivered a moving speech at the Nobel Prize banquet in Stockholm, Sweden. He mentioned Reed's human experiments and sacrificed volunteers in Cuba, calling it "one of the most glorious and heroic events in history." He concluded: “I would like to pay tribute to those who are working hard in laboratories, fields and bushes, often in harsh and dangerous conditions, and have made a huge contribution to helping us understand this disease.I would also like to pay tribute to those who have given their lives to gain knowledge that is of inestimable value. They are the true martyrs of science, willing to die so that others can live better. "
Dean Cornwell's 1939 "Conquerors of Yellow Fever" is one of the "American Medical Pioneers" series of oil paintings. This painting depicts Raziel sucking up a yellow fever patient. Blood from the ants' bodies was injected into Carroll's veins to prove the hypothesis that "yellow fever is transmitted by mosquitoes" proposed by Cuban scientist Carlos Finlay
How difficult is it to develop an attenuated vaccine
1. . inactivated vaccine failed
The discovery and isolation of the virus took the development of a yellow fever vaccine a big step forward. In 1928, scientists prepared a vaccine from the liver and spleen tissue of monkeys infected with the French strain of the virus and used formaldehyde. (Formalin) or glyceryl monophenol inactivation. However, due to the lack of effective virus propagation, titer measurement and inactivation process control methods, some preparations still contained residual live viruses, and others were degraded during the inactivation process. And lack of efficacy, so researchers believe that chemical treatment of highly virulent viruses is unlikely to produce a safe and reliable vaccine. Therefore, the development of an inactivated yellow fever vaccine failed.
2. Establish a small animal model and obtain an attenuated strain.
In 1928, Max Taylor, a scientist at the Rockefeller Foundation, began to try to establish a small animal model of infection by using an infectious monkey liver suspension in the brain. After inoculating adult mice, all of them died of encephalitis , with no signs of liver damage. He then inoculated the brain tissue of these mice into rhesus monkeys , which became fatally infected with encephalitis. fever, accompanied by liver lesions. The success of the mouse model was a milestone in yellow fever research and played an important role in subsequent vaccine research.
Max Taylor's work to control yellow fever was widely recognized by the scientific community. It was recognized that
Pasteur had infected dogs, monkeys, guinea pigs and rabbits with rabies through the spinal cord or intracerebral route during the vaccine development process. Taylor followed Pasteur's approach in mice. The French strain was passaged more than 100 times, resulting in a "fixed virus." The first yellow fever vaccine was produced using Taylor's "fixed virus" obtained by passage in mice and human serum. The strain was passaged through the mouse brain up to 176 times, and human immune serum was added to protect humans from viruses that were not attenuated enough. This method was very meticulous and could be said to be the safest live vaccine developed at the time. Methods.
What is a "fixed virus"?
Neurotropism and viscerotropism are considered to be two characteristics of yellow fever viruses. To put it simply, neurotropism means that the virus can multiply in the brain and induce encephalitis. Taylor observed that when rhesus monkeys were inoculated intracerebral with yellow fever virus, the monkeys died of hepatitis rather than encephalitis. On the contrary, neurotropic viruses that have been fixed through passage in the mouse brain will no longer have the ability to cause hepatitis in monkeys. In other words, yellow fever virus is a neurotropic virus that can reproduce in the brain.
However, Taylor believes the risks of this neurotropic vaccine are too high. In 1936, he and his colleague Hugh Smith successfully cultured the Asibi virus strain in vitro. They first subcultured the virus in intact mouse embryos 18 times, then passaged it 58 times in minced chicken embryo cultures, and then continued to passage 100 times in chicken embryos with neural tissue fragments removed. (i.e., the 176th passage after the start of in vitro culture), the most important experimental passage virus strain was born, which was named 17D. Taylor and Smith observed that: this virus strain was no longer pathogenic to monkeys, and mice no longer suffered from fatal paralysis; monkeys vaccinated with the attenuated virus produced -neutralizing antibodies and were resistant to wild-type Asibi The lethal attack of the virus. These data suggest that the 17D virus can be tested on a large scale as a human vaccine without the addition of protective immune serum.
Four illustrations from the 19th century show the clinical progression of typical yellow fever: the first shows a normal appearance before onset; the second shows a face during the infection period, with flushed skin; the third shows jaundice and nosebleedsepistaxis; The fourth picture shows severe coagulation disorder and multiple organ failure .
3. From laboratory to clinical application
The first two human subjects vaccinated with 17D were Taylor and Smith because they were immune to the yellow fever virus (Taylor was accidentally infected in a Harvard University laboratory in 1929, and Smith was French neurotropic virus and immune serum obtained from goats), so they are all low risk. Thereafter, five non-immune individuals were given high doses of the attenuated virus, and despite a minor febrile reaction, all subjects had increased antibody levels in their serum. The preliminary clinical test of 17D has achieved satisfactory results, which is another milestone in yellow fever vaccine research.
Next, they tested it in Brazil, where yellow fever outbreaks are common. In January 1937, Smith began small-scale human trials. He demonstrated for the first time the tolerability and immunogenicity of of the 17D vaccine in 24 subjects who had never been infected with yellow fever, although transient viremia occurred in about half of the subjects. The scientists then expanded the field trial of the vaccine to a large coffee plantation. By August 1937, more than 2,800 subjects had been vaccinated with no apparent adverse effects: by the end of the year, a total of 38,077 people had received the 17D vaccine. The vaccine is highly immunogenic—more than 95% of subjects developed antibodies. Epidemiological observations also prove the effectiveness of the vaccine.
Yellow fever virus under an electron microscope
Now, some high-risk countries and regions where yellow fever is prevalent require travelers to obtain an International Vaccination Certificate, also known as a small yellow book, before entering the country, as a sign that they have been vaccinated against yellow fever. evidence. In addition, some countries outside yellow fever endemic areas also require individuals who have recently traveled to countries in endemic areas to provide proof of immunization before entry.
A vaccine that spans the ages: not the first, but the best
The 17D live attenuated yellow fever vaccine developed by Max Taylor is not the first yellow fever vaccine to be tested in humans, but it is the most successful to date. Of the yellow fever vaccine, more than 500 million doses have been distributed since its introduction in the late 1930s.
The story of yellow fever vaccine development is remarkable in large part because it spans multiple eras of vaccine history. The development of vaccines began at the end of the 19th century, coinciding with a period of rapid development of vaccination concepts driven by microbiological and immunological revelations; it was truly successfully developed in the 1930s, in the early days of virology . Under the circumstances, scientists could only indirectly study changes in viral genotypes when they are passed between different hosts, following biological properties (such as virulence). Often, the changes in biological phenotype caused by virus attenuation through successive passages are difficult to predict. Despite attempts to reproduce the passage process that led to the reduced viscerotropism and neurotropism of the 17D vaccine, Taylor was never able to explain the changes that occurred between the original passages 89 to 176. It can be seen that the success of the 17D vaccine development depends on the scientists' systematic and detailed application experience and keen and continuous observation on the one hand, and on the other hand it is also inseparable from luck.
The International Certificate of Vaccination or Preventive Measures (small yellow book) is a vaccination certificate required by the World Health Organization to protect the personal health of people entering and leaving the country and prevent the spread of certain infectious diseases.
In addition to the yellow fever 17D vaccine, there have historically been several highly successful live attenuated human vaccines, all based on attenuated strains of the actual pathogen whose virulence genes have been lost or mutated through in vitro passage. If it is developed through traditional methods like the yellow fever 17D vaccine, it will be very time-consuming and it is impossible to achieve success without more than ten years.If reverse genetics is used for development, such as by rationally modifying the virus (optimizing with codons or deleting functional genes) to achieve attenuation to resist innate immune recognition, the development speed will be much faster, but this is also proposed at a technical level higher requirements.
