Ed: What is the danger when someone from outside comes into contact with an uncontacted tribe like John Chau did with the Sentinelese?
Dr. Kristen Page: Any time you have a naïve population coming into contact with "outsiders" for the first time, you have a risk of disease transmission. There are numerous examples of diseases being moved around by a host (person) who shows no symptoms.
One of the more publicized recent examples is the import of Cholera to Haiti by UN aid workers responding to the earthquake. In U.S. history, the importation by colonists of smallpox, influenza, measles, and tuberculosis caused significant loss of life for indigenous peoples.
I realize that the missionary vaccinated himself and quarantined himself, but effective vaccines for parasites do not really exist. Vaccines are in development for malaria, leishmaniasis, and hookworm, but Mr. Chau would not have had access to them as they are only in the testing stage of development. Most vaccines he would have received would be for viruses. I'm not sure how long he was quarantined, but that wouldn't necessarily help prevent the transmission of a bacterium or a parasite that is patent (shedding infective stages), but not causing symptoms, because he would not have been treated for them.
Dr. Vanya Koo: Lack of immunity is always a risk for disease transmission. The Conquistadors in South and North America are good examples where the native naïve populations were decimated – some unintentionally, but some on purpose – by the diseases that missionaries brought with them.
Based on my searches, there are no 'modern' history records of a missionary transmitting an infectious disease to a previously-unreached population. Just because there are no records, though, it does not mean that it has not happened.
Ed: The examples given often involve a group of people coming into contact with uncontacted people. Is there any difference when it’s a single person instead of a group?
Dr. Page: Theoretically, no. If a pathogen enters the environment of the naïve population in a way that it can be transmitted, it does not matter how many infected individuals brought the pathogen. Of course, the more people coming in contact with the indigenous people, the more likely one would carry a pathogen.
If a missionary were to carry an asymptomatic infection with typhoid and the excrement containing the bacteria were to contact the drinking water of the local people, then transmission could occur. This would be true with any disease transmitted in feces or urine. It could also be true for vector-borne diseases.
If a vector (an organism that can transmit a parasite) were to be present, such as a mosquito, and a person carrying the parasite for malaria were to be bitten by the mosquito, then the resident mosquitos could transmit malaria to the naïve population. There are examples of malaria expanding to new ranges with refugees.
Dr. Koo: The cholera outbreak in Haiti was started by relief workers who were asymptomatic when they left Nepal but started experiencing symptoms once on the island. Haiti was a virgin soil for cholera, and a single infected person who shed bacteria into the water source was sufficient to set into motion a country-wide outbreak.
Ed: We really don’t have a lot of details about how Chau vaccinated or quarantined himself, but do these precautions really do anything anyway?
Dr. Page: I think that it does show that Chau was sensitive to the possibility that he could accidentally spread diseases, and that is important to note. Certainly, he reduced the risk of transmission of those diseases he was vaccinated against. Most likely these diseases would not have been transmitted. However, vaccines are not 100% effective, and vaccinated people can still acquire infections with pathogens they are vaccinated against, so he couldn’t reduce the risk to zero.
Influenza has a shorter incubation period (averages 2 days according to the CDC), but other viral infections can have quite long incubation periods (i.e. measles averages 14 days and polio averages 7-10 days). The diseases that Mr. Chau was most likely to transmit would not be diseases prevented by vaccination or quarantine.
We do not know how long Mr. Chau quarantined himself, but when I first read about this situation, I read that Mr. Chau had hired a boat to take him to the island. If he was on the boat with other people who were not quarantined, he broke quarantine.
Dr. Koo: The majority of vaccines are preventative and only a few are used for treatment. What Mr. Chau would have been doing is mostly protecting himself from exposure to any of the pathogens that he was vaccinated against that he might have had a chance of coming into contact with while traveling. Quarantine was a good forethought, but as Kristen mentioned, different infectious diseases would require a different time for incubation and symptom manifestation, so it would be hard to figure out if the quarantine was sufficient.
Ed: We actually know that the tribe has been contacted in the last few decades and that it had significant interaction in the colonial era. Does that impact immunity, particularly the contacts 100 years ago?
Dr. Page: If there were diseases that were transmitted during previous contacts, people would have had to survive the infection, and the pathogen would have had to persist on the island so that subsequent generations could be exposed and develop immunity. There is a genetic component to immunity, but in reality, we develop immunity through exposure to pathogens. We develop antibodies to pathogens as we are exposed to them. Vaccines work because we create a safe way for us to experience this exposure (i.e. via a killed virus or an assemblage of proteins from the pathogen). My father had Polio as a child, but I didn’t have immunity to Polio until I was vaccinated – and even then it’s not necessarily 100% immunity.
Dr. Koo: There is a genetic component to immunity to certain infectious diseases (i.e. sickle cell trait protects individuals from malaria), but exposure is the primary way we develop antibody-based immunity to pathogens. Without the presence of pathogens to keep stimulating the humoral immunity or when the vaccination against a disease is interrupted, the immunity decreases over time. Also, different pathogens create differences in the strength and length of immune response. Some antibodies are very short lived and require continuous presence of antigens to stimulate them. This link may be helpful in illustrating how herd immunity works: https://www.historyofvaccines.org/content/herd-immunity-0
Ed: If you were preparing yourself, what would you do? And how likely do you think it is that the preparation would work?
Dr. Page: If I were preparing myself to meet a naïve population, I would want to make sure I did not have any infections that were asymptomatic. I would request tests to ensure I did not have any parasites like hookworm, roundworm, schistosomes, Cryptosporidium spp., and Giardia spp., all of which are transmitted in feces. I would also have screening done for viral diseases like Norovirus (causes diarrhea) and bacterial infections to make sure that I wasn’t shedding any microbes without the symptoms of the diseases. There are so many pathogens that I could test myself for, that I’m sure that something could be missed and I would never reduce the risk of infecting the tribe to zero.
Dr. Koo: I agree with all of the above. Blood, urine, sputum, stool would all need to be tested.
Ed: What would you encourage missionaries to do as they engage isolated peoples in the future?
Dr. Page: Isolated peoples will always be vulnerable to the diseases of the rest of the world through contact. Missionaries like Mr. Chau need to understand that they can’t reduce the risk of transmission to zero. The fact that Mr. Chau considered the risk is a good start, but his actions served to protect himself more than the islanders––even if that was not his intent. A missionary would have to be thoroughly tested for any possible disease (tracking the possible history of exposure – places traveled, etc. – may help narrow down what to look for) and treated with any available prophylactic (preventative) medicine. For example, they could take deworming pills, anti-malarials, antibiotics, etc. This would be the best effort, I believe.
Ed: We don’t know what Chau did or did not do— information is still scarce – but we know we can’t reduce the risk to zero, and some have explained that just contacting an uncontacted tribe is genocidal. If he did the best version of all possible scenarios, for example, even maintaining a distance from the fisherman, how great is the health risk for the Sentinelese?
Dr. Page: I think it’s too difficult to say. There are so many possible pathogens that we could be talking about, all with different transmission dynamics. Also, we don’t know how many islanders there are or anything about the way they interact, which also plays a role in disease transmission. There are too many factors for me to put a level on the risk.