Monkeypox: is its unprecedented spread linked to the smallpox eradication of disease?
To try to understand why monkeypox is spreading beyond the borders of Africa, a continent where smallpox is endemic in several countries, we can go back in time, shortly after the end of smallpox vaccination, when populations in contact with the smallpox virus were no longer protected.
On 8 May 1980, at the 33rd World Health Assembly, smallpox was officially eradicated: "All the peoples of the world are now free from smallpox. This was the result of the WHO's 10-year vaccination campaign, during which 500 million doses of smallpox were distributed worldwide. This eliminated a disease known since ancient times, which kills 30% of people and was responsible for 300 million deaths in the 20th century.
Although the eradication of smallpox is an undeniable victory, it has two negative consequences: children born after the end of the vaccination campaign are not protected against the disease, increasing the fear of a bioterrorist attack, a problem that can be quickly solved thanks to rich countries that have stockpiled vaccine doses in their freezers. The second consequence concerns African countries where monkeypox is endemic. The smallpox vaccine also protected the local population against this second orthopoxvirus. In response to these concerns, the WHO set up a surveillance program for monkeypox between 1981 and 1986, which showed that the disease was not a serious public health problem. People who come into contact with monkeypox have no option but to protect themselves against the virus.
Monkeypox is increasing rapidly as immunity to smallpox wanes
This is a conclusion that is strangely consistent with the current situation. A 2010 study by researchers at the Los Angeles School of Public Health found that in the 30 years since smallpox vaccination stopped, the incidence of smallpox has increased 20-fold in controlled areas of the Democratic Republic of Congo, where the virus is most active. Follow-up data collected by researchers between 2005 and 2007 show that cases of reduced immunity to smallpox, more frequent contact with animals that are reservoirs for the virus, and increased human-to-human transmission are the causes of the cases. Most of the infected children are under the age of 15 and were born after the smallpox vaccination was stopped. Based on our surveillance program, we found that the risk of monkeypox was inversely correlated with vaccination against smallpox. Vaccinated individuals were 5.21 times less likely to develop monkeypox than unvaccinated individuals.
Another, the more controversial theory is that smallpox disappeared, leaving an ecological niche - a concept whose definition is disputed in several disciplines - open for monkeypox. This view is supported by James O. Lloyd-Smith, a researcher at the University of California, and is described in detail in one of his 2013 publications. He writes: "Eradication may thus lead to a quantitative increase in the incidence of another infection, but whether this leads to the emergence of an endemic pathogen depends on other factors." There is still no scientific data formally confirming this theory.
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| The cumulative annual incidence of monkeypox in 2006 in red, when the majority of cases were in unvaccinated youth, versus 1981-1986 in blue, when all age groups were vaccinated.© Anne W. Rimoin et al, Pnas |
Is there a link to the current situation?
The fact is that the end of vaccination against smallpox and the globalization that has increased in the intervening years allowed the smallpox virus to thrive and leave the African continent via infected prairie dogs as early as 2003 and several times in 2018 and 2019 before the situation we know today. Monkeypox is the most common orthopox virus in humans. Smallpox vaccine will be taken out of the freezer to prevent the spread of smallpox and to vaccinate infected people and their contacts. There are currently 23 countries outside Africa where human-to-human transmission of monkeypox has occurred. In the Democratic Republic of Congo, monkeypox is still present and 56 people have died from the disease since the beginning of the year, but so far no one has died in Western countries.
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