Monday, August 03, 2020

WHO: Modelling Human-to-Human Transmission of Monkeypox

Geographic Range of Monkeypox - Credit WHO


One of the emerging infectious disease threats we've  discussed often (see here, here, here) over the years is Monkeypox, which is endemic in the DRC and central Africa, with more more than 4,000 cases reported there in 2019, and currently over 2,500 cases (and 97 deaths), reported in the first 7 months of 2020

Nigeria reported its first Monkeypox outbreak in nearly 40 years in the fall of 2017, and while surveillance and reporting is limited, they continued to report cases up until December of 2019. Like with so many other diseases, reporting has dropped off since the start of the COVID-19 pandemic.  

Exported cases from Africa have turned up sporadically around the world. In September of 2018 the UK saw two imported cases from Nigeria (see Eurosurveillance Rapid Comms: Two cases of Monkeypox imported to the UK). A month later Israel: MOH Confirmed An Imported Monkeypox Case). And in 2019, Singapore reported an imported case.

In last year's CDC: 8 Zoonotic Diseases Of Most Concern In The United States, Monkeypox was ranked 29th; about halfway down their list. Similarly, in 2018's WHO List Of Blueprint Priority Diseases - while Monkeypox did not make the final list (n=8) - it was mentioned as a disease to watch.

Human monkeypox was first identified in 1970 in the DRC, and since then has sparked small, sporadic outbreaks in the Congo Basin and Western Africa. It produces a remarkably `smallpox looking' illness in humans, albeit not as deadly. The CDC's Monkeypox website states:

The name `monkeypox’ is a bit of a misnomer. It was first detected (in 1958) in laboratory monkeys, but further research has revealed its primary hosts to be rodents or possibly squirrels.
The illness typically lasts for 2−4 weeks. In Africa, monkeypox has been shown to cause death in as many as 1 in 10 persons who contract the disease.
Human-to-human transmission is also possible.  This from the CDC’s Factsheet on Monkeypox:
The disease also can be spread from person to person, but it is much less infectious than smallpox. The virus is thought to be transmitted by large respiratory droplets during direct and prolonged face-to-face contact. In addition, monkeypox can be spread by direct contact with body fluids of an infected person or with virus-contaminated objects, such as bedding or clothing.
According to the CDC there are two distinct genetic groups (clades) of monkeypox virus—Central African and West African. West African monkeypox - which has been spreading recently in Nigeria - is associated with milder disease, fewer deaths, and limited human-to-human transmission.

In October of 2018, in the WHO: Monkeypox Update & Risk Assessment - Nigeria, we looked at advice from the World Health Organization, and some studies that raise concerns over the future path of the Monkeypox virus.

For a more detailed look at the Monkeypox virus in Africa, and a limited 2003 outbreak in the United States - you may wish to revisit this blog from May of 2018.
MMWR: Emergence of Monkeypox — West and Central Africa, 1970–2017
Routine vaccination against smallpox - which supposedly provides about 85% protection against Monkeypox - ended in the 1970s. Today more than half of the world's population is unvaccinated, and the level of protection remaining among those vaccinated 50+ years ago is highly suspect.

A 2016 study (see EID Journal:Extended H-2-H Transmission during a Monkeypox Outbreak) looked at a large 2013 outbreak of Monkeypox in the DRC and suggested that the virus's epidemiological characteristics may be changing (possibly due to the waning smallpox vaccine derived immunity in the community).
The DRC had reported a 600% increase in cases over both 2011, and 2012.  The authors also cite a higher attack ratelonger chains of infection, and more pronounced community spread than have earlier reports.
Like all viruses, Monkeypox continues to evolve and diversify, as discussed in the 2014 EID Journal article Genomic Variability of Monkeypox Virus among Humans, Democratic Republic of the Congo, where the authors cautioned:
Small genetic changes could favor adaptation to a human host, and this potential is greatest for pathogens with moderate transmission rates (such as MPXV) (40). The ability to spread rapidly and efficiently from human to human could enhance spread by travelers to new regions.
All of which brings us to a new report, published by the Bulletin of the World Health Organization, that examines the growing threat of monkeypox as human immunity to smallpox continues to wane.

I've only selected some excerpts from a much longer report.  Follow the link to read it in its entirety. 

Modelling human-to-human transmission of monkeypox

Rebecca Grant,a Liem-Binh Luong Nguyena & Romulus Brebana a 
Institut Pasteur, Emerging Diseases Epidemiology Unit, 25-28 rue du Dr. Roux, 75015 Paris, France. Correspondence to Romulus Breban (email: (Submitted: 5 August 2019 – Revised version received: 18 March 2020 – Accepted: 12 May 2020 – Published online: 8 July 2020) 

Monkeypox is an emerging infectious disease for which outbreak frequency and expected outbreak size in human populations have steadily increased.1 The geographic spread of monkeypox cases has expanded beyond the forests of central Africa, where cases were initially found, to other parts of the world, where cases have been imported.
This transmission pattern is likely due to the worldwide decline in orthopoxvirus immunity, following cessation of smallpox vaccination, once smallpox was declared eradicated in 1980. Monkeypox could therefore emerge as the most important orthopoxvirus infection in humans.2 We use mathematical modelling to argue that, in a population with diminishing herd immunity against orthopoxvirus species, the epidemic potential of monkeypox will continue increasing.    
The clinical presentation of monkeypox facilitates outbreak investigations around incidentally imported cases. The incubation period of monkeypox is 5–21 days, followed by clinical onsets for up to 21 days. Monkeypox is not considered contagious during its incubation period and asymptomatic monkeypox infection has not been documented.
Transmission occurs through fluids secretion, mainly from the  respiratory tract or skin lesions. The distinctive symptoms of human monkeypox greatly aid in its early detection and containment. Nevertheless, secondary transmission from imported cases is possible, as evidenced by the case of nosocomial transmission to a health-care worker in the United Kingdom of Great Britain and Northern Ireland in 2018.12 Under stringent infection prevention and control measures, including case isolation, hand hygiene, use of personal protective equipment to avoid direct contact with patients and the use of standard, contact and droplet precautions, the likelihood that an imported case triggers an epidemic can be
expected to be low.
Yet, with increasing importation rate, monkeypox outbreak investigations may become a costly and poorly effective strategy, to prevent endemic disease.
We conclude that circulation of smallpox, followed by worldwide smallpox vaccination, have previously protected human populations from monkeypox epidemics. We combined historical data on smallpox and monkeypox with mathematical modelling to estimate the basic reproduction number of monkeypox, and found out that monkeypox has epidemic potential. This finding may explain the increasing number of monkeypox outbreak reports, resulting in endemic monkeypox in central African countries. Moreover, with declining immunity to orthopoxvirus species, monkeypox can pose an ever-increasing threat for health security.