How monkeypox epidemic is likely to play out – in four graphs
Published: June 21, 2022 8.13am EDT
Author
Adam Kleczkowski
Professor of Mathematics and Statistics, University of Strathclyde
Disclosure statement
Adam Kleczkowski receives funding from the UK Research and Innovation and the Scottish Government.
Partners
University of Strathclyde provides funding as a member of The Conversation UK.
The first case of monkeypox in a human was reported
in 1970 in the Democratic Republic of the Congo. Since then, there have been many monkeypox outbreaks, but they have been
self-limiting, with chains of human transmission ending without establishing epidemics. The current outbreak, however, is different. There is more human-to-human transmission, and it is over a much broader geographical area.
In mere weeks, monkeypox has spread to
37 non-endemic countries, with
over 2,600 cases. So, what is likely to happen to monkeypox in the following weeks and months?
There are
huge gaps in what we know about monkeypox, but combining what we do know with a history of other infectious diseases makes it possible to analyse likely future scenarios.
The four scenarios below are based on the following knowledge: the average number of people an infected person is likely to infect (assuming they have not been vaccinated against the virus or have had the disease before) is
2.13. This is called the basic reproduction number, or R. Herd immunity – the point at which enough people have immunity such that disease transmission can’t be sustained – is 53% (
corresponding to this value of R). And the
incubation period, the time from catching the virus to the appearance of symptoms, is between five and 21 days.
Scenario 1: Self-limiting outbreak
The 2022 epidemic appears to have started as a
super-spreader event involving a
network of predominantly men who have sex with men.
But until the current outbreak it was assumed that the
relatively low human-to-human transmissibility of the virus
makes it unlikely for the virus to spread outside the initial community.
Plot illustrating the daily number of cases under scenario 1. The size and duration of the outbreak here and in the graphs below are for illustration purposes only and do not constitute a detailed prediction of what might happen in the future. More details available at
https://statisticallyinsignificant.b...rio-modelling/ Adam Kleczkowski
In this scenario, the outbreak ends quickly once the population at risk becomes immune and herd immunity is reached locally. In the past, many people had some immunity (called “cross-immunity”) from the smallpox mass vaccination programmes of the late 20th century. So the effective reproduction number, R, can be close to or even lower than one, and the transmission will soon stop.
Behavioural changes can reduce the number R even more. For example, the
ring vaccination can form a “firebreak”, further reducing the susceptible population. Similar previous epidemics include the
Sars outbreak in 2002-04, when a quick intervention stopped the disease from spreading.
Scenario 2: All population
The
continuing spread of monkeypox in May and June 2022 suggests that the virus is moving beyond the original network.
The size of the outbreak is already well beyond the most prominent 2017-19 outbreak in the Democratic Republic of Congo (760). It is possible that
large gatherings, including raves and festivals, have created new transmission clusters.
A plot illustrating the daily number of cases under Scenario 2. Adam Kleczkowski
Scenario 2 assumes that everybody below the age of 50 is susceptible to the infection, reflecting
the end of compulsory smallpox vaccination in the 1970-80s. The virus will continue spreading, effectively searching for pockets of high-risk and non-immune communities.
Unless a combination of contact tracing and ring vaccination stops the spread, the monkeypox will continue spreading. But, given the low transmissibility of monkeypox, the epidemic may fizzle out before reaching the herd immunity threshold of 50% of the population.
Scenario 3: Becoming endemic
Complete eradication is impossible because monkeypox exists in a wide range of animal hosts. The low transmissibility also means it can survive at low levels in the population. In addition, the long incubation period and variable symptoms allow it to avoid detection. Therefore, monkeypox may have already been
spreading for a long time.
A plot illustrating the daily number of cases under Scenario 3. Details available at
https://statisticallyinsignificant.b...rio-modelling/ Adam Kleczkowski
In scenario 3, following the large outbreak, the disease will settle on a long-term, relatively constant level. Similar to the pre-vaccination
smallpox or
chickenpox.
The influx of susceptible people through birth or migration will keep the virus in the population.
Mass vaccination programmes might be needed to eradicate the disease. But relatively low monkeypox transmissibility means such programmes are likely to be highly effective.
Scenario 4: Recurrent large epidemics
The current epidemic might be the first instance of a series of outbreaks. In the long term (scenario 4), we should expect a return of monkeypox caused by future “zoonotic events” where the disease jumps from animal hosts to humans. As the cross-immunity from smallpox vaccines wanes, the epidemics can become even more substantial.
A plot illustrating the daily number of cases under Scenario 4. Details available at
https://statisticallyinsignificant.b...rio-modelling/ Adam Kleczkowski
Little is known about the potential of monkeypox to mutate. Still, there is
potential for it to evolve into a more rapidly spreading variant.
Effective vaccines for monkeypox exist and are about 85% effective. Although there are currently not enough doses to vaccinate everyone, there is no need for a mass-vaccination programme given monkeypox’s low transmissibility. Instead, vaccines should be offered to those most at risk, including communities in Africa most in contact with the wild animals that carry the virus.
https://theconversation.com/how-monk...-graphs-184578
