How monkeypox epidemic is likely to play out – in four graphs

By Adam Kleczkowski, Professor of Mathematics and Statistics, University of Strathclyde
Glasgow, June 22 (The Conversation) The first case of monkeypox in humans was reported in 1970 in the Democratic Republic of Congo. Since then there have been many outbreaks of monkeypox, but these are self-limiting, ending chains of human transmission without causing epidemics.
However, the current outbreak is different. There is more human-to-human transmission and it is over a much wider geographic area.
In just weeks, monkeypox has spread to 37 non-endemic countries, with more than 2,600 cases. So, what is likely to happen to Monkeypox in the coming weeks and months?
There are huge gaps in what we know about monkeypox, but by combining what we know with a history of other infectious diseases, it is possible to analyze 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 base reproduction number or R. Herd immunity – the point at which enough people have immunity so that disease transmission cannot be sustained – is 53% (corresponding to this value of R). And the incubation period, the time from contracting the virus to the onset of symptoms, is between five and 21 days.

Scenario 1: Self-Limiting Breakout

The epidemic of 2022 seems to have started as a super-spreading event involving a network of mostly men having sex with men.
But until the current outbreak, it was believed that the relatively low human-to-human transmissibility of the virus makes it unlikely that the virus will spread beyond the native community.
In this scenario, the outbreak ends quickly once the population at risk becomes immune and herd immunity is achieved locally. In the past, many people had some immunity (called “cross-immunity”) from the mass smallpox vaccination programs of the late 20th century. So the effective reproduction number, R, may be close to or even less than one, and the transmission will soon stop.
Behavioral changes can reduce the number R even more. For example, the ring vaccination can form a “firebreak”, further reducing the susceptible population. Similar past epidemics include the Sars outbreak in 2002-04, when prompt intervention prevented the disease from spreading.

Scenario 2: All Population

The continued spread of monkey pox in May and June 2022 suggests that the virus extends beyond its original network.
The scale of the outbreak is already much larger than the most prominent 2017-19 outbreak in the Democratic Republic of Congo (760). Large gatherings, including raves and festivals, may have created new transmission clusters.
Scenario 2 assumes anyone under the age of 50 is susceptible to the infection, reflecting the end of mandatory smallpox vaccination in the 1970s-80s. The virus will continue to spread and effectively seek out high-risk groups and non-immune communities.
Unless a combination of contact tracing and ring vaccination stops the spread, monkeypox will continue to spread. But given the low transmissibility of monkeypox, the epidemic could die out before reaching the herd immunity threshold of 50% of the population.

Scenario 3: Becoming Endemic

Complete eradication is impossible because monkeypox occurs in a wide variety of animal hosts. Its low transmissibility also allows it to survive at low levels in the population. In addition, the long incubation time and variable symptoms avoid detection. Therefore, monkey pox has been able to spread for a long time.
In scenario 3, the disease will settle at a prolonged, relatively constant level after the major outbreak. 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 programs may be needed to eradicate the disease. But relatively low portability of monkeypox means that such programs are likely to be highly effective.

Scenario 4: Recurring major epidemics

The current epidemic could be the first instance of a series of outbreaks. In the long term (Scenario 4), we should expect a recurrence of monkeypox caused by future “zoonotic events” where the disease jumps from animal hosts to humans. As the cross-immunity of smallpox vaccines declines, the epidemics could get even bigger.
Little is known about monkeypox’s ability to mutate. Still, there is potential to evolve into a more rapidly spreading variant.
Effective vaccines for monkeypox exist and are about 85% effective. While there are currently not enough doses to vaccinate everyone, there is no need for a mass vaccination program given the low transmissibility of monkeypox. 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. (The Conversation) SCY

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