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 from 2002 to 2004, when a quick intervention stopped the disease from spreading.
SCENARIO TWO: THE VIRUS SPREADS THEN FIZZLES OUT
The continuing spread of monkeypox in May and June suggests that the virus is moving beyond the original network.
The size of the outbreak is already well beyond the most prominent 2017 to 2019 outbreak in the Democratic Republic of Congo (760 cases). It is possible that large gatherings, including raves and festivals, have created new transmission clusters.
This scenario assumes that everybody below the age of 50 is susceptible to the infection, reflecting the end of compulsory smallpox vaccination from the 1970s to 1980s. 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 per cent of the population.
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