- Median scenario: Monkeypox cases in the USA and UK to grow into 2023, but likely to remain mostly confined to the MSM community
- Globally, low vaccine availability and poor reporting will contribute to increased spread.
- Key risks: mutation leading to transmission beyond sexual networks
At the end of June the consensus among Swift Centre's forecaster panel was fairly optimistic: the median estimate for the worldwide number of monkeypox cases for all of 2022 came in at around 45,000 (80% confidence interval 13,500-525,000). As of August 8th the world had already confirmed more than 30,000 cases and there are likely many more going undetected. The median estimate for 2022's global caseload is now 392,500.
These updates come from several sources. We know more about the social networks it's spreading in, and government policy responses. Cases so far are largely confined to men who have sex with men (MSM), with much higher rates among a subset of this population with greater numbers of partners and considerable prior exposure to HIV and sexually transmitted infection. That there are few cases outside the most at-risk populations presents a potential (though not guaranteed) limit to spread, while we also know more about the speed of vaccination efforts.
We also know more about the virus. While - as one forecaster noted - it’s “evolved a surprising amount for a DNA virus”, we also have a good candidate mechanism for these changes. As such, the risk of monkeypox suddenly becoming far more transmissible seems slim, though not impossible: “it seems best to treat it like most other DNA viruses (i.e. slow to mutate), but one that is able to withstand random mutations better”.
Our new forecasts cover total cases (actual cases as opposed to confirmed cases) in the US, UK, and worldwide through to Q2 2023, providing for each region an optimistic (10% of outcomes covered), pessimistic (90% of outcomes covered), and central (50% of outcomes covered) prediction.
Forecasters provided optimistic and pessimistic scenarios alongside the central 50% probability estimate. In the optimistic cases, vaccinations and behavioural changes in the networks currently susceptible to Monkeypox contain the spread, with growth tailing off from Q1 2023. More pessimistic scenarios assume a degree of spread into the general population.
This potential spillover was the central point of uncertainty for forecasters. As several noted, if cases are largely confined to the MSM community, then growth will be limited by an effective upper bound to the vulnerable population, with case numbers determined by dynamics within this community.
Several forecasters drew on the example of HIV to model the spread, with one pointing out “total HIV cases in the US account for ~0.5% of the population… this could represent an upper bound for Monkeypox based on our current information”. Others agreed, noting that “Monkeypox is only contagious during the pox stage, unlike HIV”, and that it had yet to spread to the wider population. Just as importantly, the initial spread of HIV was aided by the time in which it arrived: today, government awareness campaigns, internet access, and proactive public health measures mean that communities are better placed to take precautions.
Another major difference is the availability of an effective vaccine. As forecasters pointed out, the Smallpox vaccine “initially appears to provide protection [against Monkeypox] in 85% of those immunized”. With a working vaccine, many of those at risk will want to receive it. The major complicating factor is the possibility of a weak government response leading to avoidable cases. There is already a severe shortage of the vaccine. However, this won’t be permanent, and it is expected that by mid-2023 the supply will be 6.9 million doses. Government incompetence, as one forecaster noted, is also less relevant in severe scenarios: “if it mutates to be more transmissible, I think the policy response would be much stronger”.
US cases reported to CDC: https://www.cdc.gov/poxvirus/monkeypox/response/2022/mpx-trends.html
Vaccine distribution by jurisdiction: https://aspr.hhs.gov/SNS/Pages/JYNNEOS-Distribution.aspx
Monkeypox is the same virus in the UK as it is in the US, and the patterns of spread - primarily within MSM communities - are similar. The reasoning on upper bounds adopted for the US applies just as well to the UK. The key differences between the countries are in how the government responds, and the structure of social relationships within those communities.
As one forecaster noted, a major difference is that “money should be less of a deterrent from seeking medical help” in the UK due to NHS provision of care. While this is good for infected individuals, it means detected and confirmed case numbers will probably be higher in Britain for a given number of cases than they would be in the US. This shifts the distribution of forecasts upwards.
The British government is already offering vaccination, and as one forecaster noted “the UK is typically a high-vaccine-uptake country in general”. While “large-scale behavioural responses” seem unlikely, a highly jabbed population is one where growth is likely to be slower.
One forecaster praised the work of health authorities in focusing on at-risk populations, noting that “growth seems linear now, rather than increasing”. With a bit of luck, “the current spread might have practically stopped”.
UK Health Security Agency epidemiological overview of the Monkeypox outbreak: https://www.gov.uk/government/publications/monkeypox-outbreak-epidemiological-overview/monkeypox-outbreak-epidemiological-overview-2-august-2022
The dynamics for global cases differ considerably from those for the wealthy and developed US and UK. As one forecaster noted, even optimistic forecasts have to account for the risk of Monkeypox “becoming endemic in a developing nation and spreading outside the MSM population. Unlike the other cases, it feels like this falls within 10%”
The circumstances which make controlling the spread of the virus challenging can also make it difficult to make accurate forecasts. As one forecaster noted, surveillance and reporting globally is highly variable. For a given number of actual cases, this biases forecasts for confirmed cases downwards. The complicating factor is that the same lack of reporting means that growth is likely to be higher, providing a partially offsetting force.
As one forecaster noted, current figures show “basically only Western countries are affected”, partly due to greater acceptance of MSM communities leading to higher partner numbers and political tolerance of MSM parties which may function as superspreading events. In countries where MSM communities are at political risk there will be fewer options to socialise and transmit the disease, though vaccine availability is also likely to be considerably lower and those infected may be less likely to report their illness.
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