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Monkeypox: forecasters predict the global outlook for 2022

After two years of the COVID-19 pandemic, a new infectious disease appears to be spreading worldwide.

The Swift Centre reports that cases of the viral disease, monkeypox, have been detected in several countries: by the end of May, 57 people had been diagnosed with the disease in the UK, and another 100 around 14 other countries, including in South Africa.

However, the spread has been very different from COVID. COVID is spread through the air, by droplets on patients’ breath, while monkeypox is mainly spread by skin-to-skin contact. And because the initial outbreak arose among gay and bisexual men, most cases have, so far, been among men who have sex with men (MSM).

That said, the disease is not specific to gay men, or sexually transmitted in the strict sense of the word, and could reach the wider community. Even if it doesn’t, it’s important to get a sense of how far monkeypox could spread to try to understand the scale of the public health problem.

We asked 10 forecasters for the likely outcomes of the monkeypox outbreak.

A note on forecasting methods

The classic “super-forecasting” model of forecasting is that people will give a percentage probability for some outcome, and also how confident they are: getting a 90%-confident forecast right is rewarded more highly than getting a 55%-confident forecast right. (On the other hand, getting a 90%-confident forecast wrong is punished more severely than getting a 55%-confident forecast wrong.)

In this case, we do something similar, but, in a way, reversed. The forecasters gave a range of values they thought was 80% likely to contain the true outcome. So, for instance, if one thought it 90% likely that the number of monkeypox cases in the UK before the end of 2022 would be higher than 500, and 90% likely that it would be lower than 10,000, their 80% range is 500 to 10,000.

They also each gave a 50% value – their central, most likely estimate.

For each question, we also gave a median of the forecasters’ 10%, 50% and 90% predictions – that is, if you took all of their predictions and put them in a row from smallest to largest, we’d take the one in the middle.

They were asked four questions:

  • How many cases of monkeypox will there have been worldwide by the end of 2022?
  • How many cases will there have been in the UK by the end of 2022?
  • What percentage of those worldwide cases will be in men? (This is to give an idea of the extent to which it leaves MSM communities.
  • What percentage of those UK-based cases will be in men?

1. How many cases of monkeypox will there have been worldwide by the end of 2022?

Median 10% estimate: 13,500
Median 50% estimate: 45,000
Median 90% estimate: 525,000


As of 17 June, there have been 2,103 confirmed cases reported to the WHO. The forecasters think that even in a realistic best-case scenario, that number will only be a fraction of the eventual total: the median 10% (that is, low-end) estimate is six times higher. On the other hand, the median upper-end estimate, while a large number – more than half a million – is still negligible compared to COVID’s spread: by the end of 2020, there had been more than 80m confirmed cases of COVID-19, and many times that number of undetected ones.

But predictions varied widely. Three said while the virus spreads through skin contact now and has a relatively low R number, it is mutating, and could easily become transmissible. One said: “Multiple somewhat distinct strains have already evolved, which is concerning – that there is a fairly high amount of genetic drift occurring, which could lead to the situation turning into a more detrimental pandemic.”

Another said the virus seems to have a very high R within MSM communities, but an R below 1 outside it. (The R value is the number of people the average infected person goes on to infect. If it’s above 1, the disease will spread exponentially; if it’s below 1, it will dwindle.) But, they said, “as it spreads more, the chance that it mutates to gain a transmissibility advantage increases, quite possibly via the airways”: thus, that forecaster gave a 90th-percentile forecast of 30m infections, the highest in the group by a factor of four.

Others were more optimistic. One thought the total would be no higher than 80,000; another, 200,000. Another with a relatively low estimate (median 40,000, 90th percentile 400,000) said “there has been limited transmission beyond networks of close contacts, so far.”

Another, who had a very wide range – from 12,000 to 5m – said: “R is clearly lower than for COVID, so there is a plausible future where once governments get serious about mitigation it ceases to spread quickly. Alternatively, they might not.”

It’s worth noting these estimates are of the total cases, not just confirmed ones. (One forecaster said they “expect much of the world is vastly undercounting”.) That means it will be impossible to accurately grade who has got this question right.

2. How many cases of monkeypox will there have been in the UK by the end of 2022?

Median 10% estimate: 6,000
Median 50% estimate: 22,000
Median 90% estimate: 85,000

Note: forecaster four’s upper bound estimate of 1,000,000 cases exceeded the chart capacity

As with the worldwide estimates, there was considerable spread. One said numbers could get as high as 1m in the UK.

Generally, the feeling was the UK is better placed to control spread than most countries. “UK monitoring is on the better end,” said one, “and there are initial attempts to vaccinate.”

But, there were concerns. “It’s failed to be controlled particularly well so far, despite good monitoring,” one wrote. “There’s still opportunity for superspreader events, but awareness within LGBT communities should slow it down.”

3. What percentage of 2022's worldwide cases will be in men?

Median 10% estimate: 50
Median 50% estimate: 94
Median 90% estimate: 99

Currently, more than 99% of recorded cases worldwide have been among men, suggesting the spread has been almost exclusively among MSM.

The forecasters expected that to remain the same, although there was a chance, above 10%, that it would spread so rapidly among the general population it would wash out the initial male headstart altogether. One said once a disease gets outside the subpopulation in which it started, it can behave very differently, depending on the context: “The US has a 5:1 male/female ratio of HIV,” wrote one, “while sub-Saharan Africa has more women with Aids than men. The modes of transmission do not keep the straight community safe. There are more straight people than gay people, so once it starts to increase in that community, the potential population is much greater.”

All thought there was a 10% chance that cases among women would reach close to 50% of the total, but most thought that unlikely: only one put their central estimate at below 90% male, and four had it at 95% or higher.

4. What percentage of 2022's UK cases will be in men?

Median 10% estimate: 55
Median 50% estimate: 98
Median 90% estimate: 99

They felt the story was likely to be largely similar in the UK to worldwide, with some caveats. First, the disease was likely to be better controlled here and to spread more slowly, so the chance of it spreading widely outside MSM communities was smaller, hence the higher median 10% estimate.

Second, one said the UK’s HIV population has a higher male-to-female ratio than the global average, and used that to extrapolate to monkeypox.

Overall: “I think community spread is definitely possible, but founder effects and (reasonably) slow spread will mean the MSM community will still dominate the numbers by year-end.”

One said the UK Government’s current strategy, of vaccinating high-risk MSM, “may solve the problem on its own”. However, they said public messaging is not useful, since it doesn’t warn at-risk communities, tending to emphasise the risk to wider society rather than to MSM groups.

Conclusions

The forecaster group felt monkeypox was likely to continue spreading, in the UK and worldwide. However, unless it mutates significantly to become capable of spreading in ways other than through skin contact, its numbers would probably remain comparatively low, and it would probably stay chiefly in the MSM community.

They also felt was a risk of a much wider impact – that a novel disease which appears to be mutating relatively rapidly has a non-negligible chance of achieving new means of spreading – and that there was a greater than 10% chance of widespread transmission among the wider population, with hundreds of thousands or even millions of cases possible.

Swift Centre article – Monkeypox: the outlook for 2022 (Open access)

 

See more from MedicalBrief archives:

 

Monkeypox mutating faster than expected – Lisbon scientists

 

Phaala warns against prejudice as SA records two monkeypox cases

 

Monkeypox possibly linked to decrease in smallpox vaccine – global experts

 

CDC expresses concern over monkeypox outbreak in UK

 

 

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