Monkeypox update: Why does it continue to spread?

The #Monkeypox epidemic continues to expand both in the UK and globally. It has now been detected in 82 countries/regions, with nearly 6,200 (confirmed) cases reported.

OWD hMPXV confirmed cases

On my blog, I have introduced a very simple model which I used to explore the future scenarios of the epidemic. I have been playing a bit more with the model, to see whether it is still compatible with the data as they are released by UKHSA and OWD.

Now, I am very aware that there are many better-suited experts to analyse the current outbreak and much better models to predict its course. But, what follows is a brief summary of how I see the current epidemic.

OWD hMPXV confirmed cases, logarithmic scale

A quick look at a log scale plot shows that the epidemic clearly has had two phases: rapid initial growth followed by a slow increase. The initial increase is most likely influenced by catching up on reported cases.

But, a very detailed study of the cohort of cases done by UKHSA also shows similar behaviour in the data of cases by symptoms.

Note the log vertical scale; black line is my Scenario 3.

More importantly, the outbreak in April and May could conceivably be approximated by an exponential function – a straight line on the logarithmic plot – a tell-tale sign of a SIR model.

However, the overall picture is more of a curve than a straight line or a piece-wise linear behaviour – which would correspond to two phases.

I explain the difference between black, red and blue data points elsewhere on my blog.

What modification of a standard SIR model can describe such a behaviour? Under Scenario 3, I suggested a model in which the disease spreads initially in a small network, mostly consisting of Men who have Sex with Men (MSM).

Although my model is too simple to draw firm conclusions, I suggest that this initial size for the UK to be of an order of 1,300.

But the total number of reported, confirmed, cases has now exceeded 1,000 and the epidemic is still going strong. I believe that what we now see is the virus “leaking” outside the original network, perhaps into a wider MSM network, perhaps into the general population.

In my model, this is captured by a SIR model, with an initial population of 1,300, but also with a process that brings in new susceptible individuals. Formally, this is a SIRS model in which immune individuals (as well as infected ones) are replaced with susceptible ones.

I am not suggesting loss of immunity, or a birth/death process. Rather, I am thinking that those who already had the disease are becoming more cautious and essentially “drop out” of the contact network.

But then those who are still within the network initiate close contacts with others, thus effectively increasing the susceptible numbers by drawing people from the “reserve” pool. How does this impact the long-term dynamics?

If this process continues for some time, the epidemic will slow down a bit in the next couple of weeks, but would then continue to spread at a slower rate. In a sense, #Monkeypox will become endemic in the UK.

Of course, at some point, the epidemic will reach the end of the “reserve” pool, or will be stopped by a combination of NPIs and targeted vaccination. So, in the really long term, I am optimistic about the outcome.

But in the short term, we are in for a long haul.

30 years ago

On July 2nd, 1992, exactly 30 years ago, I landed in Dover. Although originally planned as a 12-month research post at Cambridge, it turned out to be a permanent move to the United Kingdom.

I came to Cambridge on a Royal Society-Wolfson Foundation fellowship. The idea was to work on modelling large animal populations in Africa, working with Nigel Leader-Williams and Phyllis Lee @pili_scotland.

However, due to a change of plans, the project was converted into epidemiological modelling. I had done modelling of measles dynamics in 1990-91, and the Cambridge fellowship was a return to this research topic.

In 1992/93 I had the tremendous privilege of working with Bryan Grenfell (he very kindly offered me a house sit in the first two months and on whose door I knocked at 2am on 3rd July 1992), Ben Bolker @bolkerb, and many others.

From this period, I will always remember sitting in Parkside Police Station Aliens Department, waiting to be registered as a visiting Alien.

I must confess, I half expected to see there somebody with green skin, a large head, and several eyes on stalks. Knowing Cambridge, I do not think this would have surprised the very kind lady at the Aliens Department.

Lockdowns again

There is a renewed discussion about the upsides and downsides of lockdowns in controlling COVID-19, and the alternative of protecting the vulnerable.

I stand by what I said in my The Conversation article:

(…) lockdowns can be seen as a failure of other, more gradual public health policies. A lockdown should be seen as an efficient but also very blunt public health tool, to be used in necessity but as part of a wider strategy.

But – in (another, slight) change of heart, I accept @pieterstreicher argument that the initial lockdown should have been followed by a critical evaluation and switch to other containment strategies.

Mark W has from the beginning advocated a “shielding” strategy of protecting the vulnerable; a strategy that I think would have been unworkable and disastrous if applied on its own.

The problem is that we saw lockdowns as the only solution. They gave us time and we used it to develop vaccines.

But we miserably failed to use the time to strengthen the health system. This is why the next waves were so devastating.

So, what do I think about the 2020 strategy:

  • at the time in 2020 we did not have many options to stop/slow down the epidemic;
  • the lockdown had to be applied and should have been applied much earlier;
  • it did work by giving us time;
  • we did not use the time very well.

Hindsight is a great thing, but I believe that with what we knew about the disease then, any other strategy would have led to a collapse of NHS and massive deaths.

What should we have done later – after the initial lockdown? Implement a combined strategy (some form of lockdowns, masks, quick development of treatment) instead of lurching from one extreme to another.

In our 2012 paper,… we showed that the “middle ground” strategy is the worst solution. Instead, if the epidemic can be stopped, it should be stopped by the strongest measure available. If not, we should apply “soft” management.