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.