Some interesting links

A couple of interesting links on COVID-2019.

Up to date maps and number of cases (updated daily):

A very good article on herd immunity, so related to my yesterday post:

As usual, if you have comments, please send them to me at

Coronavirus: when and how to act

There is now a lot of discussion on when and how governments should act to stop the coronavirus from spreading further and causing more damage. These discussions and arguments will continue for a long time, even well after the epidemic finishes and we know the results of the actions.

The problem of which strategy to choose is actually not a trivial one. The decisions are taken under very limited knowledge of how fast the disease spreads, how many infectious but non-symptomatic individuals already are in the country and what the economic and social effects of both the disease and the actions to control them will be.

Fortunately, there are some lessons we can draw from previous epidemics, not only human viruses, but also animal and plant epidemics.

If we were simply looking to control the infection without counting economic and social costs, arguably the best strategy would be to clamp on the spread as quickly as we can, to lock down the whole world and stop any long-distance travel for as long as the infectious period is. If a vaccine was available, we could have relaxed some of the regulations and instead introduced a mass vaccination policy. The measures would be tough, the costs horrendous, but the whole thing probably quickly over. The Netherlands followed this course in 2001 when faced with a spill-over of an animal foot-and-mouth disease from the UK. This was in contrast with the UK which in the first stages of the epidemic allowed the animals to be moved around, took a long time to act, and then implemented a very harsh strategy of preventive killing resulting in millions of healthy animals to be culled.

The big problem with the very quick ‘lock-down and smash’ approach is that at the start of the epidemic we actually know very little about it. As the governments by their nature are cautious and tend to sweep bad news under the carpet, they are not likely to react to a small outbreak of what looks like another flu. The reaction only comes later when the number of notified cases goes up, some people die, and others actually start making a fuss. By which time it is already too late.

To understand why, we need to look a bit at how diseases like coronavirus spread. It all starts with ‘patient zero’ (or several ‘patients zero’) who somehow drew a short straw in a mixing bowl of animal and human viruses (most recent human diseases have animal origins or, as we call them, ‘zoonotic’). There could be several such events, most of which would not spread further, as the ‘patient zero’ recovers or dies without the new disease being recognised. As this often happens in places with limited health services, such animal-human transmissions often go unnoticed.

Such a phase, called ‘stuttering’, can actually last a long time, resulting in a number of cases which die or recover without causing a large outbreak. It is only when we start seeing significant ‘human-to-human transmission’, things become problematic.

Thus, once in a while one of the ‘patients zero’ will infect a few other patients and start the avalanche. At the start, it is usually local, so limited to a family, a village or a region. It is also still relatively easy to stop, but nobody knows about it or realises the potential to spread.

MERS (Middle-East Respiratory Syndrome) is a good example of a virus that never actually made it; it is causing problems, people become ill and some die, but it does not spread very fast between people and hence is (still) quite localised.

There is another ‘ace in the sleeve’ that viruses use to spread. A person who contracts the infection usually does not notice this until the first symptoms appear. But even then, if the symptoms are relatively mild, we might go about not noticing anything unusual. Have you ever gone to work or school or shop or cinema not feeling very well, with a bit of a cough and maybe a running nose? The author actually sat an important exam while showing the first symptoms of chickenpox, including a very high fever. I am not sure about the quality of my answers (it was an oral exam in philosophy and we talked about Plato and chaos theory), but I managed to pass the exam only to spend the next two weeks in bed.

The effect of this ‘pre-symptomatic’ (or as we often call ‘latent’) phase is that people can travel quite considerable distances and meet a lot of people while being highly infectious but without showing symptoms. In today’s world, flights can take the disease from one end of the world to another within hours (the shortest China to Italy flight is only 9 hours 55 minutes; we call this property a ‘small world’). So, by the time the authorities notice something is going on, there might already be quite a few ‘infectious but pre-symptomatic’ individuals on their territory.

So we end up with a ‘whack a mole’ situation in which we are trying to control something in one place, for it to reappear in another.

There are also other considerations to be taken into account, like psychological, social and economic aspects of disease control. People might start to panic which can lead to political perturbations and food and petrol shortages. The cost of closing down schools can be huge as people will need to stay off work to look after children. Closing down restaurants and music halls can bankrupt owners and musicians. In addition, not knowing exactly how fast the disease spreads and for how long people are infectious, means that it is not clear how long the closures need to last. At the moment we think 2 weeks is enough, but we simply are not sure. And what politician would want to be telling people after horrendous 2 weeks that they need to do it again for another 2 weeks?

There is good evidence in both modelling and controlling other diseases that the earlier and the harder we act, the more effective and hence the shorter the outbreak will be. But even more important is that the government is very clear and open with the public why the particular decision is taken and when. One of the lasting problems of the foot-and-mouth disease in the UK in 2001 was the loss of trust between the government (and mathematical modellers) and farmers. As a result, bad feelings continue even after 19 years. We can only hope that the coronavirus outbreak will not leave such problems in its wake, although there are very good indications that the world will not be the same after the 2020 outbreak.

One thing for sure, we as epidemiologists will have even more work to do…

As usual, if you have comments, please send them to me at

Herd immunity

I heard ‘herd immunity’ mentioned on BBC Radio 4 this morning in the context of COVID-2019 pandemic. It is worth reminding ourselves what this means, as it is actually quite a controversial concept.

Its origins are from epidemiological modelling where people observed that a disease will not spread if the proportion of susceptible individuals (those who are still healthy and can contract the virus) falls below a certain value.

This can occur in different ways. The simplest one is when we have a large outbreak of a relatively benign disease so that nothing but palliative care is needed. The number of cases will at some point stop growing and eventually the epidemic will die out on its own. But the interesting point is that this will not lead to everybody having been through the infection. There will be quite a lot of people who simply do not get the disease even though they could have been infected. In a sense, they are protected by those who did get the disease and became immune. Thus, the ‘herd’ is becoming immune to disease, even though not all individuals are. Seasonal flu or measles outbreaks before vaccination era are examples of such a situation.

Another way of reaching the ‘herd immunity’ is by vaccination. You have probably heard about controversies associated with the MMR (measles, mumps, rubella) vaccine. Back in 1960s the US, the UK and other countries faced large measles outbreaks. The idea of massive vaccination was (and still is) to make enough people immune to the disease so that the virus cannot spread. For measles, this is estimated at about 90%, so that we expect that if the percentage falls below this number, new cases will start appearing. This is exactly what has been happening in many countries.

I am not going to go in this post into a discussion of merits or dangers of MMR vaccination – this is a subject for another post or posts. But I want to point out that the key element in both MMR and COVID-2019 idea of ‘herd immunity’ is that a large proportion of the population is asked to take a – usually very small – risk in either vaccinating (MMR) or going through coronavirus infection (COVID) for the public good. In other words, I am asked to take a risk to protect others from possibly facing a larger one.

This was made quite explicit this morning in the BBC Radio 4 programme:

‘Sir Patrick Vallance says aim is to create herd immunity and broaden peak of epidemic’

says Guardian this morning. The government’s idea is to slow down the epidemic but not to stop it so that the NHS is not overwhelmed, but enough people – possibly – get immunity to prevent future outbreaks.

There is a problem with this approach in that it has been shown that voluntary measures to create herd immunity simply do not work. People (consciously or subconsciously) evaluate risks of getting complications from mild coronovirus vs. benefits of protection against complications and at some point decide that this will not work for them. This point is given by a balance between perceived individual costs (falling ill, getting complications, passing on coronavirus to the elderly Mum) and perceived population benefits (not having an outbreak in autumn). When the first perceived cost is larger, people will not play the game and the ‘herd immunity’ fails (as it did for measles). There is a large body of evidence and theory using simulations (our own research) as well as the game theory that supports this behaviour.

This is a fascinating time for epidemiological researchers. As different countries implement different control measures, we start seeing differences in the way COVID-2019 epidemic is slowing down. One thing is certain, in the future the epidemiologists will need to very closely work with psychologists and economists. In our recent paper we said:

So, are we equipped to deal with the next pandemic? According to Blackburn et al. [98], the answer currently is a qualified ‘no’. However, the tools and processes may already be available to allow a more emphatic ‘yes’ to be the answer. Our future success in preventing and combating pandemics requires close collaboration across disciplines and systems.

So what is the solution? We are facing a crisis and there is a lot of unknowns. The key thing is for both politicians and experts to be very open about what they do and what they do not know about the outbreak. In the meantime, we need to do what we can to slow down the spread.

As usual, if you have comments, please send them to me at