The known unknowns of Ebola in the Democratic Republic of the Congo

Hello and thanks for downloading the More or Less podcast with a program that looks at the numbers known, the numbers unknown and a known unknown number of numbers. And I'm Tim Harford. On the 17th of May, the World Health Organization declared that a new outbreak of the Ebola virus in the Democratic Republic of the Congo was an international emergency. Ebola virus is an extremely nasty viral disease with a high death rate. But despite the severity, little is known about the number of infections in this current outbreak, in part because this particular species of Ebola is a rare one. Loyal listener Richard Ellis got in touch after reading a BBC news article that said, Modelling by the MRC Centre for Global Infectious Disease Analysis suggested there had been a significant under-detection of Ebola cases. They stated the true number could be over a thousand. Richard was curious about how they came to these numbers. I'd love to understand more about how this kind of disease modelling works. I mean, how do researchers estimate the number of unreported cases and how reliable are these models likely to be in practice? I'm also curious about what it means if the true number of cases is already much higher than reported. To answer Richard's questions, we spoke to Dr Ruth McCabe. She's an infectious disease epidemiologist at Imperial College London and she worked on the modelling behind those estimates. Let's start with what we know. Ebola is an incredibly serious disease. It's what we would call a viral hemorrhagic fever. It typically has quite a high mortality and it spreads between contact with bodily fluids from an infected person. Bodily fluids including sweat, saliva and blood. So one of the ways in which it can transmit for example is if there's not sort of strict infection prevention measures in hospitals. So for example, a bedsheet science change between patients. That is one way in which Ebola could be transmitted. There are six species of Ebola virus, four of which can infect humans. Only one has a vaccine available. This species does not. This outbreak of Ebola that we're seeing just now is caused by the Bundabudgeo species of Ebola. There have only been two outbreaks of this recorded previously and it is different to the species that caused the very large outbreak in West Africa in 2014 to 2016. Like all species of Ebola, the Bundabudgeo species is zoonotic meaning it comes from animals. It's primarily found in fruit bats and can jump between animals and humans in so-called spillover events where a human comes into contact with the infected animal's blood, urine or flesh. One of the most notable things about this current outbreak is the size that it had gotten to by the time that alert had been signalled. It was approximately 60 suspected deaths and 200 suspected cases when this was then signalled into the wider community. That's quite large and if you look at that in comparison to other outbreaks of Ebola, this is sort of one of the largest outbreaks that there have been at the point of detection. This may be because in a country such as the Democratic Republic of the Congo, the east of which is currently suffering from violent conflict and which is very poor, detecting diseases can be difficult. So in terms of definitive numbers of infections, that's already incredibly difficult to be actively counted in the data. So for example, seeking health care, receiving health care, having that positive test which obviously can be logistically challenging, especially in an area in the eastern DRC. This is about as much as we know. We're now stepping into the land of known unknowns and this is where the modelling comes in. Researchers work backwards using the data from the two previous outbreaks. If they know how deadly a disease tends to be and how many people died, they can estimate how many people must have been infected. Based on the previous two outbreaks that we have seen of this Bundabudjo species, the case fatality ratio is around 33% with sort of broad range of uncertainty there between 26 and 40%. So it is a severe disease. So if you know that 60 people have died from Ebola, you can estimate that around 200 people must have been infected. But how many people might have been infected who aren't showing symptoms? Now what we learned in the previous two outbreaks is that the incubation period was between six and seven days. There's a tinderly between becoming infected and passing away. People will typically not die for days or even weeks after they're infected. This introduces a delay in reporting. So we have to account for that in our modelling as well. Since the alert reached the international community, the number of infections has exploded. However, this might not mean that transmission is rapidly increasing. We don't actually know how quickly this outbreak is growing right now. One of the reasons for that is because of the sort of increased surveillance activities, the increased response activities. That means that as the data are coming into us, it's quite difficult to know if the changes in the numbers that we're seeing are indeed just because of how the virus is spreading compared to how it is being reported and how it's being picked up in the numbers. So the team are constantly checking data against previous outbreaks and new information coming in. We're using independent pieces of data to essentially piece together what is happening in terms of the total outbreak size. Since the outbreak, neighbouring Uganda has identified three cases. This gave the team another piece of data. We have then looked at the border crossings between Uganda and between the DRC and then essentially used that to generate a probability that a case would travel, which we could then work back to get at how big of an outbreak would we need to have seen for it to be plausible that there are three cases exported into Uganda. I'll listen to this curious. If it's all been underestimated before, what does that mean going forward? So this is a really rapidly evolving situation. The numbers are changing every single day and that means that the estimates that we are producing are going out of date incredibly quickly. At the time of recording, bringing together those pieces of data that we do have, it suggests that as of the 22nd of May, there was between 950 to 1600 cases of the Bundabudjo species of Ebola in the Eastern DRC. And that is in comparison to around 870 suspected cases at that day. So this is really important in terms of informing the response just now. It signals about the level of case detection that has to be undertaken that's currently being undertaken in the affected regions. It then signals the resources required to then follow up with all of their contacts as well. It informs the provisions for healthcare that are required, how many beds might be needed, how many staff are then going to be required so bravely to care for these patients and what is the personal protective equipment needed then to ensure that they are safe. All of these sorts of logistical things can help to be informed by these sorts of numbers. Despite all of the work being done by Ruth and the team, there are still a lot of unknowns. We can make an educated guess at the number of people per infected group who may die, but we currently don't know how many people are infected or how many are coming into contact with infected people. One of the difficulties is that people are most infectious at the time of their death. And if the family doesn't know or chooses not to report that they died of Ebola, the disease can fly under the radar. Each Ebola death can lead to many more infections. There's a lot of uncertainty surrounding those numbers. That's not necessarily a weakness, that's just a reflection of the situation that is ongoing just now. There's other assumptions that we need to know about how quickly the outbreak is growing and we just don't know that yet. That's all we have time for this week. Thank you to Dr Ruth McCabe. Do keep your questions and comments coming in to more or less at bbc.co.uk. Until next week, goodbye.