Skip to 0 minutes and 13 secondsMARK JIT: Today I'm going to be talking about one of the essential tools that has been used to understand the spread of infectious disease outbreaks, such as the current outbreak of COVID-19 that we're seeing in the world today. I'm going to be talking to you about models-- mathematical, statistical, and economic models. Models are simply a simplification of a very complex object. In this case, the complex object is the real world-- 8 billion people in the world all interacting with each other, getting infected, interacting with the pathogen-- the COVID-19 virus-- and interacting with their environment.
Skip to 0 minutes and 51 secondsThis is far too complicated for us to understand just with our unaided minds so mathematical, statistical, and economic models help to simplify this by setting up relationships between different aspects of the world.
Skip to 1 minute and 13 secondsModels have been an essential part of understanding and responding to the outbreak of COVID-19. At the beginning, models were used to understand what was happening in Wuhan, where the outbreak first started, and how infections spread from Wuhan to other parts of China and other parts of the world. Now that we're having outbreaks in many parts of the world and epidemics in many parts of the world, the models have been used to understand some of the key features of the virus that we need to know in order to know how to respond to it. For instance, how long are people infected? How many people might they infect when they get infected? Can people infect other people even before they show symptoms?
Skip to 1 minute and 57 secondsThat's essential to understand to know whether we can stop people from transmitting to other people by actually finding them quickly and making sure that they are isolated before they spread to other people. We're also using models to understand how large the epidemics will be in different parts of the world and whether we have the health care resources that we need in order to make sure that people have enough space in hospitals, enough space in intensive care units to make sure they get the treatment that they need.
Skip to 2 minutes and 31 secondsThe rate at which the disease spread is influenced by how often people come in contact with one another. And, obviously, within very closed environments, such as on a cruise ship or in prison, there's a lot more close contact between individuals and these are some of the places where we've seen big outbreaks of COVID-19. So the spread of the virus can also be influenced by what's happening in the country or in the location. So, for instance, if schools are closed or if workplaces are shut and people start working from home or if people stop travelling or stop going out, then this will reduce the number of contacts people have with other people. And that will slow down infectious disease spread.
Skip to 3 minutes and 19 secondsCountries with COVID 19 cases report the number of cases that they've detected but these numbers are probably much less than the actual number of cases that are in the country because many people who are sick have mild symptoms or maybe no symptoms at all and, therefore, they don't report themselves as being sick to be tested for COVID-19. As a result, modellers have to look for other hints about the true number of cases.
Skip to 3 minutes and 48 secondsFor instance, they look at the speed at which these reported cases are increasing in order to understand how fast the virus is spreading or they look at other outcomes which might be better recorded like the number of deaths due to COVID-19 or the number of cases that are picked up because people leave the country and go to another country. So we put all these clues together to try to get a good understanding. And to do that one really important number we try to estimate is R0. And if we know this number R0 and we know the length of time that someone is infected, then we can have a good guess as to how fast the virus will spread.
Skip to 4 minutes and 28 secondsWe also want to know the number of deaths. And we have to be careful when we look at the number of deaths that is being reported because deaths are a lagging indicator. So you might be sick for a week or two weeks before you'll die and, therefore, the number of deaths we see today probably reflects what's happening with the epidemic about one or two weeks ago rather than what's happening now. So we have to be careful when we calculate the ratio between cases and deaths-- what we call the case fatality ratio-- to take into account the fact that actually there is this lag between the cases and deaths.
How can modelling help?
Professor Mark Jit discusses how modelling can be used to understand how an outbreak may progress, informing the interventions needed (recorded 10th March 2020).
Intended learning outcomes
- Explain how mathematical and statistical models can be used to understand disease outbreaks including COVID-19, and the effects of social behaviour and immunity on transmission and control
- Critique the limitations of mathematical and statistical models and the level of uncertainty in cases and deaths
© London School of Hygiene & Tropical Medicine 2020