Mathematician Alex James, from Te Pūnaha Matatini & the University of Canterbury, explains the art and science of modelling the coronavirus pandemic.

Since the COVID-19 pandemic began, it would be fair to say that many of us have become obsessed with numbers.

The number of cases of coronavirus, the number of deaths and so on.

Mathematician Associate Professor Alex James is no different - but for her there are some numbers that are more important than others.

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Alex James is a mathematician at the University of Canterbury and an investigator with Te Pūnaha Matatini, New Zealand's Centre of Research Excellence for Complex Systems and Data Analytics.

She is interested in creating mathematical models for what are known as complex systems, including ecology and infectious diseases.

When it came to the coronavirus pandemic, Alex and colleagues at Te Pūnaha Matatini decided to use their collective expertise to model the situation in New Zealand as it developed. They have produced a number of models, which they are providing to health officials to help with decision-making.

These models complement others being worked on by groups such as Michael Baker and Nick Wilson at the University of Otago, amongst others.

The mathematical modelling of a disease such as coronavirus is not about precise predictions.

"We're not always trying to predict exact numbers," says Alex. "We're often thinking 'in two weeks' time is it going to be going up? Is it still going to be going up? Is it going to be going down?' It's more general questions like that rather than what will the number exactly be."

Alex says that she thinks mathematical modelling is both an art and a science.

"The art is to think this is complicated, there are a million different things going on here - but what are the most important ones? Can I boil this down to just two or three ideas and capture a lot of that complexity in just a small number of features."

One magic number

"In epidemiological models, models of disease spread, we have one magic number," says Alex. "We call it R0 or R nought." It is also known as R zero, the reproduction number or reproductive ratio.

"R nought is 'if I have an infection, how many people am I likely to infect'?"

An R0 of two means that on average one person will spread the infection to two others. Anything above one means the number of cases will begin to increase exponentially. An R0 of less than one means the infection will eventually die out…

Go to this episode on rnz.co.nz for more details