Predicting the Upredictable
How a U of T epidemiologist uses computer simulation to prevent disease spread
If you’re trying to bring some order to the seeming randomness of infectious disease outbreaks, mathematics might come in handier than biology or chemistry.
“Disease spread is mathematical in nature,” said David Fisman, a Professor of Epidemiology at the Dalla Lana School of Public Health and the Institute of Health Policy, Management and Evaluation.
For most communicable diseases like influenza, measles and sexually transmitted infections, one case can cause more than one other case. Mathematicians call this exponential growth. Clinicians call it an epidemic.
“We can develop computer model simulations to make predictions that help guide policy around antibiotic and vaccine distribution,” said Fisman.
Fisman and his colleague at the Dalla Lana School of Public Health — U of T’s first new faculty in 15 years — are working with clinicians, policy-makers and public health providers to analyze and predict infectious disease outbreaks in Toronto. By using the best-available data to create simple but accurate models of disease, Fisman’s team can determine when diseases will peak, decline, what groups will be the most vulnerable and what interventions will be most effective.
Fisman and his team are simulating an outbreak in Toronto of whooping cough — an infectious disease poised to make a comeback. Based on the simulation, Fisman found the best strategy is to target vaccinations at older people, even though children and youth are most likely to need hospitalization. By testing different policy options in the model, epidemiologists can determine the ideal target percentage of the population covered by vaccination and the ideal timing of the vaccination.
Proportion of Individuals with vaccine induced immunity to pertussis
“It seems counterintuitive, but often our models suggest intervening in one group to protect another, more vulnerable population,” he said, referring to the whooping cough model outcome.
Fisman admitted that simulations are not substitutions for clinical studies, but they are a valuable tool to analyze complex health economic systems.
Whooping cough isn’t the only disease that public health officials are watching. Ten years ago there were no cases of measles in most of the Western world. Quebec, France and the UK have all seen resurgences of measles and mumps — diseases that were supposed to be controlled — in recent years.
“In the absence of risk, it’s hard to motivate people to get vaccinated. It’s a funny thing because good news often means an absence of things occurring,” said Fisman, who noted horror fiction author H.P. Lovecraft’s famous quote:
“The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown.”
So what can we do to encourage more people to get vaccinated?
“It’s got to start with showing up,” said Fisman, referring to experts who don’t engage anti-vaccination movement leaders. “We can’t mail it in. Public health advocates must actively participate in the debate and have the conversation face-to-face if we want to be heard.”