Using reinfections as a measurement tool to control epidemics

By Jennifer Stranges

A new study from the Keenan Research Centre for Biomedical Science’s summer student program has explored a potentially more effective way of measuring the severity of an infectious disease outbreak: reinfections.

The study was led by Joshua Feldman, a Keenan Research summer student in his second year of his master’s degree at Harvard’s Data Science program, and was overseen by Dr. Sharmistha Mishra, an infectious disease physician and mathematical modeler at the Li Ka Shing Knowledge Institute of St. Michael’s. We spoke with Feldman about the study, the use of syphilis as a case study, and the significance of the findings for public health.

What did this study explore and why?

Public health agencies typically use the number of diagnoses as a way to track the severity of an infectious disease outbreak. This study explored how counting the number of people who get sick multiple times could be a more effective way of measuring how difficult it will be to control an epidemic.

Why did the study use syphilis epidemics as a case study?

Over the last 20 years, there has been a global resurgence of syphilis. It is common in these epidemics for individuals to be infected with syphilis multiple times. We were interested in studying whether the large number of multiple infections is related to these epidemics being difficult to control. This line of inquiry led us to studying reinfections in general.

Your study found a relationship between proportion reinfection and the basic reproduction number (R₀). What does that mean and why is that significant?

We found that as the fraction of infections that are reinfections increases, R₀ (or how difficult it will be to control an epidemic) also increases. R₀ increases faster and faster as the fraction grows. In other words, epidemics in which reinfections are common will be challenging to eradicate and high rates of reinfection are especially concerning. This is significant because we can use reinfections to identify epidemics that may pose a challenge for public health agencies and potentially track progress over time.

What impact do you hope these findings have on disease surveillance and public health?

We hope that public health agencies will begin including data on reinfections as a routine part of disease surveillance for pathogens that cause multiple infections. In doing so, public health teams could gain a better understanding of settings and populations where more resources may be required to achieve local epidemic control. Hopefully these findings will help us better respond to epidemics, which means fewer people getting sick!

This paper is an example of how St. Michael’s Hospital is making Ontario Healthier, Wealthier, Smarter.

About St. Michael’s Hospital

St. Michael’s Hospital provides compassionate care to all who enter its doors. The hospital also provides outstanding medical education to future health care professionals in more than 27 academic disciplines. Critical care and trauma, heart disease, neurosurgery, diabetes, cancer care, care of the homeless and global health are among the Hospital’s recognized areas of expertise. Through the Keenan Research Centre and the Li Ka Shing International Healthcare Education Centre, which make up the Li Ka Shing Knowledge Institute, research and education at St. Michael’s Hospital are recognized and make an impact around the world. Founded in 1892, the hospital is fully affiliated with the University of Toronto.

About Unity Health Toronto

Unity Health Toronto, comprised of Providence Healthcare, St. Joseph’s Health Centre and St. Michael’s Hospital, works to advance the health of everyone in our urban communities and beyond. Our health network serves patients, residents and clients across the full spectrum of care, spanning primary care, secondary community care, tertiary and quaternary care services to post-acute through rehabilitation, palliative care and long-term care, while investing in world-class research and education. For more information, visit www.unityhealth.to.