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Ashleigh Tuite and David Fisman are epidemiologists and professors at the Dalla Lana School of Public Health at the University of Toronto. Dr. Fisman is also a practising physician at the University Health Network in Toronto.

How many people might get infected? How many people might die? When will this all end? There is understandably substantial fear and anxiety about what the world is going to look like in the weeks and months to come. As we deal with a global pandemic of COVID-19, we are faced with having to make individual and societal decisions about how to mitigate the coronavirus’s impact in the face of considerable uncertainty.

As infectious disease epidemiologists, we often use a tool called mathematical modelling to understand and predict how diseases will spread in populations. As novel as COVID-19 is, it adheres to the same rules as other communicable diseases. A fundamental property of communicable diseases is that we need both infected people to spread the disease and susceptible people to become infected. An epidemic ends when the number of immune people in a population passes a critical threshold (known as the herd immunity threshold). This threshold is unique to each disease. One way we can achieve herd immunity is through vaccination. There is no vaccine for COVID-19, although there is hope that one is on the horizon, with a Phase 1 clinical trial already under way.

In the meantime, we have tools that can be used to minimize how quickly the disease spreads in the population. It can be helpful to think of the COVID-19 pandemic as a forest fire. Those infected with COVID-19 are sparks being thrown off and those uninfected are the fuel. We know that the fire is going to spread, but we want to control the burn as best we can. There is no precise recipe for how to do this. The experiences in other countries have demonstrated what does and doesn’t work. Quick, decisive implementation of measures such as increased testing of putative cases, rapid isolation of cases, quarantine of those exposed to cases and social distancing measures all have a role to play. Models, such as one we developed with The New York Times, show how strength of our public health response and the timing of that response shape the epidemic curve. Strong measures put in place early are most effective.

In Canada, the past several weeks have represented what a colleague has referred to as our “golden time.” We have seen how quickly the situation has progressed in other countries and increasingly understand what is to come here. Despite the time we’ve had to prepare, there remains inconsistency in messaging and until Tuesday, some indecision on how stringently and comprehensively to engage in social distancing measures in Ontario. Ironically, it was also on Tuesday that one of us had his first experience participating in the care of a patient with respiratory failure due to COVID-19.

It is clear, from both modelling studies and the experience of countries such as Italy and Spain, that we need to act early to save lives. The time to implement wide-scale social distancing is before you have a crisis. If we wait until our hospitals are overwhelmed to react, we will have squandered our golden time and it will be too late. There are delays between a person becoming infected, developing symptoms and becoming sick enough to require hospital care. These hospital cases represent a snapshot of the epidemic several weeks ago. Since that time, the disease has continued to spread and we can anticipate a rapid ramp-up in cases, and associated health care needs in the weeks to follow. In an overstretched health care system, our ICUs soon become full and people will be denied care and die unnecessarily. That reality is unfolding in Italy. Indeed, because of this unseen pipeline of infection, even today’s stronger disease control action will take some time to have an impact; while it may reduce infections moving forward, it does nothing to reverse infections that have already occurred, and which have yet to make their way in increasing numbers to our hospitals.

Are there uncertainties? Absolutely. This is a new pathogen. There is much we don’t know. One key unknown is the effect of warmer weather in the Northern Hemisphere; if, as we see for flu, disease spread is attenuated in the summer, we have a temporary reprieve from COVID-19. But as with flu, if its spread does diminish in the summer, we can expect it will return with the colder weather.

Mistakes will be made. That is all right and to be expected. We are in an unprecedented situation and will undoubtedly need to course-correct. Is it possible that we will overreact? Certainly. If we succeed, we will also be criticized for being excessively forceful, because the cases that would have occurred, won’t. As a friend said recently: Nobody ever put up the headline “Airliner lands safely.” But we have seen the consequences of waiting to react until the health care system shows signs of distress and we cannot in good conscience allow that to unfold here.

There is no single measure that will control COVID-19. We can change the course of this pandemic, but it will require all of us to make dramatic, disruptive and potentially prolonged changes to our lives to reduce transmission and protect the most vulnerable in our communities. We have a choice. We act early, pro-actively, or we act late, reactively.

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