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A participant in a COVID-19 vaccine trial receives an injection in Kansas City, Mo., on April 8, 2020.The Canadian Press

Question: I’ve read that it could be one to two years before we have a vaccine that will guard against COVID-19. Why is it going to take so long?

Answer: The development of any vaccine can be compared to a long and challenging marathon with an uncertain outcome – and that is especially true when dealing with a new pathogen.

The purpose of a vaccine is to expose the body’s immune system to some portion of the virus so it can prepare in advance for a real attack. For instance, a vaccine might include an antigen, or protein, from the surface of the virus. But finding the antigen that will trigger an effective immune response is easier said than done.

“Science cannot be rushed,” says Rob Kozak, a clinical microbiologist at Sunnybrook Health Sciences Centre in Toronto. Researchers must follow well-established regulatory protocols that are designed to ensure a therapy is both effective and safe.

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Viruses are constantly mutating and evolving. The strain of a virus circulating in Canada might be slightly different from the one in China or Europe. This means the antigen must produce immunity against all strains, or variants, of the virus.

Once an antigen is selected, it has to be tested in animals before human trials can begin. Finding the appropriate animal model also presents challenges. The animal needs to respond to the virus – and the vaccine – in the same way as people.

Fortunately, researchers can look to previous vaccine studies for clues on how to respond to the current pandemic, including selecting appropriate animal models and viral targets.

The COVID-19 illness is caused by a coronavirus, officially known as SARS-CoV-2. In recent years, humanity has been challenged by two other deadly coronaviruses – Severe Acute Respiratory Syndrome (SARS) in 2003 and Middle East Respiratory Syndrome (MERS) in 2012. During both of these outbreaks, researchers started to develop vaccines. In the case of SARS, the work was never completed partly because the virus ceased to pose an immediate threat; it seems to have morphed and disappeared. MERS vaccine trials are continuing.

Previous SARS research reinforces the importance of doing thorough testing in animal models before any potential vaccine is given to human volunteers.

In one study, an experimental SARS vaccine made lab animals worse, says Arinjay Banerjee, an emerging-viruses researcher at McMaster University in Hamilton.

“This study showed that when mice were vaccinated and then challenged with the pathogen, there was an enhancement of the infection,” he says. “The vaccinated mice developed disease more rapidly and died more rapidly than the unvaccinated mice.”

Another study revealed that some investigational SARS vaccines produced negative side effects in some types of animals (such as ferrets) but not in others (such as mice). For that reason, many researchers are convinced that a vaccine should be tested in two different types of animals, Kozak says.

All this preclinical work is time consuming. Laboratory animals require specific time periods to develop a response to the vaccine and then to react to the virus. The clock cannot be made to run faster, Kozak says. And if an experimental vaccine fails, a research team could find itself again at the starting gate.

After a vaccine has successfully passed animal testing, it is then tried in a small group of healthy volunteers. This is known as a phase-one clinical trial. It’s basically a safety check to make sure the vaccine does not cause serious side effects.

If the vaccine clears this critical hurdle, trials are expanded gradually to include more people who are observed for longer periods of time in order to gain a better understanding of its risks and benefits.

Scientists around the world are already exploring various ways to deliver a COVID-19 vaccine. “Each of them have their advantages and disadvantages,” Kozak says.

“To be honest, I don’t think we are going to have just one vaccine,” he adds. “In fact, I hope we don’t. I hope we have three or four amazing candidates that all work basically as well as each other, and that could be critically important because you don’t want to be dependent on only one company to provide for the world.”

Like other experts, Dr. Kozak estimates it will take between one to two years to develop a vaccine. And once a vaccine does exist, special production facilities will have to gear up operations to meet the global demand. That, too, will take time.

All of which means that a “quick fix” vaccine is not on the immediate horizon.

Paul Taylor is a Patient Navigation Adviser at Sunnybrook Health Sciences Centre. He is a former Health Editor of The Globe and Mail. Find him on Twitter @epaultaylor and online at Sunnybrook’s Your Health Matters.

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