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A nurse prepares to administer the Pfizer-BioNTech COVID-19 vaccine at Guy's Hospital in London, on Dec. 8, 2020.POOL/The Associated Press

The question: I have an autoimmune disease and need to take drugs that suppress my immune system. Should I get a COVID-19 vaccine when it becomes available? I worry that the shot won’t protect me from getting infected. Or worse, I might actually catch COVID-19 from the vaccine.

The answer: You can rest assured that a COVID-19 vaccine won’t give you the illness that it is designed to prevent. And that is certainly true for the first one of these shots approved for use in Canada.

Made by Pfizer Inc. and its partner BioNTech, it is based on a new type of vaccine technology using “messenger RNA” – or mRNA. To create immunity, the vaccine contains some of the genetic instructions for SARS-CoV-2, the novel coronavirus that causes COVID-19.

“It’s only one strand of RNA for making a part of the virus – not the entire virus – and it’s not infectious,” says Rob Kozak, a scientist and clinical microbiologist at Sunnybrook Health Sciences Centre in Toronto. What’s more, the other leading vaccine candidates don’t contain infectious agents either, “so there is no risk of these vaccines making you sick with COVID,” Kozak says.

However, he adds that it’s harder to predict how well the vaccines will work in people with underlying medical conditions.

Preliminary data from trials of healthy adults indicate some of the experimental vaccines are effective in 90 to 95 per cent of those who receive two doses.

An encouraging sign is that some of the trials included volunteers over 65 who tend to have less-efficient immune systems than younger folks.

Kozak notes that a promising vaccine from AstraZeneca and the University of Oxford has produced a comparatively strong immune reaction in older adults. “This suggests the vaccines will probably work to some degree in people who are immunocompromised.”

Nonetheless, we won’t know the true protective capacity of the vaccines until there are results from trials involving a broader segment of the population, including individuals with various medical conditions.

Indeed, many questions remain unanswered.

The main goal of a vaccine is to get your immune system ready to fight off a specific microbial invader.

Traditionally, vaccines have used weakened or dead viruses – or parts of viruses – to train the immune system. When exposed to a harmless deactivated virus in this way, the body will produce antibodies and other immune cells that are primed for a rapid attack if they encounter the real live pathogen.

In recent months, the threat posed by the global pandemic has helped accelerate the development of new and innovative vaccine technologies – including mRNA.

In addition to Pfizer and BioNTech, Moderna is also banking on this approach. All the mRNA vaccines contain the genetic code for producing “spike” proteins – the distinctive protrusions that dot the surface of the coronavirus.

When delivered in a vaccine, the mRNA enters the body’s cells and instructs them to churn out spike proteins. Human cells immediately recognize these are “foreign” proteins and alert the immune system to start generating protective antibodies.

The key advantage of an mRNA vaccine is that the synthetic raw materials can be produced faster than some traditional vaccines, such as flu shots, which rely on viral samples grown in eggs.

On the downside, mRNA is inherently unstable. The Pfizer vaccine, for instance, must be stored at -70 C, thus creating huge challenges for distributing the doses.

Another uncertainty is that we don’t how long the vaccinations will last. Immunity may wane over time, requiring booster shots down the road.

But the more immediate question is whether the vaccines will actually block infection or just reduce the severity of the illness.

Even if a vaccine only minimizes symptoms, that may be sufficient to prevent a surge in COVID-19 cases from overwhelming the health care system, says Chris Labos, an epidemiologist and cardiologist in Montreal.

“If the symptoms are reduced, then fewer people will need to be admitted to hospital, and fewer of them will be taking up ICU [intensive care unit] beds and ventilators.”

On the other hand, if you are inoculated and only have partial immunity, you might later contract a mild case of COVID-19 and still be capable of spreading it to others. As a result, we may need to take protective measures to guard against infections until enough people are vaccinated to create collective “herd” immunity.

“Just because a vaccine is rolled out, it doesn’t necessarily mean we stop wearing masks or keeping our distance,” Labos says.

Kozak agrees: “I think masks are going to be with us for a while.”

Paul Taylor is a former patient navigation adviser at Sunnybrook Health Sciences Centre in Toronto and 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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