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COVID-19: What are mRNA vaccines and how do they work?

Dr. Theresa Tam, Canada’s Chief Public Health Officer

Dr. Theresa Tam, Canada’s Chief Public Health Officer, issued the following statement today:

The end of 2020 marked a turning point for Canada in our fight against COVID-19. We witnessed Canadian approval of the first two vaccines for COVID-19 – the Pfizer-BioNTech vaccine and the Moderna vaccine, and rollout of initial doses of these vaccines across the country. Knowing the first Canadians had been vaccinated against COVID-19 gave us something positive to focus on at the end of a difficult year, and continues to fuel our hope, strength, and resolve for the year ahead.

Now, at the beginning of 2021, these recently approved vaccines are top of mind for many Canadians, and there is a keen interest in learning more about them. The Pfizer-BioNTech and the Moderna vaccines are both mRNA vaccines. For many Canadians, this may be the first time you have heard about this type of vaccine and it is understandable that you may have questions. So what are mRNA vaccines? And importantly, do they work and are they safe?

Today’s statement aims to answer these important questions by providing the latest information we have at this time. We will be exploring some of these issues in greater detail, as new evidence emerges.

How Vaccines Work

Let’s start with the basics of how vaccines work. In a nutshell, vaccines help build up your immunity to a disease and protect you from getting sick. They train your immune system to recognize when disease-causing pathogens, such as viruses, enter your body so that your immune system is ready to mount a defence against them, if you are exposed at a later time. This defence, or immune response, includes the production of special proteins called antibodies that help to destroy the pathogen. In the future, if you are exposed to the same pathogen, your immune system will remember the pathogen and destroy it before it can make you sick.

mRNA Vaccines

mRNA vaccines contain strands of genetic material – the mRNA – inside a special coating. This special coating protects the mRNA from being broken down by enzymes in your body. The mRNA delivers an “instruction set” to the cells in your body to make proteins that look like the “spike proteins” studded on the outside of the coronavirus. Your immune system identifies these proteins as ones that are not usually found in your body and begins building an immune response to them by making antibodies. Once the proteins are produced, the mRNA breaks down and is cleared from your cells.

mRNA vaccines build on scientific advancements made over several decades, including understanding the structure of mRNA, and advancements in technology to determine the genetic sequences of viruses. More recent progress on mRNA vaccine technologies has focused on a challenge that lies in the nature of mRNA itself – it is very fragile and breaks down very easily outside of a cell. Researchers have discovered that enclosing mRNA in microscopically small capsules of a fat-like substance, called lipid nanoparticles, helps to protect the mRNA from being broken down by enzymes in your blood and allows it to be delivered safely to your cells. While this innovation helps to prevent the mRNA from being destroyed too quickly in your body, it means that these vaccines need to be shipped and stored at low and ultra-low temperatures to preserve their special formulation.

COVID-19 mRNA Vaccine Development and Regulatory Review

For COVID-19, developing an mRNA vaccine was facilitated by rapid decoding and sharing of the SARS-CoV-2 genetic code, the virus that causes COVID-19, which was published online in January 2020. This allowed scientists around the world to begin designing mRNA vaccines against COVID-19. Within weeks, vaccines were ready to test in animal models, with human clinical trials following in March 2020.

Health Canada received Pfizer-BioNTech and Moderna vaccine applications to permit use in Canada on October 9th and 13th, respectively. Every vaccine submission that Health Canada receives is reviewed to ensure that the vaccine is safe, effective, and of high quality, and that its benefits outweigh potential risks. In addition, Health Canada makes sure that the vaccines are manufactured according to established high quality international standards (good manufacturing practices).

Health Canada and the Public Health Agency of Canada also review how the manufacturer will undertake safety monitoring to minimize any identified risks. Manufacturers are required to continue providing information to Health Canada on the safety, efficacy and quality of the vaccine following vaccine roll out. Pfizer and BioNTech, as well as Moderna, will be following clinical trial participants for at least two years after receiving the second dose of each respective vaccine. For additional information on the regulatory approval of these vaccines, you can visit the COVID-19 vaccines and treatments portal.

Side Effects or Adverse Reactions

As we noted in a previous statement, it is quite common to experience some mild to moderate side effects (also known as adverse reactions) after receiving a vaccine, including pain at the injection site, fatigue, headache, chills, fever, and joint and muscle pain. These effects are transient and reflect your body’s natural response as the immune system gets activated. Health Canada and the Public Health Agency of Canada, in collaboration with the provinces and territories, international partners, and the manufacturer, will be monitoring vaccine safety through a number of systems and will warn Canadians about any potential side effects of concern.

Maintaining Public Health Measures and Seeking Credible Sources of Information

As vaccines roll out across the country and as the Canadian and global medical, public health and research communities continue to actively monitor the emergence and impact of these strains, it is more crucial than ever to remain vigilant. This means keeping up with all public health measures – maintaining physical distance, washing your hands frequently, wearing a mask, and staying home, even if you have mild symptoms. In addition, while the Pfizer-BioNTech and Moderna vaccines have been shown to be highly effective at preventing COVID-19 illness and serious outcomes, research is still ongoing into how effective they may be at preventing transmission of the virus from people who have been vaccinated to others.

It is also very important that Canadians continue to ensure that they are using trusted and credible sources when they seek information about COVID-19, including that relating to mRNA vaccines. There is a lot of misinformation circulating online, which can easily confuse or mislead Canadians. For additional trustworthy information about COVID-19, the Government of Canada website,, is a good place to start. You can also find reliable information on your provincial and local health agency website, as well as from international agencies like the World Health Organization and the Red Cross.

Looking back at 2020, it is incredible to reflect on how far we have come in terms of our knowledge of COVID-19, as well as the unprecedented collaboration and innovation, both in Canada and internationally, that has characterized the COVID-19 response. While I know it was an extremely challenging year for everyone, I was consistently impressed by Canadians’ collective strength and resilience. It has helped to motivate me and in turn, I will strive to provide you with factual and evidence-based health-related information as we navigate 2021 together.

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