Can mRNA vaccines alter your DNA? More of your COVID-19 vaccine questions answered

As the first shots of coronavirus vaccines are delivered in Canada, CBC News talks to the experts to answer your questions about this kind of vaccine and the clinical trials that got them to this point, including: Is the Pfizer-BioNTech vaccine safe for children and pregnant women?

Also: Is the Pfizer-BioNTech vaccine safe for pregnant women and children?

A pharmacist gives a woman the first shot in a safety study of a potential vaccine to protect against COVID-19 in March. Clinical trials for coronavirus vaccines were largely designed to see whether the vaccines reduce chances of getting symptomatic disease. (Ted S. Warren/The Associated Press)

The information in this article was current at the time of publishing, but guidelines and advice can change quickly. Check with your local public health unit for the most-current guidance, and find the latest COVID-19 news on our website.

We're answering your questions about the pandemic. Send them to, and we'll answer as many as we can. We publish a selection of answers online and also put some questions to the experts during The National and on CBC News Network. So far, we've received more than 59,000 emails from all corners of the country.

As the first coronavirus shots got into arms across Canada, CBC News talked to the experts to answer your questions about how the vaccines got to this stage, such as: What does "90 per cent efficacy" mean in a clinical trial? And what kinds of volunteers have been in clinical trials or excluded? 

What is the effect of mRNA vaccines on DNA? Is it true that it alters DNA? 

The mRNA vaccines like those developed by Pfizer-BioNTech and Moderna don't use DNA.

RNA is like a temporary photocopy of DNA and is used to make proteins. Proteins are the building blocks that cells use to grow and repair themselves. Once the protein is made, the RNA is degraded.

"In humans, the progression is always DNA to RNA to protein," said Dr. Zain Chagla, an associate professor of medicine at McMaster University and an infectious disease physician in Hamilton, Ont.

"You can't go from RNA back to DNA in human cells. It's not possible because we just don't have the machinery to deal with it. So there should not be concerns about the RNA vaccine somehow getting into the human DNA and transforming it."

There is one kind of microbe that can produce DNA from RNA — retroviruses such as HIV. And the DNA generated by retroviruses can sometimes integrate into cells, Matthew Miller, an associate professor of infectious diseases and immunology at McMaster University, said in an email. 

That has led some researchers to propose that infection with a retrovirus might theoretically provide the proteins necessary for integration of mRNA, although that's considered "extremely unlikely."

Miller said that RNA from retroviruses is structurally different from mRNA.

"So even if one was infected with a retrovirus, the mRNA in the vaccine would not alter our DNA."

Jason Kindrachuk, Canada Research Chair in molecular pathogenesis of emerging and re-emerging viruses at the University of Manitoba, also thinks the risk of integration is tiny. The amount of mRNA in the vaccine is far lower than the amount generated in a natural infection, he noted in an email, and trials of either this type of COVID-19 vaccine or mRNA vaccines under development for other diseases have shown no indications that integration could be a risk. He added there are no noted contraindications for people with HIV on the product monograph, and the only thing it says about immunocompromised patients is that they could have a diminished immune response. 

If participants in Phase 3 clinical trials are not intentionally exposed to the virus, how can vaccine efficacy be calculated?

People in the clinical trials were divided into two groups — those who got the vaccine and those who got a dummy shot. They would have gone to work and school and had social interactions as they normally do. Because Phase 3 clinical trials were mostly done in places with very active pandemics, such as the U.S. and Brazil, a certain number of participants would have been exposed to the virus that causes COVID-19 in their daily lives.

Efficacy was calculated by comparing the number of infections among those who got the vaccine and those who got the dummy shot.

Efficacy is how well the vaccine works in clinical trials. Effectiveness is how well a vaccine or a product will work in real world settings.

How do we interpret "90% efficacy" of a vaccine in a clinical trial? Does this mean 90 out of 100 people will have immunity to the virus? 

No. The clinical trials so far generally weren't designed to measure whether people were immune to the virus. They were designed to see whether it reduced their chances of getting symptomatic disease. Ninety per cent efficacy means those who got the vaccine were 90 per cent less likely to get symptomatic COVID-19 than those who got the dummy shot. 

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However, the trials did not generally test for asymptomatic infection, so it's not clear how many people actually had immunity to infection. 

What kinds of volunteers have been in clinical trials and who has been excluded so far? From an efficiency point of view, can a vaccine protect different persons differently?

Most trials so far have included adults.

Vaccines are often less effective in older populations, which is why there are higher-dose flu and shingles vaccines for seniors.

Both Pfizer-BioNTech and Moderna said the efficacy of their vaccines was consistent across age, race and ethnicity, and gender demographics. Pfizer-BioNTech also specified the vaccine was 94 per cent effective in adults older than 65. 


Neither company reported results specifically for people with chronic conditions.

Chagla said as the vaccine rolls out to millions of people worldwide, we'll learn more about people in different age groups or who have other illnesses.

"We'll get a better sense of whether or not this vaccine does prevent people from transmitting and shedding COVID-19 or just makes their symptoms much better and are still transmissible," Chagla said. 

Dr. Joanne Langley, a pediatric infectious disease specialist in Halifax who co-leads Canada's vaccine task force, said companies are planning trials in children and in pregnant women, which are expected to start in this country in 2021. International trials are also expanding to include children under the age of 16, vaccine makers say.

Do we know if the Pfizer-BioNTech vaccine is safe for pregnant women, breastfeeding women and children?

So far, there isn't any specific data on pregnant and breastfeeding women, so vaccine makers can't say for sure whether the vaccine is safe for that group or not.

However, Dr. Michael Gardam, a Toronto infectious disease physician who is the senior medical advisor for Health PEI, said: "There's nothing about this vaccine that would suggest that it would cause any problems in pregnant women." Pregnant women will likely have the option to receive the vaccine.

Biotechnology company Moderna protocol files for COVID-19 vaccinations sit in binders at the Research Centers of America in Hollywood, Florida, in August. The mRNA vaccines like PFizer-BioNtech and Moderna's don't use DNA. (Chandan Khanna/AFP via Getty Images)

Gardam recommends talking to your doctor about it.

As for children, pediatric trials are just beginning. Both Pfizer-BioNTech and Moderna are starting to test children between 12 and 17 this year, and younger children next year. This is necessary before giving the vaccine to children because children often need a slightly different dose or formulation than adults because they have more active immune systems and may show stronger immune reactions.

Have a question? 

Send your questions about this and other COVID-19 vaccine candidates to

With files from CBC's Cheryl Brown


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