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The COVID-19 vaccines approved in Canada so far (Pfizer-BioNTech, Moderna, AstraZeneca and Johnson & Johnson) are safe, effective and widely available. Here’s what you need to know about vaccine ‘hangovers,’ getting the second shot, mix-and-matching vaccines, optimal timing for immunity and more

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People get their shots at the CAA Centre in Brampton, Ont., one of the largest COVID-19 vaccination centres in Canada.Nathan Denette/The Canadian Press

Am I eligible for a first or second dose? Check provincial rules here.

Getting the second COVID-19 vaccine shot

If I received a first shot of the Pfizer-BioNTech product, can I switch to Moderna for my second? Or vice versa?

From a logistical perspective, that depends on availability and policy in your home province. You might not have a choice. But from a scientific perspective, experts agree it shouldn’t be a problem to mix-and-match COVID-19 vaccines that use the same platform, as the Pfizer and Moderna shots do. Both use messenger RNA technology to trick immune cells into displaying a piece of the coronavirus’s telltale spike protein, spurring the immune system to make antibodies against SARS-CoV-2, the virus that causes COVID-19.

The Pfizer and Moderna shots have performed almost identically in clinical trials and real-world studies. “They are based on the same technology; they have very, very similar spike segments [and] they trigger the body to do the exact same thing,” said Zain Chagla, an infectious-disease physician at McMaster University.

In guidance published June 1, Canada’s National Advisory Committee on Immunization (NACI) said it’s best to offer the same mRNA product twice, if possible, but “when the same mRNA COVID-19 vaccine product is not readily available, or is unknown, another mRNA COVID-19 vaccine product recommended for use in that age group can be considered interchangeable and should be offered to complete the vaccine series.”

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Prime Minister Justin Trudeau is inoculated with AstraZeneca's vaccine at an Ottawa pharmacy on April 23.Blair Gable/Reuters

What if I took the Oxford-AstraZeneca vaccine first? What should I do for my second dose?

On June 17, the National Advisory Committee on Immunization said it is now recommending people who got the Oxford-AstraZeneca vaccine first should get Pfizer-BioNTech or Moderna for their second shot.

The guidance is based on growing evidence that a second dose of an mRNA vaccine produces a stronger immune response, and because of the risk of a rare but sometimes fatal blood-clotting disorder called vaccine-induced thrombotic thrombocytopenia, or VITT, associated with getting AstraZeneca. The advice doesn’t mean people can’t still choose AstraZeneca if they want, or if they are allergic to the mRNA vaccines.

Provincial governments stopped offering AZ, a viral-vector vaccine, as a first dose because of the risk of VITT.

VITT risk falls – but doesn’t disappear – after a second dose of AZ, so you may want to factor that into your decision. The risk is 1 in 55,000 after a first dose, according to Ontario’s COVID-19 Science Advisory Table and about 1 in 600,000 after a second dose, according to the most recent British data.

When it comes to effectiveness, two shots of AZ don’t work as well as two mRNA shots at preventing symptomatic cases of COVID-19, but the brands appear to be near equals in forestalling hospital admissions, even in cases involving the Delta variant. Less is known about the effectiveness of mixing an AZ dose with an mRNA shot, but preliminary studies from Spain and Germany that measured antibody levels in the blood suggest a mixed regimen might actually work better than giving the same vaccine twice.

Will I have more side effects from the second dose?

That’s difficult to predict. People’s immune responses to the vaccines depend on their individual genetic differences, so it’s hard to say whether your side effects will be more or less severe after the second dose, said Jorg Fritz, associate professor in the department of microbiology and immunology at McGill University.

Moreover, how people respond has more to do with their genes than based on which vaccine they receive, Dr. Fritz said.

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An illustration of how cells respond to the AstraZeneca vaccine by learning to produce spikes of protein that the immune system can learn to recognize and fight if the real virus appears.University of Southampton/via REUTERS

What does it mean if my second dose makes me feel worse than my first?

The reaction to vaccines made by Pfizer and Moderna, the two most widely administered mRNA shots in Canada, is often more pronounced after a second dose.

“Feeling poorly for a couple of days is assurance that the vaccine has worked,” said Cecile King, an immunologist and associate professor at the University of New South Wales in Australia. “The effect is temporary and not dangerous.”

By the second dose, the immune system already knows what a COVID-19 spike protein (the virus’s calling card) looks like, according to Dr. King. With many cells already activated, the effect tends to be greater.

Another player is a class of molecules called interferons, which are unleashed naturally after vaccination and help prime the immune system to form long-term immunity to the virus. Interferons trigger a host of other responses that explain many of the reactions people experience after receiving a vaccine.

“Am I surprised that people are experiencing these symptoms following an mRNA vaccine? Not at all. That’s the interferon response,” said Eleanor Fish, a University of Toronto immunologist who specializes in the action of interferons.

If I’m a male 16-24, should I get a second mRNA vaccine?

Experts agree it’s increasingly likely that heart inflammation is a rare side effect of the mRNA vaccines produced by Pfizer or Moderna, after the U.S. and Israel reported higher cases among young men after a second dose. But what that means for young men considering a second dose is a “tough issue,” David Juurlink, a drug-safety researcher and head of the division of clinical pharmacology at the University of Toronto.

In Canada, the number of reported cases of myocarditis (an inflammation of the heart muscle) and pericarditis (an inflammation of the membrane around the heart) after vaccination has been small, according to Health Canada, and it is not yet known whether these cases are related to the vaccines.

In an e-mail, Health Canada said no regulatory action has been taken in relation to the issue, and the agency encourages people to get immunized with any of the COVID-19 vaccines that are authorized and available in Canada.

Nevertheless, Dr. Juurlink said he is inclined to wait for more data before proceeding with second doses for his own children. “I think we’ll know more a few weeks from now,” he said. Meanwhile, he said, priority for second doses should be given to people in older age groups who are at higher-risk of COVID-19.

However, Anna Banerji, a pediatric infectious-disease expert and associate professor at the University of Toronto’s faculty of medicine, said she still believes it’s best to get young people fully vaccinated. Even though parents and teens should be aware these cases seem to be tied to the vaccines, she said, they should also consider “the risk of getting severe illness with COVID and the repercussions of having a kid spread it to their family.”

What’s the best timing for immunity?

While vaccine manufacturers had initially recommended a 21- to 28-day gap between doses, the National Advisory Committee on Immunization (NACI) extended the recommended interval between doses to 12 weeks in March to ensure as many people could be partially immunized as possible, given a limited supply of vaccines.

Now that vaccine scarcity is no longer such a problem, provinces are allowing people to receive their second doses earlier. In Manitoba, for example, people are asked to wait at least 28 days before booking an appointment for their second dose. (The province recommends – but does not require – those who received an AstraZeneca vaccine, or its India-manufactured counterpart Covishield, for their first dose to wait at least eight weeks before a second dose of any vaccine.)

Although it is very clear people should not receive their second doses any earlier than three to four weeks, it is much less clear what the optimal timing is for vaccine efficacy beyond that period, Dr. Fritz said.

“There are no big studies where that has been looked at across age ranges,” he said, nor have there been any large studies on the impact of dose intervals on how long a strong immune response lasts.

On one point, there is unanimous agreement, he said: It’s important to get a second dose.

In February, AstraZeneca and co-inventor Oxford University released data on their vaccine, showing greater efficacy when a second dose was given after 12 weeks compared with less than six weeks.

But since none of the vaccines give sufficient protection against the Delta variant after only a single dose, it’s best not to wait, said pharmacologist Sabina Vohra-Miller, founder of the Unambiguous Science website and co-author of a new guide for people who received a first dose of the AstraZeneca vaccine.

Getting a second dose quickly “ends up being more advantageous, even if the efficacy may not be at the most optimal timing for intervals,” she said.

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Montrealers wait in line to receive a COVID-19 vaccine.Graham Hughes/The Canadian Press

How do COVID-19 vaccines work?

Which COVID-19 vaccines are available in Canada?

So far, Canada’s health regulators have authorized four vaccines for fighting COVID-19: Most of the shots now being administered are the Pfizer and Moderna products, which work by delivering genetic information that is taken up by cells and used to manufacture coronavirus proteins. The proteins, in turn, teach the immune system to recognize and attack the real virus when it shows up.

Some people have received the AstraZeneca product for their first shot and may get it again for their second, but provinces have largely stopped offering that product for the first shot due to concerns about VITT, a rare blood-clotting disorder linked to the drug in a handful of cases.

Johnson & Johnson, the only vaccine that requires just one dose, has not been cleared for distribution. Health officials have been investigating safety concerns about how it’s manufactured.

Do COVID-19 vaccines stop transmission of the virus?

On a public scale, vaccines do a great job at slowing the spread of SARS-CoV-2, the virus that causes COVID-19. They’re also really effective at keeping people from getting so sick that they end up in hospital or die. But there’s a chance that even a vaccinated person may have a mild or asymptomatic form of the virus and spread it to others, particularly the unvaccinated. That’s why it’s important to keep protective measures such as physical distancing and masks in place until enough of the population is vaccinated.

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Empty vials of Moderna lie in a kidney dish at a rest stop near Drayton, N.D., where Manitoba-based truckers were vaccinated.Dan Koeck/Reuters

What’s in the COVID-19 vaccines?

Health Canada’s list of approved vaccines outlines their ingredients and possible allergic reactions some people might have. Note the difference between the medical ingredients (substances that make a drug do its essential job) and non-medical ingredients that might be used to preserve it or improve its effects. In the Pfizer and Moderna products, the medical ingredient is mRNA; in the others, it’s a relatively harmless virus called an adenovirus, which triggers a response from the immune system.

Will I get a ‘vaccine hangover’?

Maybe, but maybe not. Many people do experience a “hangover” of fevers, chills, fatigue and soreness after they receive a vaccine, but researchers are still figuring out why it happens to some people and not others. Younger people and women appear to be somewhat more likely to develop hangovers. It is not, as commonly believed, a sign of how well the vaccine is working; one doesn’t need to experience a reaction for the vaccine to work. If you do end up with a hangover, physicians say it’s safe to take over-the-counter pain medication to treat it.

I’ve heard COVID-19 vaccines affect fertility. Is that true?

No. That’s a myth that spread last year when some social-media users alleged that the Pfizer vaccine could interfere with syncytin-1, a protein involved in placenta development, because it resembles SARS-CoV-2′s protein spikes. The idea that the vaccine could latch on to syncytin-1 is “completely a conspiracy theory,” Supriya Sharma, Health Canada’s chief medical adviser, told The Globe and Mail in late 2020. “There’s no evidence that it is similar enough to even be remotely in the realm of possibility.”

How long will immunity to COVID-19 last after vaccination?

A team led by Tania Watts, an immunologist at the University of Toronto, has focused on the T cell response to COVID-19 to see how long the memory of infection is retained. A paper published by Watts’ team now shows that T cell memory of COVID-19 can endure for at least nine months. (T cells perform a variety of tasks, such as killing off virus-infected cells and alerting other parts of the immune system to begin making antibodies.)

While it is too early to be certain, the persistent effect suggests that immunity to the coronavirus will last longer than for seasonal coronaviruses, which account for a portion of common colds, and which can reinfect people after a relatively short time.

Will I need a third dose or annual dose?

Third shots could become necessary for the elderly and people with HIV, certain cancers and autoimmune disorders. Small studies from the U.S. and France hint at some benefits, but the scientific jury is still out on whether a third shot makes a significant difference for transplant recipients on anti-rejection drugs – the group that appears least likely to derive protection from COVID-19 vaccines.

Researchers at Toronto’s University Health Network are hoping to provide some clarity with what is believed to be the world’s first randomized controlled trial offering third doses to transplant recipients. Results are expected in a few weeks.

For now, the National Advisory Committee on Immunization is waiting for more evidence before it considers recommending third doses for any immunocompromised populations, said Mark Johnson, a spokesman for the Public Health Agency of Canada.

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People line up at a walk-in clinic in Montreal to get the AstraZeneca shot on April 21, when Quebec's age cutoff for the drug was lowered to 45.Paul Chiasson/The Canadian Press

AstraZeneca and blood clots

If I get the AstraZeneca vaccine, how will I know if I develop a blood clot?

Based on Heath Canada figures and international reports, the chance of developing VITT after a first dose of the AstraZeneca vaccine is extremely low – approximately 1 in 50,000. However, the condition can be serious, particularly if not identified and treated promptly.

Blood clots may develop between three and 34 days after vaccination. Symptoms include severe, persistent headache, blurred vision, seizures or difficulty speaking, shortness of breath, severe pain in the chest, abdomen, limbs or back, swelling, colour change or coldness in the arms and legs and small bruises or blood blisters under the skin. Anyone experiencing one or more of these symptoms after vaccination should seek medical attention and be tested for VITT.

Vaccines and children

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JJ Fixman, 14, gets a first shot from Stara Sinanan at a Montreal clinic on May 22.Graham Hughes/The Canadian Press

Are COVID-19 vaccines safe for children?

It’s safe for children as young as 12 to get some types of COVID-19 vaccines. Others may be approved later, but first researchers need to finish their studies to verify they’re safe and effective in younger children. The more pressing question is not safety but supply: Since children are at such low risk of severe COVID-19 symptoms, medical experts disagree on whether it’s better to reserve doses for Canadian adults who haven’t yet received their second shot or people in developing countries who have yet to receive their first.

Is there a preferred vaccine for kids?

In Canada, Pfizer’s is the only product approved for 12-to-17-year-olds, and vaccines for children under the age of 12 are not expected until mid-autumn at the earliest.

Is it safe for my unvaccinated kids to see their fully vaccinated grandparents?

There’s no national guidance about that in Canada, but it’s something other countries have addressed directly. In the United States, the Centers for Disease Control and Prevention says fully vaccinated people can safely visit with each other, and even with some unvaccinated people, as long as they’re not at risk of severe COVID-19, which children would not be as long as they don’t have immune disorders or other illnesses. Canada could borrow guidance like that, Isaac Bogoch, a Toronto physician on Ontario’s vaccine task force, told The Globe. “That’s about as safe as it’s going to get in this era, so I think we could at least be mindful of what the data shows and create public-health guidance.”

Are indoor, unmasked playdates safe if all the parents and caregivers are vaccinated?

The Public Health Agency of Canada hasn’t issued any clear guidance on what Canadians can do after they are fully vaccinated, which is defined as two weeks past a second dose. Unvaccinated children can still contract and transmit COVID-19, which means families can’t drop their guard right away, especially if adults have received only a single dose, says Nisha Thampi, a pediatric infectious disease physician at CHEO, a children’s hospital in Ottawa.

It also depends on the rate of infections in the community; if rates are low and falling, vaccinated parents should feel comfortable letting their children take part in indoor playdates and small group activities and sports, says Srinivas Murthy, a critical care and infectious disease physician at BC Children’s Hospital.

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Pregnant women wait to get a dose of the Chinese-made Sinopharm vaccine in Colombo, Sri Lanka's largest city, on June 9.LAKRUWAN WANNIARACHCHI/AFP via Getty Images

Vaccines, pregnancy and at-risk medical groups

Is the vaccine recommended for pregnant women?

Canada’s health agencies urge pregnant Canadians to get their shots as soon as possible, which is a big shift from late 2020, when they said there wasn’t enough information to recommend it. Preliminary monitoring data from the United States has shown no increased risks to pregnant people from the vaccine, and at the same time, some mothers-to-be have contracted COVID-19 and faced the more serious risks of hospitalization or death. As a result, new mothers have been urging others in Canada to be vaccinated.

Post-vaccination life

Ontario’s staged reopening started Friday with patios busy with diners enjoying eating together but with extra space between tables. Restaurants say they hope this is the start of continued reopening for the province.

The Globe and Mail

Do I still have to wear a mask if I’ve received two doses of the vaccine?

Some provinces already plan to allow people to go maskless in certain settings.

As of June 25, Quebec will relax the rules for fully vaccinated people; it has already exempted students from wearing masks in class. In Alberta, Premier Jason Kenney announced on June 18 that the province will be lifting its province-wide masking mandate - along with virtually all its public health measures - on July 1, though some communities may still require them. Saskatchewan, moving ahead similarly, will be lifting all its public health restrictions on July 11.

Each province is taking its own approach and pressing federal health officials to provide clear national guidance for fully vaccinated people.

Keeping workers safe from COVID-19

What measures are effective at keeping workers safe from COVID-19?

The U.S. Centers for Disease Control and Prevention pegs the risk of transmission from contact with a contaminated surface as “low,” at less than one in 10,000. Riskier environments are spaces with poor ventilation, where aerosols can build up, scientists say. Workplaces should focus on improving ventilation as a way of mitigating against airborne COVID-19 transmission, commentary in the Canadian Medical Association Journal said in early June.

Workplaces vary, and solutions to improve air quality differ. But where and when possible, windows and doors should be opened regularly. In cases where it’s hard to improve ventilation, and after consultation with an expert, HEPA filters can lower the concentration of some viruses in the air. Move outside when possible, and adjust existing systems to allow more fresh air intakes and to minimize recirculation. Workplaces can stop investing in Plexiglas barriers, which do not prevent aerosols from wafting over and under them, experts say.

Can my workplace mandate the COVID-19 vaccine?

Whether employers can require vaccination is likely going to be industry and context-specific.

Employment lawyers say workplaces that carry a high risk of transmission of COVID-19, such as long-term care facilities, have stronger cases for a vaccine requirement than low-risk work settings. Few are currently making it a requirement.

“I think there will be workplaces that will be able to justify [a mandatory vaccination policy] if it’s challenged, but for all employers to assume they can do it I think is a mistake,” said Dan Bokenfohr, a partner in the labour and employment practice group for McLennan Ross LLP.

Instead of mandating the vaccine, workplaces may implement non-punitive measures to encourage uptake. One Toronto accounting firm is planning on allowing vaccinated employees back into the office and forcing unvaccinated employees to continue working remotely. In Ontario, LTC workers must provide proof they’ve been vaccinated or offer a documented medical reason for not being vaccinated. If they’re unable to do either, they are required to take part in an educational program on the benefits of getting vaccinated and the risks of abstaining from immunization.

Employees can also refuse to disclose their vaccination status, creating a tricky balance between employee privacy rights and workplace safety.

“There’s not going to be any black-and-white answers, I don’t think, at least not for a while,” said Melanie McNaught, a partner at Filion Wakely Thorup Angeletti LLP.

Will Canada adopt a vaccine passport?

The Canadian government is deferring to the provinces to decide whether to implement proof-of-vaccination systems. Ontario is the latest province to reject the idea of a domestic COVID-19 vaccine passport.

Manitoba has been issuing proof-of-immunization cards to residents since June, while Quebec says it will use vaccine passports starting in September, but only in communities or non-essential sectors experiencing outbreaks, such as gyms, bars and spectator events. If cases in the province remain low, the passports will not be required.

Alberta and Saskatchewan, however, have turned down the idea of a vaccine passport, citing privacy laws, and B.C. Premier John Horgan and Provincial Health Officer Bonnie Henry have both said they aren’t supportive of such passports to access services.

Have a vaccine question you need answered? Email

More reading

How many COVID-19 cases are there in Canada and worldwide? Latest maps and charts

Canada vaccine tracker: How many COVID-19 doses have been administered so far?

When do COVID-19 restrictions ease in my province? The latest rules across Canada

Compiled by Globe staff

With reports from Kelly Grant, Ivan Semeniuk, Andrea Woo, Marieke Walsh, Wency Leung, Tavia Grant, Paul Taylor, Chantelle Lee and The Canadian Press

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