TORONTO -- The coronavirus vaccine rollout has given some hope for an end to the pandemic, especially as Canada ramps up its immunization efforts, but much remains unclear as new variants spread.

The Pfizer-BioNTech vaccine was the first to circulate​ among priority groups in 2020, followed by the Moderna shot in late December. Health Canada approved a third candidate, the Oxford-AstraZeneca vaccine, on Feb. 26, and approved a one-shot vaccine from Johnson & Johnson’s Janssen Pharmaceuticals on March 5. A fifth vaccine by Novavax is still under review. Whether the vaccines will mean a return to normal remains to be seen. Meanwhile, the latest federal modelling data suggests variants of the coronavirus could spark a third wave long before most of the country is vaccinated. readers submitted their burning questions about COVID-19 immunization and we posed them to infectious disease experts Drs. Isaac Bogoch, Zain Chagla, Sumon Chakrabarti, Anna Banerji, Marla Shapiro, Hassan Masri, and Amy Tan. Here’s what they said. Note: Questions have been edited and condensed for clarity.

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It is free. The federal government expects to be able to offer free vaccination to all Canadians. Health officials have procured the most doses per capita for Canadians than any other country, but are also committed to supplying vaccines to low- and middle-income countries who weren’t able to order doses.

The Canadian government assembled a team of experts and officials months ago to prepare for vaccine rollout, including logistics and prioritization. From an operations standpoint, a Canadian military general is heading the rollout. From a prioritization standpoint, the National Advisory Committee on Immunization (NACI), a long-established body that includes public health, infectious disease, and immunization policy experts, created a vaccine prioritization framework, released in early November. The framework determined that people and staff at long-term care facilities should be immunized first, then other health care workers and vulnerable populations, and finally the general population. For more details, including whether you can choose which vaccine to get and who gets priority, please read our explainer, Your COVID-19 vaccine rollout questions, answered.

It’s possible that vaccination could be required in certain education, health care and travel circumstances, said Chagla, and there is precedent for it.

In parts of South America and Africa, travellers are required to present a “yellow fever vaccination certificate” to prove they are immune from the virus and won’t set off an outbreak. In Canadian schools, some provinces require children and adolescents to be immunized from a variety of illnesses before they can attend school, unless they have a medical exemption.

“I think you’re going to see some fairly forward-facing industries like travel industries, health care, schools, where this is probably going to be part of the mandate,” said Chagla, noting that mandatory vaccinations get “dicey” when it comes to employment law.

While no one can be forced to get an eventual COVID-19 vaccine, ethicists have urged for more clarity around what the rollout of vaccines will mean, including for millions of people who may choose not to get one.

“That is an ethical concern because what will likely happen to people within that group is more and more opportunities may slowly be shut off to them,” Kerry Bowman, a bioethicist and assistant professor at the University of Toronto’s faculty of medicine, told CTV National News.

Common side-effects will have already been identified since most adverse events associated with vaccines occur within a day of receiving the shot, said Bogoch, the most common of which is a sore arm.

“There are certain rare vaccine side effects that can occur two to four weeks after vaccination. They’re not very common at all,” he said, adding that early data suggests the Pfizer and Moderna vaccine candidates, which use a relatively new mRNA technology, have not resulted in any notable side effects other than people feeling “fatigued or a little unwell” in the day following vaccination.

The AstraZeneca and Janssen (Johnson & Johnson) vaccines are viral vector-based, and the side effects seen during clinical trials were similar to other types of vaccines.

A number of European countries did pause their use of the AstraZeneca vaccine amid reports of blood clots. Health Canada said there was no indication that the vaccine was the cause and that none of the identified batches under investigation in Europe were shipped to Canada. AstraZeneca Canada said the company completed a new safety review of 10 million patients who received the vaccine and uncovered “no evidence of an increased risk … in any defined age group, gender, batch or in any particular country.”

Chagla said he trusts that Health Canada, which is constantly monitoring safety reports, would step in if there was any concern.

No. The vaccine is preventative, not therapeutic. Someone with COVID-19 should remain isolated until full recovery and later seek vaccination. “The recommendation is typically about 90 days or longer after an onset of the coronavirus to get the vaccine to offer longer-term immunity,” Shapiro told CTV News Channel. “It would act as a typical vaccine does for future forward immunity.”

No. The federal and provincial governments are not implementing widespread routine screening before vaccinating individuals. “That would not be cost-effective,” Shapiro told CTV News Channel. Instead, health officials developed a prioritization framework to determine who gets vaccinated first. “You would be getting the vaccine regardless, as long as you’re [part of the group] that’s stratified.”

Yes. While research shows that COVID-19 antibodies are often detectable several months after infection, there have been several reports of reinfection.

“We know that those credible cases of reinfection are pretty much the tip of the iceberg,” Bogoch told CTV’s Your Morning. “It’s probably happening a lot more frequently than we would think.”

There has been no evidence of “sterilizing immunity,” adds Shapiro, meaning we can’t say if anyone infected with COVID-19 will be “immune for life.” Whether or not you have COVID-19 antibodies from past infection, the vaccine is still recommended, she told CTV News Channel.

It’s likely that the majority of people with allergies will be able to receive the Pfizer-BioNTech vaccine and others, but experts stress that Canadians should go over the list of ingredients with their physician. On Dec. 12, Health Canada issued a notice that people with severe allergies to the ingredients in the Pfizer product should avoid the shot altogether.

“If you’re allergic to any of those ingredients you shouldn’t be getting this vaccine and luckily we have other vaccines that are in the pipeline and are hopefully going to be available in Canada soon,” Bogoch told CTV’s Your Morning.

Yes. Guillain Barre syndrome is an uncommon condition in which someone’s immune system attacks their nerves and can result in weakness, tingling and paralysis. “It’s a rare event. Most of the time people don’t know why Guillain-Barre happens, but it’s not something that usually happens on a repetitive basis,” said Banerji. "Someone who's had Guillain-Barre, if they have any underlying conditions, [those conditions] may make them at risk for severe COVID-19 infection. If someone's at risk of severe disease from the virus, they should get the vaccine."

The messenger RNA vaccine by Pfizer-BioNTech does not contain preservatives. Mercury-based preservatives like thimerosal, which researchers have shown is safe in low doses and does not cause autism, have not been used in the majority of vaccines since the late 90s.

The Pfizer and Moderna vaccines are both “messenger RNA” or mRNA vaccines, which is a new kind of vaccine technology that is like “giving an instruction booklet for your cells to make antigens,” said Chakrabarti in a phone interview with last month. In the absence of the coronavirus, a person’s cells would still be programmed to develop the antibodies that fight the virus off.

The AstraZeneca and Janssen vaccines use a more well-established approach that utilizes a harmless virus like the ones that can cause a common cold as a delivery system. The vaccine alters the virus, called a modified adenovirus vector, to mimic COVID-19 and thereby produce an immune response in the human body without making you sick.

Probably not. While study participants represented a wide spectrum of ages and ethnicities, they were also all generally healthy people, said Chagla in a phone interview with in November. In the past, when vaccines for other illnesses rolled out, they may have had strong data from clinical trials too. “But they’re not necessarily as good as patients get more complex, as their medications get more complex and their immune system diseases are more complex,” he said.

Nonetheless, a real-world study published in February that looked at the Pfizer vaccine’s efficacy in more than 500,000 recipients found it 62 per cent effective at preventing disease after one shot and 92 per cent after both shots were administered. This was better than what many were expecting. Data from other trials, however, have demonstrated “miraculous” protection of at least 90 per cent just from the first dose.

In clinical trials, Pfizer was 95 per cent effective one week after the second dose in a study of about 44,000 participants; Moderna was 94.1 per cent effective two weeks after the second dose in studies of 30,000 participants. AstraZeneca was about 62 per cent effective two weeks after the second dose, while Janssen’s was 66 per cent effective two weeks after its single-dose vaccination.

It’s too early to know. The Pfizer and Moderna vaccines require two doses, separated by two weeks, but the second is not considered a “booster” shot, which are given to prevent immunity from waning.

“We don’t have a clue how long immunity will last,” said Bogoch over the phone with in November. “We’ll have clues about this longer-term after people are vaccinated to see how long immunity will last.”

Booster shots are typically given several years after the first, though children will often receive them earlier to provide a more “robust immune response,” said Chagla. “For most adult vaccinations you have a series [of doses], and things like the pneumonia shot or tetanus shot you do at five, 10-year intervals after getting it just knowing that immunity wanes in some of the older populations,” he said.

As with any vaccine that requires multiple doses, the first dose exposes the body to the COVID-19 virus to kickstart an immune response. The second dose “gets the engine running quicker,” infectious disease specialist Anna Banerji told “The second time the body sees the virus it responds quicker because the antibodies are there and you can have a much more immediate response.”

It is still possible to contract COVID-19 after the first or second dose, but data from leading vaccines trials suggest they are effective in preventing infection.

The vaccines all have different requirements for transportation and storage. Pfizer’s vaccine needs to be kept at -70 C during those stages to remain effective, but it is not administered into someone’s veins at that temperature, assured Bogoch.

“God, that would suck. You’d get frostbite. That would cause tremendous damage,” he said. “The Pfizer vaccine is stable for about five days in a conventional refrigerator. The Moderna vaccine needs to be stored at -20 C but it’s stable in a refrigerator for about 30 days.”

The AstraZeneca and Janssen vaccines can be stored between 2 C and 8 C, which experts consider a major logistical advantage for areas of the country that don’t have access to ultra-cold freezers.

There are a variety of methods by which people can be vaccinated. Where someone lives may determine which vaccine they receive by which method.

“It’s going to depend on what vaccine is locally available, what resources are in that region to actually administer them,” said Chagla, noting the different storage temperatures required of the vaccines.

Each province and territory has its own rollout plan, though in general, vaccines are by appointment and based on prioritized groups. Some pharmacies are also helping with the vaccination process. Check your provincial or territorial website for more information.

If someone is vaccinated and they come into contact with the virus, one of two things is going to happen, said Bogoch: “You’re either going to get the infection, or you’re not.”

Early data from COVID-19 vaccine trials suggests that the probability of getting the infection is significantly lower in vaccinated people than in unvaccinated people. Similarly, the early data suggests severe illness is also less common in people who are vaccinated.

“It’s extremely important to note that we don’t have the data available, and we're only going by the snippets of data that we do have available,” he added.

The National Advisory Committee on Immunization strongly recommends that everyone continues to practise all public health measures regardless of vaccination, because there is currently not enough data regarding the effectiveness of COVID-19 vaccines in preventing asymptomatic infection and reducing transmission of SARS-CoV-2, or the duration of the vaccine protection. There is also emerging evidence on the decreased efficacy of the vaccines against variants of concern.

Yes. While a minority of people get sick and die from COVID-19, that small group of people can still overwhelm the hospital system, said Chakrabarti.

“Once that happens, there’s a ripple effect that has effects on everybody’s health,” he said, noting cancer screening appointments are cancelled and elective operations are postponed. This past winter, some of the largest hospitals in Canada were nearing capacity, threatening widespread cancellations of non-emergency surgeries that would further extend a health care backlog.

While some have suggested protecting the elderly and letting the virus “rip” through society to allow for natural herd immunity, Chakrabarti says it’s very difficult to protect all vulnerable people and the consequences of such a strategy can’t be understated.

“The cost of doing that is that you would overrun hospitals and there would be all sorts of collateral damage,” he said.

Likely yes. Since the Pfizer-BioNTech and Moderna vaccines are not “live-virus” vaccines, there is no concern that a suppressed immune system will get COVID-19 from these shots, said Shapiro. “The concern is that they may not mount as large an immune response, but there is no reason to think that they will not be able to be given the vaccine,” she told CTV News Channel. “Looking at previous histories of this population, we worry about [them] getting sicker with the disease so usually they're a population we like to immunize.”

Maybe, but it’s unlikely that individual people will get each of the Pfizer, Moderna, AstraZeneca, and Johnson & Johnson vaccines, or some other combination of vaccines, during the first rollout, said Chakrabarti.

“This stuff will be honed in the coming years as we start to get to know more and more. There is a precedent for using vaccines of slightly different mechanisms to give you a synergistic type of effect,” he said, noting that high-risk populations are inoculated with both pneumonia vaccines on the market for a more powerful affect.

There’s also added benefit to having multiple vaccines on the market, whether or not an individual person receives multiple shots. The differences in the storage requirements alone provide important benefits, particularly to the developing world, noted Chagla.

“Right now it’s just a race to get a vaccine on the market rather than necessarily saying ‘What’s the optimal strategy?’” he said. “As things go on, we’re probably going to get more and more data to say which one is better in kids, which one is better in the elderly, which one is better in certain settings, or with certain medications. That’s the benefit fo having multiple on the market.”

While some experts expect a feeling of “normal” to return at some point next year, a complete return to normal might not come until 2022.

“The rollout of the vaccine is a gargantuan task,” said Chakrabarti. “I do completely expect that things will be back to normal sometime in 2022. I think 2021 will be a gradual and stepwise improvement in our daily lives and the case count.”

Chagla expects people will begin to “step off the brake” once the most vulnerable populations are vaccinated in the early months of 2021. Summer could be close to normal, he added. “We’re going to see ‘normal’ starting to show up more and more and more as some of those higher-risk settings get more and more vaccinated,” he said.

Yes. Until more of the population has been vaccinated, it’s likely that mask mandates will remain in place across much of Canada. While the COVID-19 vaccines being distributed have proven to be very effective at preventing infection, it’s still possible that inoculated individuals could be asymptomatic and unknowingly spread the disease. “There’s nothing there [in Pfizer’s data] that suggests that people are less transmissible,” Chagla told in December. “All we know is individuals who got the vaccine had less symptomatic disease than people who didn’t get the vaccine.”

The National Advisory Committee on Immunization strongly recommends that everyone continues to practise all public health measures since there is not enough data yet on how effective the vaccines are at preventing transmissions. There is also emerging evidence on the decreased efficacy of the vaccines against variants of concern.

The approach to distributing the two-dose vaccines (Pfizer, Moderna, AstraZeneca) varies, but a number of jurisdictions across Canada began delaying administering second doses. This came after the National Advisory Committee on Immunization (NACI) recommended that the interval between the first and second shot be extended by up to four months when vaccine supplies are limited. The new advice is based on early real world data which showed sustained high levels of protection. In Quebec, for example, officials decided from the beginning to use all available vaccine supplies to provide more people with their first dose. Ontario switched its strategy to follow NACI’s recommendation and after the province was criticized for its slow rollout.

The COVID-19 vaccines currently in rotation have not been tested in children and most teens. Last month, Pfizer-BioNTech released more data from its clinical trials in which there were no people younger than 16 among all 43,448 participants. "They can't really say this vaccine is recommended for an age group that hasn't been tested yet," Dr. Anna Banerji, an infectious diseases pediatrician and associate professor of pediatrics at the University of Toronto, told in a phone interview last month.

The timeline for children becoming vaccinated in Canada depends on the results of clinical trials, availability of the vaccine and each province’s rollout of the drug, but targeting the start of the 2021-22 school year seems appropriate, according to the experts. Testing for the 12 to 15 age group are well under way outside Canada, so we may see a phased rollout for children as the clinical trial results come in. In Canada, authorities have approved a clinical trial for the Janssen vaccine in children ages 12 to 17.

None of the COVID-19 vaccines currently being administered in Canada are "live virus" vaccines, meaning they can't cause infection even in people with compromised immune systems. Other vaccines that use live virus, such as the yellow fever shot or MMR shot, aren't given to people with compromised immune systems because they could get infected. "But if you have a normal immune system, your body can mount an immune response," said Dr. Chakrabarti.

There are several scenarios that put people at risk of severe disease from COVID-19, including having diabetes, high blood pressure, a transplant, and undergoing chemotherapy treatment. But elderly people are prioritized above all because data still suggests they are most at risk. "If you take the entire pile of risk factors, the one that stands out by far is age," said Dr. Chakrabarti. "A completely healthy 80-year-old is much more likely to get severe illness from COVID-19 than a 40-year-old who's on immunosuppressive medication."

The initial vaccines were made to target the original SARS-CoV-2 virus. The development of more variants depends on how well the virus is under control, said Dr. Hassan Masri, an ICU physician and Associate Professor of Medicine at the University of Saskatchewan. "If you look at the variants, they are variants of places where the virus was very much out of control," he told CTV News Channel in February. "The chances of another variant coming alive is less likely to do with the vaccine, but more related to whether we will have excellent control of our numbers."

Thousands of people in Canada have received both already, as have likely millions in the U.S., U.K. and other countries. There are plenty of vaccines that people get within the same month, said Victoria, B.C., palliative care and family physician Dr. Amy Tan. "They're different mechanisms, they're different immune responses. Everything looks to be fine," she told CTV News Channel in February.

The Serum Institute of India (SII) vaccine COVISHIELD is a version of the AstraZeneca one. The company partnered with SII, the world’s largest vaccine manufacturer, to supply its vaccine to India and other countries under an exclusive sub-license. It was sponsored for use in Canada by Verity Pharmaceuticals, the Canadian importer and distributor. While they were approved as two separate vaccines, Health Canada said it reviewed the manufacturing information for both vaccines and found the manufacturing processes and controls and the quality characteristics of both to be comparable.

The 62 per cent efficacy is based on a four-week gap between the first and second dose, says Dr. Supriya Sharma, the chief medical adviser at Health Canada, adding that there is some data that suggests that rises to 85 per cent if the gap between doses is increased to 12 weeks. Sharma notes that the same vaccine can have multiple trials with different methods and varying results.

For now, it is recommended that the vaccine be used for those under 65, due to insufficient data on older recipients. There are also some concerns around its effectiveness against the B.1.351 variant that is predominant in South Africa. Federal bodies stress that all three vaccines can lower hospitalizations and deaths due to COVID-19 and getting the AstraZeneca shot early is better than waiting for Pfizer or Moderna, which could take longer to reach some groups.

For perspective, Sharkawy points to the efficacy of flu vaccines.

“A great match between a circulating flu strain and the vaccine in a given year might not exceed 60 per cent. If the flu vaccine is delivered widely in the community, we see dramatic reduction in every bad outcome,” he told CTV News Channel in February.

There are measures in place to make sure that your second dose is the same as your first. While the U.K. has changed its guidance to authorize mixing and matching COVID-19 vaccines in “extremely rare occasions” such as when the same vaccine is not available, Canadian experts caution against the practice due to concerns it may compromise the overall efficacy of the vaccine. It is unlikely to be dangerous, but more data is needed before implementing such an approach, Bogoch says.