Like many new parents, Meggan Larson was anxious to learn everything she could about keeping her kids healthy. So, she joined community groups and forums on Facebook, hoping to learn the ins and outs of healthy living as she navigated her new role as a parent.

What she found instead were piles of warnings about vaccines.

"It seemed like everywhere I looked, there was [a post about] another child being injured by vaccines — and not a child getting polio," she said. "It was definitely convincing."

As a result, Larson became "vaccine hesitant."

Unlike strident anti-vaccination activists known as "anti-vaxxers," those who are "vaccine hesitant" don't consider themselves against vaccination necessarily; they are just unsure about the safety and/or efficacy of vaccines.

It seemed like the more you clicked on links that people would post, the more stories you would see. And it just seemed so loud. - Meggan Larson

The number of vaccine-hesitant parents worldwide is growing. Last year, the World Health Organization named vaccine hesitancy a Top 10 threat to global health .

"You just kind of go down a rabbit hole of information," said Larson. "It seemed like the more you clicked on links that people would post, the more stories you would see. And it just seemed so loud."

WATCH | CBC Marketplace's Asha Tomlinson is joined by associate medical officer of health for Toronto Public Health Dr. Vinita Dubey, answering your questions on vaccinations and myths on YouTube.

About half of Canadians have some level of hesitancy, according to the Wellcome Global Monitor, a survey by a U.K.-based health foundation that looks at global confidence in immunization.

In its latest 2018 survey, only 53 per cent of Canadians chose "strongly agree" when presented with the statement "vaccines are safe" while 58 per cent strongly agreed with "vaccines are effective" and 76 per cent strongly agreed with "vaccines are important."

Tracking biases

Marketplace sifted through studies and arguments, examined anti-vaccination documentaries and attended an anti-vaxx rally in Washington, D.C., to get a sense of the information being distributed.

The team also spoke with more than a dozen vaccine-hesitant and anti-vaxx Canadians and heard from many more via email. Many of the arguments and scientific studies seemed reasonable, so Marketplace ran them by specialists.

WATCH | Why these sisters regret not vaccinating their children

Meggan Larson and her sister, Tara Hills, were vaccine hesitant until Hills's daughter contracted whooping cough, which quickly spread to her siblings and cousins. Now they want other parents to learn from their mistakes. 2:23

Journalists further consulted with toxicologists, vaccinologists, microbiologists, psychologists and academics throughout this investigation, all of whom confirmed the safety and efficacy of vaccines.

So, why does anti-vaxx messaging remain so effective at sowing doubt?

Dr. Olivier Drouin, a Montreal pediatrician, says that the reason why anti-vaxx messaging is so effective at sowing doubt is because of the way it manipulates our emotions and biases. (Olivier Drouin)

Some experts point to our cognitive biases, mental shortcuts that help us understand and remember information. The way that misinformation is disseminated on social media manipulates us — intentionally or not — into wanting to believe it.

Dr. Olivier Drouin, a Montreal pediatrician with a background in behavioural psychology, sees these biases at play with parents of his patients.

"They just care deeply about their child, and so those cognitive biases are amplified," he said.

Safer to do nothing

One bias that vaccine misinformation plays on is our omission bias, or our tendency to prefer inaction over action when we are faced with potential negative consequences, because we will feel less personally responsible for the outcome.

Larson says she can relate.

"For me, the worry is paralyzing initially because I don't want to hurt my kids," she said.

Larson said the misinformation she'd read was convincing, so she was delaying making a decision about vaccination until she felt more sure.

"My kids will be fine for now while I figure out what I'm going to do," she recalls thinking. "And the years just go by — and suddenly, you have a five-year-old who's never had a single vaccine."

The power of a personal story

Another bias that makes us susceptible to misinformation is our identifiable victim bias, or our tendency to feel more connected to the emotional story of a single victim, rather than a larger cause backed by statistics.

If a charity wants to inspire people to donate to its cause, for example, it might tell the story of a single victim rather than explaining why a cause is important through figures and facts.

Even though she'd seen the statistics proving the benefits of vaccines, Larson said the individual horror stories were more impactful and sowed doubt.

"I was like, 'Gosh, this is happening all the time, I can't do this to my children,'" she said.

Much of the anti-vaccination information spread online is anecdotal.

When parents come to Drouin with doubts about vaccines, he says, they often start with an anecdote they read or a video they saw.

"They always start with the story of somebody — either a celebrity, or some relative, or their hairdresser, or somebody in their environment who had a personal story — so they feel that connection," he said.

"I think that's one of the strongest biases I see when it comes to vaccinations."

Fear is more powerful than stats

Another big issue, Drouin says, is a bias called probability neglect, or our tendency to disregard numbers when there's a highly emotional potential outcome — even an extremely unlikely one.

"If I'm with my patient, and I speak about the risk of [requiring] a transfusion, for example, it's one in a million. Well, you know, they're not listening to the number," said Drouin.

His patients immediately start to worry about their child having to be in the ICU and being really sick, he said.

"The minute I mention the risk, it's forgotten," he said. "They're focusing on the vividness of the image it brings to mind."

The bigger the number, the harder it is to put into context, he says.

"What is one in a 100,000 versus a one in a million chance? Like, it's really kind of a strange number to think about," he said. "We don't even really have context about what it means, even though something might be 100 times more likely."

Larson says she struggled with the numbers even after she had decided to vaccinate.

"It was still scary because I'm like, 'Oh my gosh, I hope they're not the one in a gazillion who is going to have some horrific reaction," she said.

Questioning the rhetoric

So, what convinced Larson to vaccinate? She had started questioning the rhetoric behind the information she was reading and was talking to her doctor about it.

"I started clicking on the sources that anti-vaccine campaigns were linking [to] and realized that most of them were garbage," she said.

The scientific studies were not the hard evidence she thought they were.

"Once you actually followed it to the conclusion, it didn't actually state at all what [the messaging] said it did."

WATCH | Meggan Larson's children, along with her sister's kids, came down with whooping cough:

Meggan Larson and her sister Tara Hills were vaccine hesitant until one of their children contracted whooping cough, which quickly spread to siblings and cousins. 3:49

Just when she was about to book an appointment to vaccinate, Larson's unvaccinated niece came down with whooping cough — and it quickly spread to all five of her niece's siblings, as well as to Larson's three kids.

It was a scary reminder of how contagious many vaccine-preventable diseases are, she said.

"Coming across someone with whooping cough or polio, I mean, you're going to get it, it's a miracle if you don't."

All of Larson's kids fully recovered and have since been vaccinated, along with the children of her her sister, Tara Hills. Hills spoke out about vaccine misinformation back in 2015, but it has taken Larson four years to go public about her experience.

"The only reason that I wanted to share my story is because I don't want people to sit in fear anymore," she said.

Here are some of the most pervasive concerns Canadians have about vaccines, and why experts say we shouldn't worry about them:

1. Vaccine ingredients

It's true that vaccines contain chemicals that would be considered toxic at very high doses. But so does pretty much everything humans interact with daily. Even water is toxic in high enough quantities.

Although the idea of a "chemical" may sound harmful, everything we eat can be broken down into its chemical makeup. A banana, for example, contains dozens of chemical compounds.

The vaccine ingredients that most people are concerned about — mercury, formaldehyde and aluminum — are present in very small, non-toxic quantities.

Mercury was previously used as a preservative in some vaccines in the form of thimerosal. It was one of the first ingredients that parents took issue with, and there was so much public concern that it was removed from almost all vaccines, despite the fact that it never existed in quantities that would be considered problematic.

Vaccines only ever contained 50 micrograms of thimerosal (equal to 25 micrograms of mercury) per 0.5 millilitre dose. For comparison , that's about the same amount of elemental mercury in a three-ounce can of tuna. Nowadays, mercury only exists in certain types of flu shots.

Formaldehyde is another ingredient often of concern. There is only between 0.1 mg and 0.003 mg of formaldehyde in a single dose, depending on the vaccine. Compare that to a 200-gram pear, which contains 12 mg of formaldehyde, or 120 times as much. Formaldehyde is actually a chemical that your body creates itself as part of the metabolic process.

Aluminum is another ingredient people are concerned about. It is also found in very low quantities in vaccines for diseases such as HPV and hepatitis . Aluminum is one of the most common metals found in nature and is present in air, food and water and breast milk . Keep in mind that very few vaccinations contain aluminum, and those that do, only contain 0.85 mg per dose. Compare that to 10-20 mg of aluminum in one tablet of buffered aspirin. Antacids can contain anywhere from 104 to 208 mg of aluminum per tablet, or more than 100 times more aluminum than any vaccine. The reason it is added to vaccines is because it helps create the immune response needed to develop immunity.

There’s no reason to worry about the safety of vaccines, say experts, but misinformation distributed online by members of the anti-vaccination movement can often muddle the message for vaccine-hesitant parents. (itsmejust/Shutterstock)

Vaccines work by injecting a dead, weakened or partial version of the virus into the body so the immune system can learn what it looks like and how to fight it the next time it is encountered — before it gets deep within the body and causes physical harm.

Adjuvants , like aluminum, make that dead virus look more threatening than it is in order to activate an immune response, prompting the body to learn how to fight the disease instead of immediately flushing it out.

It is important to note that all traces of aluminum (and the rest of the ingredients in the vaccine) would still be flushed completely out of the body within a day or two, leaving behind only the lessons learned by your immune cells. The ingredients don't build up in the body, therefore, receiving many vaccines will not stack up the levels of aluminum or other ingredients.

Learn more about aluminum and other adjuvants here .

2. Vaccines and autism

The claim that links vaccines and autism originates from a now-discredited 1998 study published in the prestigious Lancet medical journal by former gastroenterologist Andrew Wakefield titled "Ileal-lymphoid-nodular hyperplasia, non-specific colitis and pervasive developmental disorder in children."

There were 13 authors listed on the study, with Wakefield being the lead author. But in 2004, 10 authors retracted the interpretation of the study after " serious allegations of research misconduct " were brought to the attention of The Lancet, including allegations of biased selection of patients and unethical, invasive procedures.

The journal also found Wakefield had unreported conflicts of interest, namely, that some of the research was funded by lawyers who were acting on behalf of parents suing vaccine manufacturers. As a result, Wakefield lost his medical licence.

Anti-vaxxers claim the MMR vaccine can cause autism, but all scientifically reputable studies have found no link. (Rafferty Baker/CBC)

Many studies have since been done to test his hypothesis, but all scientifically reputable studies found no link. A 900-page analysis by the American Institute of Medicine (IOM) looked at all relevant studies and meta-analysis, finding that "the mechanistic evidence regarding an association between MMR vaccine and autism is lacking."

This may sound like the science is simply insufficient, but microbiologist Jocelyn Srigley explained that in scientific terms, "lacking" is tantamount to saying evidence is not just weak, but absent altogether.

The biggest and most recent study looking for a correlation between vaccines and autism followed 657,461 children born in Denmark from 1999 through 2013. This study compared MMR-vaccinated with unvaccinated children and found "no increased risk for autism." This study even looked at sub-factors, such as sibling history of autism, autism risk factors or other childhood vaccinations to see if those factors could combine with a vaccine to create a higher risk. But again, it found no increased risk.

3. Big pharma and money

Pharmaceutical manufacturing is a profitable industry, but vaccines are not big money-makers compared to other drugs.

Only two to three per cent of the global pharmaceutical profits come from vaccines, despite the fact that most people on earth are vaccinated. In fact, most childhood vaccines have expired patents , and it's estimated that once a generic drug enters the market after a patent expiration , name-brand sales drop by 80 per cent .

Physicians often lose money on the administration of vaccines, and some governments have to incentivise drug companies to keep making them because of the fact that the profit margins are so low.

Vaccines take lots of time and money to develop because the regulatory standards are higher than for other drugs since they are being injected into healthy people. They may make some money in the first few years, but they also have to carry the burden of cost for all the years of research and development.

Vaccines prevent millions of people from getting seriously sick every year. If you're worried about motives, keep in mind that Big Pharma makes a lot more money off people who are sick than people whom vaccines have kept healthy.

4. Prevalence and danger of vaccine-preventable disease

Even though we rarely encouter certain diseases anymore, such as pertussis or polio, they are only kept at bay because the vast majority of the population is vaccinated .

If you talk to older generations, they'll tell you stories of their friends who became deaf, blind, disabled or died from the diseases that vaccines have eliminated in North America.

In areas where vaccination rates drop past a certain threshold, we can see resurgence of these diseases, which can be picked up and brought back to countries that had virtually eliminated them.

For example, Samoa had a significant drop in vaccination rates over the past few years due in part to anti-vaccination campaigns and an MMR scare after a nurse accidentally mixed up two shots with an expired anesthetic, resulting in two deaths. When measles made it into the community, it quickly infected thousands of people; 81 of those infected died, and almost all of them were babies and young children .

Authorities arrested an anti-vaxxer for spreading misinformation during the epidemic while the government was frantically trying to get everyone vaccinated.

The only disease that has been entirely eradicated worldwide is smallpox . This was because of a very effective mass-vaccination initiative in the 1960s that managed to drive the disease into extinction. Polio is a candidate for eradication , with only three endemic countries left.