For parents, making a decision about vaccinating your child can be tough, given the cacophony of opinions out there. So what do you need to know?

Myths about vaccine safety are both powerful and prevalent. In parts of Canada vaccination rates have dropped, and in some communities more than 40 per cent of seven-year-olds do not have all their shots. (Watch "Shot of Confusion," Friday, Nov. 28 at 8 p.m. ET / 8:30 p.m. NT on CBC Television or online at cbc.ca/marketplace. You can also join Marketplace co-host Erica Johnson and Dr. Peter Lin for a web chat about vaccines tonight at 6 p.m.)

Statistics around inoculations are a patchwork because while some communities collect data, Canada doesn't have a national tracking system for vaccinations.

It does track disease, though, and earlier this year the Public Health Agency of Canada issued a warning about an unusually high number of cases of measles. This is a disease that is preventable if people are vaccinated against it.

Measles is very contagious and can be quite serious, leading, in severe cases, to brain damage and death. However, some people still avoid getting vaccinated, often due to misconceptions.

So why do myths about vaccines persist the way they do?

“It's hard to unscare people," Shannon MacDonald, a registered nurse and adjunct assistant professor at the University of Alberta who researches vaccine trends, told Marketplace co-host Erica Johnson.

remain unconvinced. "These diseases are the thing to be afraid of, and the vaccines are just by and far the safest option," says Shannon MacDonald, a registered nurse and adjunct assistant professor at the University of Alberta who researches vaccine trends. (CBC) Scientists have tried to debunk these myths, they've "done study after study after study," MacDonald says. But some parentsremain unconvinced.

"It's like the alligator in the sewer. You heard it, you don't know if it's true, but what if it's true? Maybe you'd better not go down the sewer."

Marketplace investigated what parents are being told by alternative health-care providers about vaccine safety and alternatives.

Dr. Lin is a CBC health columnist and has been a practising family physician for more than 25 years.

Here are some of the most pervasive myths and what the science actually says.

Dangerous ingredients

Myth: Vaccines can contain ingredients that are dangerous

Diseases that we have vaccines for, including measles and pertussis (whooping cough) can be deadly. According to the World Health Organisation, measles is one of the leading causes of death, globally, among young children. (CBC) Fact: "The one that people often hear about is thimerosal," says MacDonald. Thimerosal is an organic compound containing a form of mercury that is used as a preservative to prevent contamination.

Thimerosal is not present in most vaccines – only in some flu and hepatitis B vaccines in small amounts – and it's not dangerous, says Dr. Lin.

"The World Health Organization looked at it, the CDC [Centres for Disease Control] looked at it, Health Canada looked at it and they found no connection to any diseases."

"But parents seem to still be concerned about it," says MacDonald. Thimerosal is broken down by the body to ethylmercury, which is different than methylmercury, which is the chemical actually known to cause harm, she says.

"To try to equate ethylmercury to methylmercury is like saying that ethyl alcohol, which I have in my glass of wine, is the same thing as methyl alcohol, which is the antifreeze that I put in my car. So they are not the same thing.”

MacDonald says there is no evidence ethylmercury is harmful, and there has been considerable research showing that.

But because some parents were alarmed, thimerosal was removed anyway from many vaccines, just to try to assuage concerns.

Connection to autism

Myth: Vaccines can cause autism and other disorders

Fact: According to PHAC: "Medical researchers and scientists around the world have studied information collected over many years to see whether there is a link between the MMR [measles, mumps and rubella] vaccine and autism -- a lifelong developmental disorder. They have not found any evidence of a link."

Dr. Lin says that the timing of MMR vaccines has led to this anecdotal link in many peoples' minds.

"Kids get the shot around the same time that you would see if the child was responding to you. So many parents thought it must be the shot."

"There are some celebrities that keep it going. And if they talk about it, those kinds of single cases fuel the fire. A lot of people think, 'I heard something about that,' and if you hear it twice you think it must be real."

MacDonald says that even though researchers have thoroughly debunked the link between autism and vaccines, doctors should do a better job communicating that.

"You need to communicate in a way that people can understand that we have done multiple studies; we have looked at different study designs in different populations; in different parts of the world, and we have found no link between autism and MMR vaccine."

Overwhelm the immune system

Myth: Getting too many vaccines could overwhelm my child's immune system

Fact: Dr. Lin says this is a common misconception. "A lot of parents believe that, but we look at it this way: Your child is exposed to a lot of germs, they crawl on the floor and they get more germs that way than they do from a vaccine."

In some parts of Canada, vaccination rates have dropped, and in some communities, more than 40 per cent of seven year olds do not have all their shots. (Michael Buholzer/Reuters) "What vaccines do is they train the immune system. We want to train your army against the bad diseases like polio," says Dr. Lin.

"If you look at what is actually in the vaccines today compared to the vaccines kids were getting 20 years ago, they're actually getting less," says MacDonald.

"The number of vaccines are more, but we refined vaccines to the point that the actual – shall we say 'germs', the antigens that are put in the child, are at a much lower number."

According to PHAC: "The vaccines that are currently recommended do not overload or weaken your immune system.

"Infants and children are exposed to many kinds of germs every day, through normal eating, drinking and playing. Scientists estimate the immune system can recognize and respond to hundreds of thousands, if not millions, of different organisms."

Risk of disease is low

Myth: Some vaccines aren't necessary because the risk of disease is low or not serious

Fact: Diseases that we have vaccines for – including measles and pertussis (whooping cough) – can be deadly. According to the World Health Organisation, measles is one of the leading causes of death, globally, among young children.

According to PHAC, "Vaccines in Canada are effective and safe – much safer than the many diseases they prevent. These diseases can lead to pneumonia, deafness, brain damage, heart problems, blindness, paralysis and carry a risk of life-long disability or death."

Dr. Lin says that these considerations helped motivate scientists to develop specific vaccines in the first place.

"When vaccines were developed, they only developed vaccines for diseases that are serious and common. That's why we don't have a vaccine for the cold, but we have one for polio."

The effectiveness of some of these vaccines are part of the reason why so many hesitate now.

"We don't remember how bad some of these diseases actually are," Dr. Lin says. "Now many don't even know what polio is, and that's why people think it's not important" to be vaccinated against it.

"I have worked overseas and in intensive care with children, so I have a very real image of what the outcomes of these diseases can be," says MacDonald. "I understand that parents in Canada today just have not seen these diseases, so they are trying to balance risks that they have no concept of with stories that they read either in the media or on the internet that vaccines are unsafe.

"These diseases are the thing to be afraid of and the vaccines are just by and far the safest option."

Unnecessary risk

Myth: A strong immune system can be just as effective as vaccination with less risk

Fact: Having a strong immune system is important, but making sure that community immunization rates are high is essential to protect everyone, a concept called herd immunity. "If we can vaccinate everyone and protect everyone, now the virus has no place to get a foothold," says Dr. Lin.

And, if people choose not to vaccinate, we're all more vulnerable.

"Right now we have managed to keep all these diseases at bay through herd immunity," says MacDonald.

But if you lower vaccination rates, herd immunity doesn't take effect, and it is important for those who can't be immunized, like children who are on immunosuppressant therapies or who are too young to be vaccinated.

(Friday, Nov. 28 at 8 p.m. ET / 8:30 p.m. NT on CBC Television's Marketplace, Erica Johnson examines the confusion surrounding vaccine myths and reveals how trusted health practitioners may be putting your kids at risk. Join us for a live video chat on Thursday, Nov. 27 at 6 p.m. ET to voice your opinions on the issue and ask our experts your questions about vaccines.)