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A student receives a measles vaccine injection in Switzerland during a 2009 measles outbreak in that country.

Valentin Flauraud/Reuters

Despite recent measles outbreaks in Canada – which spread into Burnaby, B.C., this week – some parents remain unsure of whether vaccines are safe. Dr. Noni MacDonald, a pediatric infectious-disease specialist at the IWK Health Centre in Halifax, answers common questions she hears from vaccine-hesitant parents.

Are babies being vaccinated too young? Is the current vaccination schedule designed mainly to target babies as soon as they enter the health-care system?

The vaccination timing has been chosen to maximize the benefits and minimize the risks of disease. The problem with tetanus and diphtheria, for example, is that you need three immunizations before you are protected. Tetanus is in the dirt and the dust, and it takes just a picogram of tetanus toxin in a tiny cut that you can't even see to make a baby very ill. Even with today's best medical attention, 10 per cent of children who get tetanus poisoning will die. Children need three tetanus shots before they're nine months old and crawling around, which is why we give them at two, four and six months.

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Is there any harm in a delayed vaccination schedule, starting at one year?

Scientific evidence and reports from the Institute of Medicine [a non-profit organization and the health arm of the U.S. National Academies] show that when you delay vaccination, you increase the risk of disease. The highest risk for dying of pertussis is under six months of age.

Babies with strong immune systems don't need vaccinations, do they?

Some people believe that a healthy lifestyle will give them a strong immune system, but this isn't about your lifestyle. These are infectious diseases and you can't protect yourself unless you're immunized.

Some vaccines contain mercury and aluminum. How can that be safe?

There is a big difference between ethylmercury and methylmercury, which affects the developing brain and is the kind that accumulates in large fish, such as tuna. Ethylmercury does not accumulate in the body, nor does it have the same side effects as methylmercury, which is not used in vaccines. In Canada, the only vaccines that contain ethylmercury (in the preservative thimerosal) are certain influenza vaccines, which are not used on infants, and some hepatitis B vaccines, depending on the province. Even so, the National Advisory Committee on Immunization concluded in 2007 that there is no legitimate safety reason to avoid the use of thimerosal–containing products for children or older individuals, including pregnant women. As for aluminum, it has been in vaccines for decades, and has been shown to be safe. The aluminum found in pots and pans has a different chemical format.

Celebrities such as Jenny McCarthy and Kristin Cavallari have access to the best medical care – and they are choosing not to vaccinate their kids.

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This is what I tell parents who are swayed by celebrities: If your child had a brain tumour, who would you choose to operate on your child – Jenny McCarthy or a neurosurgeon? You would pick the neurosurgeon. So I ask, why would you trust someone who has no training and has never studied [vaccines] extensively with a decision about immunization, when your child could die?

Older vaccines may be proven safe, but what about newer formulations that have not stood the test of time?

Many people don't realize that immunizations are followed much more closely than drugs are. In Canada, we have an active surveillance program that documents any adverse effects. When immunizations first came out in the early 1700s [with the smallpox inoculation], people had concerns – and down through the ages, the fears have changed but vaccines have remained a source of concern. But let's not forget that 25 years out of our 30-year gain in life expectancy between 1900 and 1999 are due to clean water, improved sanitation, antibiotics and vaccines.

This interview has been edited and condensed.

Correction

An earlier version of this interview contained incorrect information about the presence of ethylmercury in vaccines. This version has been corrected.

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