Vaccines have become modern-day suits of armour, protecting humanity from deadly pathogens that used to cut us down in great numbers. But with many common illnesses now apparently vanquished, some parents question the need for vaccinating their children, fearing the potential side effects of the shots more than the diseases themselves.

In part, the anti-vaccination movement has been fuelled by a growing tendency to question traditional medicine and embrace alternative therapies.

For Alison Gowan, it was an "easy choice" not to vaccinate. The 33-year-old music teacher, who lives with her husband in Kingston, Ont., had given birth to both her boys at home and wanted to raise them in a "natural" manner. "Vaccination was not the way we wanted to go," she says.

Conversations with the midwives who helped to deliver her babies raised concerns about the long-term health implications of vaccinations, she says. "I worry about how it is going to affect our bodies later in life."

Ms. Gowan is bright, articulate and well educated. Her husband is a university sociology professor. Her own mother was a nurse. She feels she knows enough to make the right decision for her boys, who are now 8 and 10.

"Measles and mumps are pretty minor diseases when you are in a developed country with good, clean water, decent food and that sort of thing," she says. The risk of side effects from the vaccinations just didn't seem worth it, given "the very, very rare possibility" of her boys acquiring the diseases.

Such talk strikes fear into the hearts of public-health officials, who say a vigilant guard against the microbial world must be maintained.

Dr. Peter Nieman, a Calgary pediatrician, is critical of parents who forgo vaccinating their children. "I think it's quite selfish of them to come along for a free ride," he says. "They benefit from the fact that the majority of other people got their kids immunized."

Unvaccinated children are partly protected by what is known as "herd immunity." Diseases can't easily spread through a community when most members act as poor hosts for the pathogens.

But Dr. Nieman warns that deadly diseases can make a quick comeback whenever vaccinations dip below a critical level.

That has happened most recently in parts of England, where up to 20 per cent of children entering grammar school have not been vaccinated. Reported cases of the measles have jumped to the highest level in nearly a decade. "We are approaching the danger zone where measles could once again become an endemic disease in the United Kingdom," British researcher Vincent Jansen says.

And Canada may also be entering a danger zone of its own. Last year, Health Canada commissioned a poll of parents with young children. It found that at least 90 per cent of one-year-olds had received the recommended number of shots, according to the vaccination records kept at home by the parents. However, only 75 per cent of two-year-olds and 72 per cent of seven-year-olds had all their shots.

Dr. Arlene King, director of immunization and respiratory infections at Health Canada, isn't jumping to any conclusions about the potentially alarming findings. The parents' records might simply be wrong, she says. But to be sure, Health Canada is going to have another poll done in the New Year, and officials are checking to see if parent records match those kept by medical offices.

Dr. King says the vast majority of Canadian parents support vaccination. Still, a vocal -- and potentially influential -- minority continues to attack vaccinations, especially on websites, which blame immunizations for a wide range of things, including asthma and autism.

Edda West, co-ordinator of Vaccination Risk Awareness Network in Winlaw, B.C., says her daughter came down with "a full-blown case of severe, severe measles" shortly after being vaccinated in 1977. "I was told it was a coincidence. I don't believe that."

Ms. West says her daughter went on to develop "life-threatening allergies" to insect stings. "My instinct, as a parent, is that something happened to her immune system in that time," she says.

She believes soaring rates of asthma and other disorders are linked to the increasing reliance on vaccines. "A huge concern is too many vaccines starting too early in life when a child's immune system is very vulnerable and very fragile."

But Ian Gemmill, chairman of the Canadian Coalition for Influenza Immunization, says it's a myth that children's immune systems are being overloaded by too many shots. "Giving your child a vaccine . . . is a drop in the bucket compared to the many, many organisms that they are exposed to all the time."

Dr. Gemmill says vaccination programs have become victims of their own success. "When they make diseases go away, people don't see how bad they were," he says. Even ordinary diseases like the measles can take a heavy toll.

Dr. Ronald Gold, author of the book Your Child's Best Shot: A Parent's Guide to Vaccination, says about one in a thousand children who get measles will die. An equal number of children develop encephalitis -- an inflammation of the brain. About a third of them die and another third will be left severely brain damaged.

"That doesn't sound like a big number, but if everyone gets measles, then a lot of children are being harmed."

Even so, vaccinations have been at the centre of repeated controversies in recent years. A 1998 article in the British scientific journal The Lancet suggested that the three-in-one shot vaccination for measles, mumps and rubella (known as the MMR) could trigger autism and bowel disorders. Dr. Andrew Wakefield, who wrote the article, speculated that a mercury compound, used as preservative in some of the vaccines, might be a source of trouble.

Various studies have since exonerated MMR and the mercury preservative known as thimerosal. A Danish study, for instance, found that autism rates continued to rise even after thimerosal was withdrawn from use in that country.

Dr. Paul Offit, chief of infectious diseases at the Children's Hospital of Philadelphia, said the mercury levels in vaccines are far lower than what a child would be exposed to naturally. What's more, the compound used in vaccines is known as ethylmercury -- and is not the same as the more toxic methylmercury found in the environment. "An important difference is that the body eliminates ethylmercury far more quickly than it eliminates methylmercury."

Nonetheless, thimerosal is gradually being withdrawn from most vaccines -- as a precautionary measure. Since March, 2001, it has been removed from vaccines routinely given to Canadian children, Dr. Gold notes.

Dr. Gold and other immunization advocates readily acknowledge that vaccines are not without some risk. "But it's a very rare event to have an allergic reaction after a vaccination."

An older form of the combo shot for diphtheria, pertussis (whooping cough), tetanus and polio has been criticized for provoking temporary fevers, swelling, crying episodes and convulsions in some children. Part of the vaccine has been reformulated and side effects are now down significantly, medical experts say.

However, assurances of new and improved vaccines come as cold comfort to parents such as Heather Zischler of Peterborough, Ont. At the age of 41, she recently gave birth to her first child and "agonized" over the decision not to vaccinate.

"No one has been able to prove that the stuff that goes into these vaccines is safe for everyone and there is always going to be someone who reacts to it. I just don't want it to be my son," Mrs. Zischler says.

But Dr. Offit says that saying no to vaccinations is not a risk-free choice. "It is just a choice for a different risk -- and arguably a much more serious risk of getting these diseases that are still out there."

Paul Taylor is a Globe and Mail assistant national editor, responsible for health and science coverage.

Vaccines at work

The following figures compare the annual number of cases of a disease in Canada before vaccines were introduced with those recorded last year.

Polio

Before vaccine: 20,000

2002: 0

Diphtheria

Before vaccine: 9,000

2002: 0

Rubella

Before vaccine: 69,000

2002: 16

Mumps

Before vaccine: 52,000

2002: 197

Hemophilus influenzae b (Hib)

Before vaccine: 2,000

2002: 48

Whooping cough

Before vaccine: 25,000

2002: 2,557

Measles

Before vaccine: 300,000

2002: 7

-- Health Canada