Ontario is making three more vaccines mandatory for schoolchildren, a move that cements the province's hard line against parents who decline to immunize their children.

As of this September, Ontario students will need to be vaccinated against whooping cough, meningococcal disease and, if they were born after 2010, chickenpox. All three are already publicly funded and part of the province's immunization schedule.

"We need to make sure that we protect kids in our schools and others in the public," Health Minister Deb Matthews said in an interview.

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Ontario already bars students who have not received their shots for six diseases, including measles, unless their parents apply for an exemption on religious or philosophical grounds – something fewer than 2 per cent of families pursue, according to Arlene King, Ontario's chief medical officer of health.

She suggested it is much more common for parents to unintentionally let their children's vaccinations lapse, particularly if the shots are not necessary to be admitted to school.

For instance, Ontario's vaccination coverage rate for the mandatory measles shot was 89 per cent for seven-year-olds in the 2011-2012 school year. That figure was 76 per cent for the whooping cough vaccine, which was optional until now.

"The requirement for immunization against these diseases at school entry is a tremendously important catalyst for people to just check to see whether their kids' immunizations are actually up to date and complete," Dr. King said. "When there isn't that catalyst and impetus, there may be an impression that [the three additional vaccines] may not be as important as the other six, which isn't the case."

On average, there are 38 cases of invasive meningococcal disease, 8,900 of whooping cough and 66,000 of chickenpox reported in Ontario annually.