Too few Hamilton schoolchildren are vaccinated to give herd immunity to measles, which is spreading in parts of Ontario.

The vaccination rate is 93 per cent in Hamilton for children between the ages of seven and 17.

That's better than the 85 per cent in Niagara, which has its first case of measles in nearly 20 years. An unimmunized woman in her early 20s is recovering in hospital after being admitted with complications from the potentially deadly disease.

However, Hamilton's rate is still below the 95 per cent required to provide community immunity to those who haven't been vaccinated.

It's significant considering Ontario public health officials are investigating whether measles is spreading between Toronto and Niagara. Toronto has an outbreak with five cases so far, diagnosed in three adults and two children under the age of two.

The Niagara woman was in Toronto twice in late January. Normally, measles in Canada is related to travel but the woman had not been outside of the country recently.

It's not known how the six became infected and there is no apparent connection between them. As a result, public health officials assume there will be more cases.

"We have not been notified of any contacts in Hamilton," said Dr. Jessica Hopkins, Hamilton's associate medical officer of health. "That being said, both investigations are ongoing, so that could change."

Niagara's public health department is offering vaccination clinics and warning residents they may have been exposed to measles if they were in Unit C at Greater Niagara General Hospital, at the MacBain Community Centre or at the Walmart on Oakwood Drive in Niagara Falls at certain days and times.

"There is no Hamilton exposure," said Carrie Beatty, spokesperson for Niagara Region Public Health.

The cases in Ontario, combined with an outbreak at Disneyland in California, are raising questions about why more children aren't vaccinated against the highly contagious disease.

"We have a very effective vaccine," said Dr. Michael Gardam, director of infection, prevention and control at Toronto's University Health Network. "This is one of those diseases that is on the list of things that we could, theoretically, remove from the earth."

Instead, measles is creeping back. The Public Health Agency of Canada reported that 2013 saw the fifth highest number of reported measles cases since 1998. The number of cases was even higher in 2014.

Before now, the last case of measles in Niagara was in 1996. Hamilton had an outbreak a year ago, with two cases in March 2014 linked to a third Ontario case.

The disease is still one of the leading causes of death among younger children globally.

"It's entirely preventable," said Gardam. "Even one death in a child with measles who had an opportunity to be vaccinated and wasn't vaccinated seems like a tragedy."

The number of measles cases in Canada has plummeted because of vaccinations. Between 1991 and 2012, there were 14,883 cases in Canada. That compares to 313,686 cases between 1924 and 1933.

The vaccine is considered to be nearly 100 per cent effective, although one of the Toronto adults diagnosed had been immunized. Three others weren't vaccinated and one received only one dose instead of the required two.

Adults born between 1975 and 1990 were only given one dose of the vaccine, and it's recommended they get the second shot.

It takes seven to 21 days for those exposed to develop symptoms, which means new cases could crop up until at least Feb 23.

Symptoms start out as a cough, runny nose, red and watery eyes and fever. A rash starts on the face after about four days and moves down the body. White spots might also appear in the mouth.

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It's estimated one person spreads measles to 18 to 20 others. In comparison, one person spreads the flu to two to four others.

"If you do not have protection against measles, it is insanely contagious," said Gardam. "It's one of the most infectious contagious diseases we have … it's an airborne transmitted virus. All you have to do is breathe the air."