The infected person had received the recommended two doses of vaccine, said Lenore Bromley, media relations manager for Toronto Public Health. While the measles vaccine is considered highly effective, about five per cent of people who get two doses may not be fully protected.
News of the fifth Toronto case came after health authorities in Ontario's Niagara region disclosed they had found a case, a woman who had never been vaccinated.
Meanwhile, public health authorities in Quebec's Lanaudiere health district were awaiting test results on eight suspected measles cases there. The suspected cases are linked to one another and one person is believed to have contracted measles outside the province, spokesperson Pascale Lamy said in an email.
No connections have been discovered among the six Ontario cases, suggesting there are probably undiagnosed illnesses in the Toronto region. And officials acknowledged the current crop of cases may have already sown the seeds for the next one.
"Will we see secondary cases? If I were a betting person, I would say yes," said Dr. Shelley Deeks, medical director for immunization and vaccine preventable disease for Public Health Ontario.
In the Niagara region, authorities were scheduling public health clinics to offer measles vaccines after confirming their case. The woman, in her early 20s, was hospitalized with complications from measles. She is recovering.
"Our phones have been ringing off the hook," said Dr. Valerie Jaeger, local medical officer of health, who added that doctors have also been alerted that measles may be spreading locally.
It's not known how any of the Ontario cases contracted the virus. None had been out of the country recently. But the woman from the Niagara region had been in Toronto twice in late January, when the first four cases in the city were diagnosed.
Jaeger said her region too is bracing for more cases.
"In terms of progression from this case, we would not be out of the woods until Feb. 23," said Jaeger. "Then we would know whether there had been any person-to-person transmission from this."
The incubation period — the time it takes for an exposed person to develop symptoms — is seven to 21 days, though most infections become apparent in 10 to 14 days.
Jaeger said whenever a measles case is found, public health investigates the immunization status of the person's household contacts. If they have not been vaccinated, they are offered vaccine. If they refuse, they are instructed to isolate themselves until 21 days has passed since their last exposure to the infected person.
Jaeger said the measles vaccination rate for elementary school children in the region is only 85 per cent, which is not high enough to keep the virus from spreading if it makes its way into that population.
It's estimated that 95 per cent vaccination coverage is needed to maintain herd immunity, the term used to describe the situation in which enough people are vaccinated that the virus can't continue to spread.
People born before 1970 — when measles vaccine use began — are assumed to have previously had the disease. Canada was able to stop domestic spread of the virus in the late 1990s. As a result, any measles cases that occur now are sparked by virus importation — in a returning Canadian or an infected visitor — or spread from the person who imported the virus.
— Follow @HelenBranswell on Twitter