Authorities in Ontario's Niagara region are scheduling public health clinics to offer measles vaccines after a woman was diagnosed with the highly contagious disease.
"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.
The unidentified woman, who is in her early 20s, was hospitalized with complications of the disease but is recovering. She was unvaccinated.
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It's not known how she became infected. Jaeger said the woman had not recently been outside the country.
However, she was in Toronto twice in late January, during the period when four cases of measles were diagnosed in the city. Public health officials in Niagara and Toronto will work with Public Health Ontario to look for links between the patients.
The fact that five cases of measles without obvious connections to one another have popped up in southern Ontario leads authorities to assume the virus is spreading and more infections will occur.
"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.
That work was done for this case, Jaeger said, but she would not reveal if any contacts were considered susceptible to measles.
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.