TORONTO -- Canada's top doctor is urging Canadians who haven't been vaccinated against measles to get their shots in the wake of a B.C. outbreak of the disease and the always present danger of cases being imported into the country by travellers.

Dr. Theresa Tam, Canada's chief public health officer, said measles is a "serious and highly contagious disease" and that getting inoculated is the best way to avoid getting sick -- and transmitting it to others who may be unprotected.

Tam's comments Tuesday come in the wake of a cluster of nine cases of measles in Vancouver that began in recent weeks after an unvaccinated Canadian child contracted the disease on a family trip to Vietnam.

"There's always a risk of measles importation into Canada," Tam said from Ottawa. "We eliminated measles in Canada (in 1998), but what we're seeing is importation from when people go travelling to another country and then bring it back.

"But when they bring it back to pockets of underimmunized groups, then we're going to see the potential for more people to be infected in Canada itself."

Infection with the measles virus starts with a high fever, coughing, sneezing and red eyes, followed by the development of a blotchy, painful rash, said Dr. Sarah Wilson of Public Health Ontario. "The rash starts on the face and moves down onto the trunk, to the chest and the back, and then it spreads to cover the whole body."

Measles can lead to such complications as ear infections, blindness, pneumonia and encephalitis (swelling of the brain). It can also be fatal.

The disease is highly contagious, spreading through virus-laden droplets after an infected person coughs or sneezes.

"I think one of the features of measles that makes it so exquisitely infectious is that it's air-borne," said Wilson, noting that viral particles can remain in the environment for up to two hours after being introduced.

"Those measles virus particles can stay suspended in the air and anyone who's unvaccinated who then goes into that doctor's office or restaurant or airport lounge, they can acquire measles."

Another complicating feature of measles is that it can be contagious for four days before the rash arises, said Tam.

"Prior to that, when you're developing the fever, the cough, the runny nose, you're infectious and people don't even know it's measles," she said. "And that I think is another reason why it can spread before you know it."

But Tam said getting two doses of the measles, mumps and rubella (MMR) vaccine is virtually 100 per cent effective in preventing the disease within a given population.

Some people cannot be vaccinated, including infants under six months of age, people with certain underlying health conditions and those undergoing chemotherapy -- meaning they must rely on high levels of immunity within their communities to prevent infection with the virus.

To achieve that protection -- called herd immunity -- a community needs a vaccine coverage of about 95 per cent of residents, Tam said. At the national level, Canada has close to 90 per cent coverage, but average uptake of the MMR vaccine varies from province to province and region to region.

Canadians born before 1970 are assumed to have natural immunity to measles, a disease long considered a childhood rite of passage, despite its dangers.

For those born after 1970, who aren't sure if they've been vaccinated, there is no increased risk of adverse effects from getting the MMR vaccine, said Wilson. The two-dose inoculation provides lifelong immunity.

"The vaccine against the measles virus is one of the most effective vaccines that we have," noted Tam.

"So I think what we want to stress is that right now is really a good time to make sure you are up to date with two doses of measles vaccine if you were born after 1970, because we are seeing these importations," she said.

"And if we don't get vaccine coverage up to higher levels in Canada, throughout Canada, for community protection, we're going to continue to see these pockets (of cases)."