Health officials in Vancouver are scrambling to track down a list of individuals who may have been exposed to measles on a weekend flight from Beijing to British Columbia, according to local media. Authorities have contacted about half of the passengers aboard Air China Flight CA991, which arrived in Vancouver Saturday with two high school students who later tested positive for measles and would have been contagious on the plane.

"We were able to get a flight manifest ... and a list of passengers who were sitting closest to the individuals infected and we've spent time trying to reach them, as well as flights crews," Reka Gustafson of Vancouver Coastal Health told CBC News. Authorities this week warned all British Columbians to make sure they were up-to-date on their immunizations.

“This is more than anything else a really important reminder that if you are out and about and breathing, you should have two doses of measles vaccine to be protected,” Gustafson told the Canadian Press. Residents who were fully immunized didn’t have to fear exposure, health officials said.

Measles is a highly-contagious disease that causes fever, severe rash and, in serious cases, brain damage, deafness, or even death. Before vaccines were widely available, large epidemics of measles were common in Canada and occurred every two to three years, according to the public health group Immunize Canada. In the first half of the 20th century, measles led to roughly 350,000 cases, 5,000 hospitalizations and 50 to 75 deaths in Canada every year.

Today, the number of annual measles cases in Canada has fallen by over 99 percent following a robust immunization effort in the 1960s. In 1995, Canada introduced a two-dose immunization regimen, which has proven to be more effective against outbreaks.  

Neither of the students who contracted measles was fully immunized against the virus, according to Global News. The students – one boy and one girl – were likely exposed to the disease around the same time, health officials said. They were diagnosed after visiting separate doctors within a few days of arriving home in Canada. It was unclear whether the students had become infected while abroad, as the virus can incubate for up to 21 days.