Toronto Public Health says that two doses are needed for lifelong immunity.
"If you've only had one dose, again, it won't protect you as much," said Dr. Vinita Dubey, when speaking with CBC News on Monday.
Most people born between 1970 and 1992 will likely have been vaccinated in childhood. But they may not have received the second immunization that is recommended for adults.
Click on the video above to see a full report from the CBC's Shannon Martin.