2015 Ontario Measles Outbreak Underscores the Need For Immunizations

Researchers have released a new report on an unusual outbreak of measles that occurred in the Canadian province of Ontario in 2015, highlighting the importance of epidemiological investigation and surveillance in the effort to eradicate the virus.

While the World Health Organization (WHO) notes that there were 134,200 measles deaths worldwide in 2015, Canada has largely eradicated the virus through widespread vaccination. According to the Centers for Disease Control and Prevention (CDC), the measles, mumps, and rubella (MMR) vaccine protects against the 3 highly contagious viruses; a 2-dose schedule is 97% effective at preventing measles, while a single dose schedule is 93% effective. Measles — marked by high fever, cough, oral Koplik spots, and a telltale rash – is no longer common in countries such as Canada and the United States where immunization rates have helped populations obtain herd immunity. However, outbreaks of the virus can still occur in these countries, particularly in under-vaccinated communities, and as global travel connects individuals with parts of the world where measles is still common.

In a recent report in the CDC journal Emerging Infectious Diseases, a team of researchers in Canada have examined an outbreak of measles that occurred in the province of Ontario in early 2015. The outbreak period occurred that year from January 25, when the first case was reported to Toronto Public Health, to February 17, and involved a total of 18 cases reported by 4 public health units. Investigators confirmed the immunization status of 14 out of the 18 patients; of these patients, 3 adults and 6 children were not immunized, while 2 adults were fully immunized. None of the infected individuals reported recent travel, and the investigators were unable to identify a “source patient,” though molecular epidemiologic analysis suggested that all cases likely came from “a single importation of measles virus genotype D4.”

The report notes that the outbreak was “unusual” in that cases involved “a unique strain of virus and no known association among primary case-patients.” Furthermore, investigation yielded no answer regarding the common source of the virus, though the high rate of measles vaccination in Ontario likely helped prevent a larger outbreak. The cases did occur in mostly unimmunized or under-immunized individuals, in a part of the province with a multicultural population and many international travelers.

“This outbreak is an excellent example of why it is important that everybody ensures that their immunization status is up to date,” senior author Shelley L. Deeks, MD, interim chief of Communicable Disease, Emergency Preparedness and Response for Public Health Ontario, told Contagion®. “We live in a global and interconnected world. We are continually exposed to other people in our everyday lives. For a disease such as measles, that is extremely contagious and transmitted through the air, an individual can be exposed to an infected person and be at risk of disease with limited interaction.”

Dr. Deeks notes that measles can be transmitted through limited contact with a contagious individual, a noninfected individual can catch it by sitting beside someone on a plane who is infected, or walking into a room an hour after an infected individual has left; it’s also easily spread to others through coughing and sneezing.

Canada experienced a total of 196 cases of measles in 2015. Two-dose vaccination coverage of 95% or higher is recommended to achieve measles herd immunity, the authors noted in the report. “Herd immunity is important as there are some individuals who are not able to receive the vaccine,” said Dr. Deeks, such as individuals who are immunocompromised or who are too young to receive vaccination. “If most of the population is vaccinated, then the disease will not continue to spread because we no longer have a pool of susceptible people. As measles is very infectious, we need a high proportion of the population to be vaccinated to stop the ongoing spread of this disease.”