What happens without measles immunity

Measles is one of the most contagious diseases in the world. Those who don’t have immunity, from either vaccination or a previous infection, have a 90 per cent chance of getting infected after being exposed.

Tricky to contain, the virus can easily spread before somebody realizes they’re infected. It’s also incredibly efficient at finding and sickening those who are susceptible. It can transmit through coughing and sneezing. It can stay on surfaces for up to two hours. It can even spread just through breathing.

“The virus can hang in the air after an infected person has left a room,” said Shelly Bolotin, director of the Centre for Vaccine Preventable Diseases at the University of Toronto’s Dalla Lana School of Public Health. “When they leave, the virus can still be in the air for up to two hours. If another person walks in the room, they’re at risk of being infected.”

Before vaccines, between 10,000 and 90,000 people were infected with measles in Canada every year. After the launch of routine immunization programs in the early 1970s, cases dropped by more than 99 per cent.

While measles has been eliminated in Canada since 1998, isolated cases occur when the virus is introduced through travel. Bolotin said Canada’s elimination status means that “although cases can arise from travel-related importation, the virus doesn’t continue to circulate in Canada for a long period afterwards.”

As of Thursday, there were more than 40 measles cases in Canada. Last year saw just 12 cases nationwide.

With measles on the rise in Canada — and outbreaks in many countries around the world — public health leaders have for weeks been calling on people to ensure their immunizations are up to date amid warnings that vaccination rates have declined.

Canada’s National Immunization Strategy has set a vaccination coverage goal of 95 per cent for measles. But the most recent federal data shows just 79 per cent of kids had two doses at age seven in 2021.

Bolotin stressed that while the impact of measles has largely faded from Canadians’ collective memories, the disease can be life-threatening, especially for the most vulnerable.

The infection affects many organ systems, with about 20 per cent of cases needing hospital care, she said, adding that the disease also harms the immune system.

A two-dose series of the measlesmumps-rubella (MMR) vaccine or the measles-mumps-rubella-varicella (MMRV) vaccine will protect up to 99 per cent of people against the disease.

Beyond offering individual protection, Bolotin said it’s crucial to vaccinate as many people as possible to guard a community against measles. She said those with immunity from a previous measles infection also help block the virus from spreading.

“For vaccine-preventable diseases, the more infectious something is, the higher proportion of people need to be protected to keep it in check.”

The Star has created three scenarios that show how quickly measles can spread in an unprotected community — and why it’s important to have high rates of immunity to protect everyone against the virus.

1. All population is susceptible

In a population with no immunity, each person with measles on average can transmit the virus to between 12 and 18 other people. The number of people infected can be higher or lower depending on the makeup of a community. That’s because there is a higher proportion of susceptible individuals — the newborn infants — entering the community.

In this animation, which assumes each case infects another 18 people, a single measles infection rapidly turns into an outbreak that sickens more than 5,800 people over three generations, which is roughly the span of up to two months.

2. Population is 75 per cent immune

When a majority of a population has immunity — but has not met the 95 per cent coverage goal — many people remain vulnerable.

“It slows the virus down, but it doesn’t stop it,” Bolotin said. “A measles case is mostly encountering protected people but occasionally they come in contact with people who are not protected and spread the virus to them. We see that 75 per cent immunity in a population isn’t high enough to prevent an outbreak.”

She said this animation illustrates that high levels of immunity are needed to provide indirect protection to those who can’t be vaccinated, including infants and those with compromised immune systems.

“Getting vaccinated means we’re putting ourselves between them and the virus.”

3. Population is 95 per cent immune

When measles is introduced into a community that has 95 per cent immunity, the virus has few places to spread. While there are still some susceptible individuals, the infected person is much less likely to encounter them.

“Rather, they’re more likely to be surrounded by immune people who cannot be infected,” Bolotin said.

With 95 per cent immunity, transmission is effectively halted. The virus has very few places to go.