Measles in 2025: your questions answered

In early 2024, measles was on the rise in Canada and rapidly becoming a major public health concern in many jurisdictions across the country. Caroline Colijn and I wrote up a mathematical model to see what outbreaks could look like (how big they get and how long they could last) under different levels of vaccination coverage and public health responses. It’s a year later, out paper was recently published (Feb 2025) and unfortunately it is more timely this year than last. As of writing this (March 9, 2025), Canada has seen 227 cases so far this year. The total for all of 2024 was 146.

The target vaccination coverage for measles is 95%. If 95% of people in your community are vaccinated (two doses of measles containing vaccine), then an individual who has measles returns to your community (and hopefully isolates) we would not expect to see a chain of infections take off (i.e., an outbreak).

Many communities across the country have very high vaccination rates for measles, in fact, it’s the highest among routinely administered vaccinatations at 87% uptake nation wide; however, many communities have not met the tagert of 95% – outbreaks can happen at 87%, and there are communities (certain schools for example) with much lower coverage.

What you need to know

How does the measles virus spread?

Measles spreads by human-to-human transmission. This means you have to be in contact with someone who has measles to catch it. The MMR vaccine is 97% effective. In Canada, measles outbreaks can only occur if a case has been imported to your region (ie, by travel – you must get it from somewhere) AND the affected person(s) are in contact with people who are unvaccinated. If a case enters your community, the affected individual isolates and 95% of the community is vaccinated, then an outbreak is extremely unlikely (i.e., herd immunity). What we worry about is when the individual returns to a community that has many unvaccinated people in it. Measles is highly contagious and unvaccinated people who are in contact with the virus will likely get the measles.

• Measles spreads through the air, and on contact surfaces. The virus lives in the nose/airways of the infected person and comes out when they cough or sneeze.

How can public awareness and intervention help stop outbreaks?

Often when there is a measles outbreak, public health agencies will run awareness campaigns to educate the public. These are critical to stopping the spread.

• popup vaccination clinics: those who are unvaccinated or do not know their immunity status (if you do not know if you have received one or both doses of the MMR vaccine, receiving another dose will not harm you and once you have two doses you are protected for life – no need for additional boosters). If you are unvaccinated, you can even receive a dose once you have been exposed (but I would not encourage people to wait until then).
• promotion of self isolation: if you think you may have been exposed it is very important to self isolate since you can be contagious before you show symptoms (i.e., rash).

Who is already immune?

• If you have had two doses of an MMR vaccine you are not at risk.
• If you were born in Canada before 1970 you are assumed to have acquired immunity. This is because the first measles vaccine became available in Canada in 1964 and was rolled out in schools thereafter. Before the vaccine, measles ran though the population and many children got measles. Once you have immunity you are usually immune for life.
• If you have had the measles you are likely immune.
• One dose of the vaccine is around 87% effective at protecting you against the measles. It is recommended to have both doses.

Who is most at risk?

• Children who have not received two doses of a measles vaccine, particularly school-aged children. If you are in the same airspace as someone with measles and you are not vaccinated you have a very high chance of contracting the virus. Measles is contagious even before the onset of symptoms, vaccination is the only way to ensure protection.
• “Measles parties” are not safe. While it is true that getting the measles will mean that you are immune to getting the measles again, THIS IS DANGEROUS. Having the measles can lead to serious complications including hospitalisation and death.

Our paper

What did our study look at?

In McNichol et al. (2025) we model measles outbreaks in small communities – a school, church congregation, etc. We think this limit on community size is reasonable in the sense that most people [in Canada] are vaccinated and outbreaks tend to occur in a pocket of unvaccinated individuals. Many historical outbreaks in undervaccinated populations fall into the projected outcomes of our model. There is, of course, scope to create more complex models that cover larger regions. Our model is informed by epidemiological data on the likelihood of measles transmission.

We look at the potential size of outbreaks and how long they might last, given the vaccination rate in the community and the communities response to public health interventions (e.g., willingness to self isolate, get vaccinated in response to awareness campaigns).

So what might an outbreak look like?

At 95% vaccination coverage, no outbreaks. At 85% (or higher) coverage, outbreaks can occur but should be manageable through public health campaigns/increased awareness – that means taking immediate action to get vaccinated, thus increasing the vaccination coverage! Under 80% vaccination coverage outbreaks will likely happen. Depending on the size of the community, a few hundred cases over two months is a realistic scenario at a vaccination rate of 60-70%. Our model shows that, regardless of vaccination coverage, taking immediate action to get vaccinated will slow or stop the spread.

How is the measles different from Covid-19?

Many scientists and health care researchers around the world tirelessly worked to develop vaccines and understand the Sars-CoV-2 virus. Measles is a virus that we already know about, know how to prevent, and have a very effective and safe vaccine for. We know exactly how to prevent measles outbreaks. The challenge seems to be getting everyone on board.

Key takeaways

1. If you are not vaccinated or do not know your immunity status, get vaccinated.
2. If you are planning to travel, especially to places with current outbreaks, make sure you are vaccinated.
3. If you have symptoms, self isolate. Speak to your health care provider about next steps and treatment.
4. Measles is highly contagious and severe complications or death can occur. The vaccine is safe and very effective.

More information

• Measles signs and symptoms

References

• J. McNichol, J. Valizadeh, S. Chaudhury, C. Colijn, Measles in Canada: modelling outbreaks with variable vaccine coverage and interventions. BMC Infectious Diseases 25, 236 (2025).

• MMR coverage in Canada, Govt of Can