Measles outbreaks in Canada, the United States, and elsewhere have been the subject of numerous recent reports.[1] [2] [3]Although these outbreaks are limited in size, measles is a serious disease – especially for children – and outbreaks are taken seriously by health officials and afforded significant attention and surveillance.
It is natural and reasonable to ask why these outbreaks have occurred. We explore the issue and accountability for measles outbreaks by considering both technical and historical information. Likely factors include the effect of governmental policy on vaccine uptake, the reasons people choose to vaccinate, and, most tellingly, the characteristics of measles itself.
Measles outbreaks in 2025
As of June 16, 2025, 3,170 measles cases have been reported in Canada in 2025. Ninety per cent of these cases have been confirmed, and they cover nine provinces and territories.[4]Most of these cases – 71% – are under the age of 18. In the previous 25 years, the number of yearly reported measles cases in Canada has varied between 0 and 752, with an annual average of 91 cases.[5]
In the U.S., as of June 13, 2025, there were 1,197 confirmed cases of measles in 35 states this year. As in Canada, most of the cases (66%) involve children.[6]Two per cent of those infected had received one dose of the measles mumps rubella (MMR) vaccine, and 3% of those infected had received two doses of the MMR vaccine. Twelve per cent of the cases involved hospitalization, and three persons were confirmed to have died from the disease.[7]
Data from the European Union indicates a directionally similar rise in measles cases from February 2024 to January 2025, with 32,265 people diagnosed with the disease.[8] Eighty-five per cent of the cases were in Romania.[9]
BIG Media’s historical coverage
BIG Media has covered measles in a variety of contexts, some of which are relevant to this discussion. In our article Analysis of COVID vaccine AEFIs, Part I , we showed the incidence of severe outcomes from measles and other serious childhood diseases, as well as the ratio of vaccine adverse events to disease morbidity. We will briefly review this as well as a July 2021 article Examining the effectiveness of restrictions and the path to COVID eradication outlining the high transmissibility and herd immunity rate needed to control measles.
In August 2021, we published Prisoner’s dilemma and vaccination , which discussed game theory and vaccination. This article is particularly relevant because – while it was talking about policy during the COVID pandemic – it discussed the need for societal co-operation in controlling disease through vaccination and warned against policies that might harm that co-operation. Lastly, on January 9, 2025 we explored the question of vaccine hesitancy and childhood vaccinations in Is vaccination hesitancy on the rise? , finding that the data was unclear. A problematic fact about reported vaccination data is that it is a snapshot in time – and of the recent past, not the present.
Measles is a serious disease
Measles is a serious disease principally for two key reasons. First, it has a high incidence of severe morbidities for those who contract the disease. We will describe the long list of these morbidities shortly. Second, measles has a very high reproductive rate; it spreads easily and quickly through an unvaccinated population.
The reproductive number, or Ro, is one of the ways that we can assess how infectious a disease is. Ro estimates how many new persons will catch a disease directly from one infected person in a population with no previous immunity.[10]The reproductive number is an estimated term, not an absolute one, and may vary somewhat from place to place based on the different behaviours of people and other factors.[11]Measles is typically thought to have a reproductive number of somewhere between 12 and 18, though some researchers have suggested the true range could be larger.[12]
Figure 1 illustrates the reproductive number of measles and a few other diseases along with a curve depicting the vaccination rate that would be required to theoretically eradicate the spread of the disease.[13] [14] [15]This chart assumes that the vaccine perfectly stops the transmission of the disease. Note that because measles is so infectious, it is thought to require between 90 and 95% vaccination rate (or immunity from vaccination and from having survived the disease).
Figure 1 – The estimated reproductive number of certain common infectious diseases and the vaccination rate required for theoretical herd immunity, assuming 100% vaccine effectiveness. Measles has a very high reproductive number and required vaccination rate.[16] [17] [18]
Measles is a respiratory illness and is spread through airborne droplets (from coughing, sneezing, or speaking). It can be contracted through breathing in those droplets, through touching surfaces upon which droplets have settled and then touching the face (transferring infectious matter to the nose, mouth, or eyes), or through direct contact with nasal or throat secretions.[19]Measles is accompanied by a high fever, a rash, Koplik spots, coughing, and conjunctivitis.[20]None of these effects is in any way pleasant, however measles has numerous other potential morbidities, including the following:[21] [22]
-
- Febrile convulsions (a convulsion in a child caused by fever): 2/100 cases
- Pneumonia, otitis media (ear infection): 1/10 cases
- Thrombocytopenia (a disorder of the blood): 1/300 cases
- Encephalitis (often resulting in brain damage): 1/1000 cases
- Death from respiratory and or neurologic complications: 1 to 3/1000 cases
- Subacute sclerosing panencephalitis (a brain disorder): 1/25,000 cases
One final morbidity from measles is an effect called “immunity amnesia”. Measles can, in effect, wipe part or much of the immunity a child has built up. Restoring lost immunities can take years, and the possibility of secondary infections following measles is high.[23]
This boils down to a disease that, if unmitigated, has both high consequence and a high probability of being contracted.
Why people choose to be vaccinated
People choose to vaccinate primarily because they decide it is in their best interest. Parents choose to vaccinate their children when they believe the benefit of the vaccine outweighs its potential harm. We presented a mathematical game theoretic discussion of vaccination decisions in Prisoner’s dilemma and vaccination , which highlighted a few additional terms and drivers behind this decision. A key factor is the chance of contracting the disease. The higher the chance the disease can be contracted, the higher the value of vaccination. Because measles is highly contagious with significant morbidities, the value of vaccine-based immunity is relatively high. Another key factor is the morbidity ratio of potential vaccine-caused negative outcomes versus the negative outcomes from the disease.
Measles vaccine effectiveness and adverse effects
There is likely no perfect vaccine, both from a measure of effectiveness and a lack of adverse effects. The current two-dose MMR measles vaccine is a live attenuated vaccine with an estimated range of effectiveness between 99% and 94% after both doses.[24] [25] [26] [27]There is some data to support a slight waning in the effectiveness of the measles vaccine over long periods of time, on the range of greater than 99% effectiveness immediately after receiving a second dose, down to about 91% more than two decades later.[28]In any case, the effectiveness of the MMR vaccine against contracting measles is high, though not perfect, and is subject to the response of individual patients.
We noted in our August 2021 article Predicting vaccination rates and outcomes for the Delta variant that herd immunity rates must be adjusted by vaccine effectiveness, with the required vaccination rate being defined by the following:
Required vaccination rate = (1-1/Ro) / E, where E is the vaccine effectiveness against transmission.
This creates an even higher required vaccination rate than in our first chart and is shown in Figure 2. The blue dots show the vaccination rates for herd immunity assuming a 95% vaccine effectiveness. This is illustrative only; however, it is not inappropriate for measles. A curve for a 50% vaccine effectiveness is also included.
This curve is not relevant in a discussion of measles, but annual flu vaccines often have such a low effectiveness, and this curve illustrates why influenza is unlikely to be fully stopped by vaccination. In related coverage from June 2021, fellow data scientist and BIG Media contributor Laurie Weston wrote an enlightening piece comparing COVID to the flu: COVID-19 is definitely not the flu .
Figure 2: The estimated reproductive number of certain common infectious diseases and the vaccination rate required for theoretical herd immunity, assuming three levels of vaccine effectiveness. The blue curve is relevant for measles.
Adverse events following immunization (AEFI) for measles have been studied extensively. We show only the serious AEFIs in Figure 3, and they do exist for measles. These include serious conditions such as encephalitis, anaphylaxis, and thrombocytopenia. The chances of these reactions are low, and the risk ratio versus the same or similar morbidities from the disease are high.[29]These high risk ratios create high value for vaccination according to our previous discussion involving game theory. To put it simply, these risk ratios equate to the disease having a much higher chance of hurting an unvaccinated person who contracts it than the vaccine has of hurting the person.
Figure 3 – Chart showing the AEFIs (morbidities) associated with measles, and the risk ratio of those morbidities and those of the disease itself.[30]The incidence of these adverse reactions to the MMR vaccine are quite low, and the risk ratios are quite high.
Why there are measles outbreaks, (1) the disease
The biggest reason that measles outbreaks occur is the nature of the disease. As we showed in Figures 1 and 2, measles spreads easily and requires a nearly perfect vaccination rate to reach eradication. In the case of a disease of this nature, many researchers consider that it cannot be eradicated, only minimized to a rate that society can tolerate.[31]Even slight reductions in vaccination rates can allow local outbreaks of measles, and they may occur before vaccination rate data is clear.
Why there are measles outbreaks, (2) people
People are also responsible for measles outbreaks. This is not blaming the victims of the disease; it is simply a factor we must account for. Some people may refuse the MMR vaccine – though these numbers are few – but others simply cannot have it. Newborn infants cannot be vaccinated, and a small element of the population are advised not to take the vaccine for specific medical reasons. Those persons are dependent on the rest of the population being vaccinated for their protection.
When we spoke about vaccination and game theory in our August 2021 article Prisoner’s dilemma and vaccination, we cautioned that to reach very high vaccination levels, societal co-operation was required. This is because the motivation to vaccinate drops as vaccination rates increase. We note that people vaccinate from their choice for their benefit (or the perceived benefit of their children). To choose to vaccinate in order to protect other people requires other things: goodwill and co-operation.
Why there are measles outbreaks, (3) policy
While the policies of Canada, the U.S., and Europe are strongly in favour of measles vaccination, other policy decisions may have indirectly harmed vaccine uptake for this disease. Coercion in the form of COVID vaccination mandates and pejorative rhetoric aimed at the unvaccinated during COVID may have harmed goodwill and trust regarding vaccination in some people. Damage to goodwill, co-operation, and trust was our concern in the game theory article we mentioned, and has been the subject of some research, although the effects of vaccine fatigue remain somewhat speculative.[32]
Accountability is not blame, but we should all consider it seriously
Measles is a terrible disease. It has several serious effects, and is highly contagious, hurting mostly children. There is a high rational value in vaccination against this disease, though no vaccine is perfect. Outbreaks of measles are likely inevitable, but with trust, co-operation, and a high level of vaccination, these can be minimized. We cannot change this terrible disease, which is the real problem. And we cannot all be vaccinated against it. Ultimately, we need societal goodwill and trust to minimize this disease, and that comes down to us as individuals, and good government policy.
It is important that we consider the importance of trust and goodwill in accounting for measles outbreaks when we formulate other policy that has the potential to injure it.
References
[1] Government of Canada, 2025, Measles and rubella weekly monitoring report, Week 23
[2] Centers for Disease Control and Prevention, June 13, 2025, Measles Cases and Outbreaks
[3] European Centre for Disease Prevention and Control, March 11, 2025, Measles on the rise again in Europe: time to check your vaccination status
[4] Government of Canada, 2025, Measles and rubella weekly monitoring report, Week 23
[5] Government of Canada, 2025, Measles and rubella weekly monitoring report, Week 23
[6] Centers for Disease Control and Prevention, June 13, 2025, Measles Cases and Outbreaks
[7] Centers for Disease Control and Prevention, June 13, 2025, Measles Cases and Outbreaks
[8] European Centre for Disease Prevention and Control, March 11, 2025, Measles on the rise again in Europe: time to check your vaccination status
[9] European Centre for Disease Prevention and Control, March 11, 2025, Measles on the rise again in Europe: time to check your vaccination status
[10] Shaw, Claire, and David Kennedy, 2021, What the reproductive number Ro can and cannot tell us about COVID-19 dynamics, National Library of Medicine, Theoretical Population Biology 137, 2-9
[11] Shaw, Claire, and David Kennedy, 2021, What the reproductive number Ro can and cannot tell us about COVID-19 dynamics, National Library of Medicine, Theoretical Population Biology 137, 2-9
[12] Guerra Fiona, Shelly Bolotin, Gillian Lim, Jane Heffernan, Shelley Deeks, Li Ye, Natasha Crowcroft, 2017, The basic reproduction number (R0) of measles: a systematic review, The Lancet. Infectious Diseases. 17 (12): e420–e428.
[13] Shaw, Claire, and David Kennedy, 2021, What the reproductive number Ro can and cannot tell us about COVID-19 dynamics, National Library of Medicine, Theoretical Population Biology 137, 2-9
[14] Guerra Fiona, Shelly Bolotin, Gillian Lim, Jane Heffernan, Shelley Deeks, Li Ye, Natasha Crowcroft, 2017, The basic reproduction number (R0) of measles: a systematic review, The Lancet. Infectious Diseases. 17 (12): e420–e428
[15] Shabir, Osman, February 16, 2021, What is Ro? News Medical Life Sciences
[16] Shaw, Claire, and David Kennedy, 2021, What the reproductive number Ro can and cannot tell us about COVID-19 dynamics, National Library of Medicine, Theoretical Population Biology 137, 2-9
[17] Guerra Fiona, Shelly Bolotin, Gillian Lim, Jane Heffernan, Shelley Deeks, Li Ye, Natasha Crowcroft, 2017, The basic reproduction number (R0) of measles: a systematic review, The Lancet. Infectious Diseases. 17 (12): e420–e428
[18] Shabir, Osman, February 16, 2021, What is Ro? News Medical Life Sciences
[19] Centers for Disease Control and Prevention, May 23, 2025, Clinical Overview of Measles
[20] Centers for Disease Control and Prevention, May 23, 2025, Clinical Overview of Measles
[21] Centers for Disease Control and Prevention, May 23, 2025, Clinical Overview of Measles
[22] BC Centre for Disease Control, 2009, Relative Risks of Diseases and Immunization, Communicable Disease Control Manual Chapter 2: Immunization, Supporting Documents
[23] BC Centre for Disease Control, 2009, Relative Risks of Diseases and Immunization, Communicable Disease Control Manual Chapter 2: Immunization, Supporting Documents
[24] Government of Canada, April 2015, Measles vaccines: Canadian Immunization Guide
[25] Centers for Disease Control and Prevention, January 17, 2025, Measles Vaccination
[26]Patel, Preeti, and Ellis Tobin, MMR Vaccine, May 5, 2025, National Library of Medicine
[27] Kim, Jerome, Florian Marks, and John Clemens, 2021, Looking Beyond Covid-19 vaccine phase 3 trials, NATURE MEDICINE, 27, 205-211
[28] Robert, Alexis, Anne Suffel, and Adam Kucharski, 2024, Long-term waning of vaccine-induced immunity to measles in England: a mathematical modelling study, Lancet Public Health, V 9, e766-e775
[29]BC Centre for Disease Control, 2009, Relative Risks of Diseases and Immunization, Communicable Disease Control Manual Chapter 2: Immunization, Supporting Documents
[30] BC Centre for Disease Control, 2009, Relative Risks of Diseases and Immunization, Communicable Disease Control Manual Chapter 2: Immunization, Supporting Documents
[31] Fine, Paul, Ken Eames, and David Heymann, 2011, “Herd Immunity”: A Rough Guide, Clinical Infectious Diseases, 2011:52(7): 911-916
[32] Su, Zhaohui, Ali Cheshmehzangi, Dean McDonnell, Claudimar da Veiga, and Yu-Tao Xiang, 2022, Mind the “Vaccine Fatigue”, Frontiers in Immunology, Vol 13, 1-8
