Measles Cases Continue to Rise in the US and Canada

Measles are continuing to make headlines as the U.S. reports 164 cases, 153 of which are from three outbreaks, with the largest occurring in West Texas; and Canada reports 78 cases in its largest outbreak in Ontario in more than a quarter century. Of serious consequence are the deaths from the virus of two unvaccinated children within the last year, one in New Brunswick, Canada, last spring, and one in Lubbock, Texas, last week.

Measles is one of the most contagious diseases known, with an R₀ value of 18. This means that, on average, one person infected with measles will spread it to 18 others in a susceptible population. For comparison, COVID-19 currently has an R₀ value (basic reproduction number) of about 3, meaning one infected person typically spreads it to 3 others. Measles is therefore six times more contagious than COVID-19.  The R0 value indicates how easily an infectious disease spreads in a population with no immunity [1].

Like COVID, vaccines can prevent infection. According to CDC, one dose of the MMR (measles, mumps, rubella) vaccine is 93% effective against measles, with two doses (the recommended dosage) being 97% effective. Additionally, those who receive the MMR vaccination according to the U.S. vaccination schedule are generally considered protected for life against measles. With the schedule being widely followed for years, the U.S. had considered measles eliminated in 2000, which meant any continuous spread of the disease had stopped for at least a year.

But as vaccine uptake began to decline with and following the pandemic, measles cases rose, with a 2024 Chicago outbreak infecting more than 60. In the current outbreaks, nearly all those infected have been unvaccinated or had unknown vaccination status, and the majority of reported cases have been in children.

The reduced uptake is being attributed, in part, to the misinformation disseminated online. But while experts can promote the need for vaccines amidst the severity of the virus – e.g., a person can still become infected two hours after someone with measles has left the room, and 40% of infected persons in the U.S. required hospitalization – science alone does not always persuade those who are vaccine hesitant. Rather it is important to determine why they are hesitant: what are their concerns, what was the source of their information, have they checked its validity, etc. Then it’s a matter of gradually imparting information and advice, preferably by a health care provider, to dismantle the misinformation and skepticism that has been built up.

Although the outbreaks have thus far been fairly localized in largely unvaccinated populations, a number of travel-related infections have also occurred due to its infectiousness. Because of this, it is also important that businesses be aware of the health of their workers. Workers who are ill should be sent home and advised to see a health care provider.

Individuals should also be sure to check the status of your vaccinations and talk with your doctor about any recommended boosters. Remember that vaccinations don’t only protect you, they can protect anyone with whom you come in contact – particularly for disease with high infectious rates, such as measles.

For more information on measles, see TAG’s Infectious Disease Fact Sheet on Measles. And give TAG a call if you need assistance.

[1] The R₀ value of a disease directly determines what percentage of a population needs immunity to achieve herd protection. This relationship follows a simple formula: the herd immunity threshold equals (1 – 1/R₀) × 100%. For highly contagious diseases like measles with an R₀ of 18, approximately 94.4% of the population must be immune to prevent outbreaks. By comparison, COVID-19 with an R₀ of 3 requires about 67% immunity, while seasonal influenza (R₀ ≈ 1.3) needs only around 23%. The higher a disease’s R₀ value, the more challenging it becomes to reach herd immunity, leaving less margin for error in vaccination campaigns. This explains why diseases with extremely high R₀ values like measles can cause outbreaks even when vaccination rates appear relatively high but fall short of the critical threshold, while diseases with lower R₀ values can be controlled with more modest vaccination coverage. Vaccination rates appear relatively high but fall short of the critical threshold, while diseases with lower R₀ values can be controlled with more modest vaccination coverage.

COVID Risk Matrix:

Influenza:

Infectious Disease News:

• Korea has enhanced their entry measures in response to an Ebola outbreak in Africa by requiring that travelers from Uganda, South Sudan, Rwanda, Kenya, the Democratic Republic of Congo, and Tanzania, who exhibit symptoms alert quarantine officials at Incheon Airport. 
• Measles outbreak currently in Texas: As of Feb 28^th, there were 146 cases, 20 hospitalizations, and 1 death reported. Additional cases are expected. 
• The CDC has published estimates of the 2024–2025 seasonal influenza vaccine effectiveness (VE), from October 2024–February 2025. Interim VE among children and adolescents aged <18 years ranged from 32-60% in the outpatient settings and against influenza-associated hospitalization, 63-78%. Among adults aged ≥18 years, VE in the outpatient setting was 36% and 54% in two networks and was 41% and 55% against hospitalization in two networks. 
• VE of 2024–2025 COVID-19 vaccine was 33% against COVID-19–associated emergency department or urgent care visits among adults aged ≥18 years and 45%–46% against hospitalizations among immunocompetent adults aged ≥65 years, compared with not receiving a 2024–2025 vaccine dose. VE against hospitalizations in immunocompromised adults aged ≥65 years was 40%. 
• The UK Health Security Agency has warned there is a new strain of norovirus circulating, which means people who have already fallen ill with the bug this winter could be infected again. The GII.17 genotype of norovirus is the predominant strain, but they are also seeing cases linked to the older GII.4 variant emerge. 
• COVID-19 survivors have a significantly higher rate of ongoing symptoms than those who had influenza, according to a recently published study, that covered 7 US Sites, in 2021–2023.