“If the number of measles in 2025 is above X, then the market resolves to Yes. Outcome verified from CDC…. If this event occurs, the market will close the following 10am ET. Otherwise, it closes by Dec 31, 2025 at 11:59pm EST.”
On its face, the question is straightforward: How many cases of measles will there be in 2025?
As of December 23, 2025, the CDC has reported that 2,012 cases of measles have been confirmed this year. Interestingly, the Johns Hopkins measles tracker currently reports that there have been 2,023 confirmed cases of measles as of December 26, 2025.
And as of writing this, the odds for over 2050 confirmed cases stands at only 63%. But why?
The Market in Perspective
The Kalshi measles market for 2025 started at 100 cases. Until this year, this was a pretty reasonable ceiling for a measles market. In 2024, the US had 285 confirmed cases, the year before that 59. And before 2010 cresting 100 was an anomaly, instead of the norm for almost a decade.
Since 2000, the measles virus has been considered “eliminated” in the United States. What fewer readers may know is, two years earlier in 1998, measles was considered “eliminated” in Canada. (Elimination is defined as an interruption of endemic measles transmission for greater than or equal to 12 months.) As of November 10, 2025, Canada’s measles elimination status was revoked after almost 27 years.
Obviously, this is not great.
So, what happened?
A Measles Explosion
South Carolina
Between December 5 and December 9, there were 27 new confirmed cases of measles in South Carolina, meaning that, on average, between those dates, approximately 6.75 people contracted confirmed cases of measles per day.
As of December 10, news sites began to cover an “accelerating” outbreak in South Carolina, with hundreds of people reported to be in quarantine at that time. The outbreak itself appears to have originated from a single church in Inman (Way of the Truth Church) and multiple schools in Spartanburg County.
As of December 10, there were 111 confirmed cases of measles in South Carolina since July 9, 2025. Of those 111:
105 people were unvaccinated, while three people were only partially vaccinated;
254 people had been placed in quarantine; and
16 were currently in isolation.
Regarding demographics, of the 111 cases:
20 were under the age of 5;
75 were between the ages of 5-17;
10 were over the age of 18; and
6 minors under the age of 18 were reported with confirmed cases of measles.
On December 12, reported cases had risen by 15 more people (since the previous Tuesday, 12/9/25) bringing the total up to 126, with 303 people quarantined and 13 in isolation.
Utah
Meanwhile, in Utah, as of December 23, 2025, there have been 142 cases. Of the 142 cases, 102 occurred in Southwest Utah, 134 were in people who had not received the measles vaccine, and 85 were under 18.
The consistent thread between almost all of the outbreaks has been that the main victims have primarily been unvaccinated children.
Arizona
The measles outbreak in Mohave County, Arizona (which as of 2024, was reported to have a 78.4% vaccination rate for the MMR vaccine), appeared momentarily at the start of December to potentially be puttering out. However, as of December 29, 2025, the website had updated, reporting 195 confirmed cases in Arizona, with new cases being actively confirmed on a weekly basis.
Texas
Earlier this year, in Texas, there were 762 confirmed cases of measles reported by Texas’s HHS. Thankfully, as of August 18, 2025, Texas’s DSHS announced that the West Texas measles outbreak had officially ended. What we also learned from this outbreak was that most of the cases were clustered in low-vaccination areas within Texas. We also know that it’s likely that the measles virus essentially crept up from Texas and across New Mexico, then through Arizona, and then finally made its way to Utah, since August.
Predicting the Spread
According to CDC guidelines, for there to be effective herd immunity from measles via vaccination, 95% of a population must be vaccinated against the measles virus. Currently, the estimated percent of South Carolina residents who are vaccinated is at 92.1%; the estimated percent of vaccinated Arizona residents is 89.3%; and the estimated percent of Utah residents who are vaccinated is 88.8%.
However, it’s important to keep in mind that these percentages do not represent an even spread across the state. Instead, you’ll find large communities with over 95% vaccination rates and higher, next to extremely small communities with vaccination rates that in some cases may be almost 0%.
When epidemiologists attempt to predict the proportion of an at-risk population that is likely to contract a pathogen within a specific time frame, they calculate something known as an “attack rate.”
The attack rate is defined as:
Attack Rate = Number of new cases /Total exposed at-risk population
(Note: An “at-risk” person is defined as a person with no immunity against the pathogen.)
The attack rate for measles is 90%. This means that there is a 9 in 10 chance that you’ll get sick with measles if you’re unvaccinated or no longer have the titers against the measles virus and are exposed to measles.
Next, measles has an R0 of ~12-18. R0 is another number used in epidemiology, which is used to express the number of people a single sick person will infect on average.
R0 < 1 typically indicates that the pathogen is likely to die out.
R0 = 1 indicates that the disease is endemic to an area or “Stable.”
R0 > 1 implies that the disease is likely to spread in the population.
To put things into perspective, the flu has an R0 ranging from 0.9 to 2.1.
A person with measles is contagious approximately four days before they begin to show a rash. Additionally, measles has been reported to remain infectious for approximately 2 hours in enclosed spaces without high-efficiency airborne-contamination removal systems, i.e., most places you normally go in public.
This depressingly growing number of cases of human tragedy, and the circumstances of their creation, ironically, also creates a dataset, which should make the math for trading on this market somewhat trivial. However…
While we’ve been tracking this outbreak for weeks and our model has consistently predicted the reported growth in confirmed measles cases, we also noticed something important which led to us holding off on sharing our findings earlier.
Even though the title of the Kalshi market would suggest that, by accurately predicting the number of confirmed measles cases in 2025, a trader could successfully anticipate the growth of these cases and therefore make informed trades, there are three large hiccups.
The rules of the market state that: The Last Trading Date of the Contract will be the sooner of the date of the first 10:00 AM ET following the occurrence of an event that is encompassed in the Payout Criterion or the last day of <year>. The Last Trading Time will be 11:59 PM ET;
The CDC website does not update in real time or even on a regular basis. In fact, it will often also add cases that occurred in the past, several weeks later; and
The measles virus takes approximately 7-14 days to incubate before an infected person begins to show signs of infection.
This means that it actually does not matter what the true number of confirmed cases for 2025 will be. Instead, the outcome of the market is dependent on whether the CDC website updates with an accurate number for confirmed cases in 2025 in time for the market’s resolution.
This turns what should’ve been a guarantee that there will be over 2,050 confirmed cases of measles this year, into a near coin flip. Because the true number of confirmed cases for 2025 will not be able to be ascertained until after the Kalshi measles market has already resolved, and because it’s not clear that even if there was a huge surge of confirmed cases between December 23rd and December 31st that the CDC website will update its results in time.
With all that said, Happy New Year folks. Stay vaccinated and get your titers checked, because 2026 is going to be a great year for measles.
