Resurgence of measles in the U.S

And why it can be a real problem for the country

If you find Complexity Thoughts interesting, follow us! Click on the Like button, leave a comment, repost on Substack or share this post. It is the only feedback I can have for this free service. The frequency and quality of this newsletter relies on social interactions. Thank you!

---

→ Don’t miss the podcast version of this post: click on “Spotify/Apple Podcast” above!

In recent months of 2025, the U.S. has experienced a surprising uptick in measles cases, with over 600 reported in just about 90 days. This stands at roughly twice the number of cases historically recorded over 2024.

Today, we try to understand the (predictable) dynamics behind this resurgence by examining epidemiological data, vaccination coverage and the role of human mobility networks.

The near past

Historically, following the 1989 outbreak, public health authorities implemented a second-dose vaccination strategy which contributed significantly to controlling the spread of measles. By 2000, measles was declared eliminated in the U.S. after showing an absence of sustained transmission (i.e., no continuous spread over 12 months). However, the reappearance of significant outbreaks in 2014 and 2019 raised concerns over long-term immunity and the potential for the disease to regain a foothold.

Number of measles cases between 1985 and 2025 in the U.S.

Current outbreak: numbers tell a story

When we compare current data to historical records, an alarming pattern emerges:

• Rapid growth: the 600+ cases recorded in 90 days during 2025 are comparable to the total number for the entire year in 2014 and nearly half of the annual cases in 2019.

• Sustained process: the weekly case counts over the last two years suggest that the outbreak is not “just” a short-term flare-up, but may be driven by a continuous underlying process.

This raises pressing questions: why is measles, once eliminated, now re-emerging with such persistence? The answer is intertwined with vaccination coverage and the complex patterns of human behavior (namely, mobility and interaction).

Number of measles cases since 2000 (when it was declared eliminated) in the U.S.

Weekly number of measles cases [by rash onset date] in the last 2 years, in the U.S. (as of April 3, 2025)

Vaccination coverage and the herd immunity threshold

For diseases like measles, which are highly contagious, the herd immunity threshold (HIT) is critical.

How herd immunity works. Figure from Wikipedia

This threshold is determined by the basic reproduction number, R_0 (to understand this number and the graph below, I recommend to follow this interactive visualization from The Guardian).

[I have prepared a short video about an interactive experiment that you can do as well (see the next section and end of this post for references)]

Under some simplified assumptions, the HIT is related to R_0 as:

\(\tau_\text{c} = 1 - \frac{1}{R_0}\)

In practice, the above formula suggests that, for a R_0 equal to 15-18, at least 93–95% of a population must be immunized to prevent measles outbreaks reliably. Recent data comparing vaccination rates across states in two periods — 2009–2010 and 2023–2024, see the two figures below — highlight a concerning shift. While only five states had vaccination rates below 90% in the earlier period, meaning that the majority of the country is above the HIT, a new analysis shows that only about a dozen states now exceed the 95% threshold needed to maintain herd immunity.

Take Texas as an example: it is reported to have vaccination levels between 90% and 95%, which is precariously below the herd immunity threshold. This coverage gap creates fertile ground for localized clusters of infection, setting the stage for a widespread outbreak.

Data corresponding to 2023-2024

Data corresponding to 2009-2010

The role of human mobility

Measles does not spread in isolation:

It moves along with the flow of human interactions. Recent network studies that capture human mobility flows between U.S. counties illustrate how interconnected our communities are. In a complex system like a nation with highly mobile populations, even local drops in vaccination coverage can trigger chains of transmission that propagate widely.

Local clusters in states like Texas can spark outbreaks in neighboring regions, especially when public health policies face additional pressures, such as administrative changes from a new government. Given the overall coverage situation as of 2023-2024, and the highly interconnected human flows in the country, we should be concerned.

Networks capturing human mobility flows between distinct counties in the U.S. [Source]

Implications for public health policy

The resurgence of measles in 2025 serves as a stark reminder of the delicate balance between pathogen dynamics and vaccination policies. With many states failing to reach the crucial 95% coverage, the potential for sustained outbreaks increases. In a networked (and hyperconnected) society, localized lapses in vaccination not only endanger individual communities but can also compromise national public health due to the seamless connectivity between regions.

The current measles outbreak in the U.S. is a prime example of how complex systems science can help to gain insights about and illuminate the pathways through which infectious diseases spread, even those once thought to be under control.

Therefore, understanding these complex interactions is not just an academic exercise: it has real implications for public health policy and the safety of our communities. One thing is clear: maintaining and surpassing the 95% vaccination threshold is essential to sustaining herd immunity and preventing the resurgence of diseases like measles.

---

Supplementary information

• Epidemiological data source: CDC

• To play and learn with interactive visualizations and simulations you can visit the Complexity Explorables website:

  • Click here for the interactive simulation about network vaccination

• Click here for the interactive simulation about how herd immunity works