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Question: what will be the maximum weekly increase in confirmed U.S. measles cases over the 12-week window May 21–Aug 6, 2026. Both forecasters note CDC data (1,842 cumulative cases as of May 7) and agree weekly increases have trended down over the last month (96 → 28) and that summer/school closures generally reduce transmission. They model a baseline weekly mean in the ~15–25 range (one centers around ~20, the other ~25) and expect the maximum over 12 weeks to most likely fall in the roughly 30–50 range (mode ≈33), with the highest probabilities concentrated around 30–45. Both teams explicitly allow for a fat right tail from localized outbreaks (international travel, summer camps, or hotspots like Utah), which could push a week up into the 60–120+ range though this is treated as lower probability. One forecaster emphasizes the max might occur in the early weeks if the downward trend continues, while both maintain substantial uncertainty driven by chance outbreaks.
Forecast: Probability distribution:
We are forecasting the maximum (not typical) week-over-week increase in the national 2026 YTD confirmed measles cumulative count across 12 weekly windows ending May 21 through Aug 6, 2026, using CDC if it issues on-time weekly updates (otherwise JHU if it posts Thursday-specific updates). As of the CDC’s May 7 update, the national cumulative total is 1,842, and multiple analysts cite that late-April to early-May week-over-week increases were already down to the few-dozen range (e.g., around +28 from Apr 30 to May 7 per USAFacts/CDC-derived figures). This deceleration, plus typical seasonal decline from late spring into summer (school-year ending), suggests the distribution of weekly increases during the target window is centered around a few tens of cases.
However, the target is the maximum across 12 weeks, which pulls the expected value upward relative to a single-week forecast, and measles outbreaks are overdispersed (localized flare-ups can create occasional “spike weeks”). The resolution rules also reduce artificial spikes from missed updates (a missed week breaks consistency and invalidates adjacent weekly deltas for that source), which tempers the extreme upper tail from pure reporting catch-up.
Comparing analysts: Forecaster 2 appears to rely heavily on rash-onset curves and Utah-specific figures that conflict with other provided context, producing a likely-understated maximum. Forecaster 1 allows substantial mass above 200, which seems too high given (a) observed deceleration and (b) the anti-catch-up consistency rules. Forecasters 3 and 4 are closer to the implied recent CDC step sizes and correctly account for a max-of-12 effect, but I keep a somewhat heavier upper tail than Forecaster 4 to reflect the chance of a summer cluster/outbreak generating a single high week. Net: median around the high-30s, with a meaningful but not dominant chance of a week in the ~70–100 range, and a small tail to ~140.
An amateur forecast is likely to (1) extrapolate from the very high YTD total (1,842) and assume large weekly gains persist, or (2) extrapolate only the most recent weekly delta and forget the question asks for the maximum across 12 weeks. This forecast improves by anchoring on the most relevant recent CDC week-to-week increments (few-dozen), incorporating seasonality and outbreak deceleration, explicitly adjusting upward for the “maximum of 12” effect, and limiting the extreme tail because the consistency rules make missed-update catch-up spikes less likely to count. Confidence in improvement: moderate (upper-tail uncertainty remains due to outbreak stochasticity).
Forecast rationale (numeric):
— Iteration 1 — The forecasts converge on a moderate summer peak rather than a renewed spring-level surge. They anchor on the recent national case trajectory—roughly +50 over 14 days, or about 25 per week as of early May—and interpret that as evidence of a post-spring slowdown. Seasonal dynamics, school closures, and the apparent winding down of a major outbreak all point to further deceleration through late May to early August.
Because the metric is the highest weekly increase over a 12-week window, both forecasts lift the central estimate above a “typical” week, putting the most likely maximum in the low-to-mid 40s, with much of the plausible range roughly in the 20s to 80s. They also agree that the distribution should be right-skewed: measles clusters are overdispersed, and earlier 2026 saw much larger weekly jumps, so a rare spike remains possible even if summer conditions are calmer.
On the upper tail, both keep most probability below 200 cases in a week, but preserve a small chance of very large weekly increases because of cluster dynamics and the historical precedent of large spikes. The main difference is emphasis: one rationale gives less weight to reporting batch effects and expects extreme weekly maxima to be rarer, while the other allows a somewhat broader upper tail from outbreak flare-ups or batched confirmations. Overall, the shared view is of a decelerating outbreak with occasional spike risk, but a maximum weekly increase most likely well below the spring peaks.
— Iteration 2 — Across the forecasts, the main reasoning is that U.S. measles case growth is likely to continue, but at a slower pace than the early-2026 surge, with the highest weekly increase in the 5/21/2026–8/6/2026 window most likely coming from a moderate outbreak-driven spike rather than a sustained national acceleration.
The collective reasoning suggests that the highest weekly increase during this period is most likely to be moderate rather than extreme, driven by the tail risk of a localized outbreak amid an otherwise slowing, seasonal decline. The forecasts agree that continued accumulation of cases is likely, but that very large one-week jumps are possible mainly as outliers rather than the base case.
— Iteration 3 — The forecasts converge on a moderate but spike-prone summer trajectory for weekly U.S. measles case increases. The main reasoning is:
The collective reasoning expects summer declines overall, but with enough outbreak volatility that the single largest weekly increase is still likely to land around ~90–100, with occasional spikes well above that remaining possible.
Summary of Forecasting Rationale
Current Trajectory and Seasonality Forecasters broadly agree that while the U.S. is experiencing a historically severe measles outbreak in 2026, the national transmission rate is actively decelerating. The massive South Carolina cluster has concluded, leaving Utah as the primary active hotspot. Furthermore, historical epidemiological patterns indicate that the end of the school year in late May and June will significantly reduce community contact rates. Consequently, the baseline number of true weekly cases is expected to taper off throughout the summer, meaning the highest underlying transmission will most likely occur in the earliest weeks of the forecast window.
Data Reporting Artifacts and Batching Because the prompt asks for the maximum single-week increase across 12 candidate weeks, rationales emphasize the statistical noise inherent in public health data. Forecasters highlight administrative “batch reporting” as a primary driver of artificial weekly spikes. State health departments frequently delay data submissions due to staffing shortages, lab backlogs, or summer holidays (such as Memorial Day and Independence Day). When these departments finally dump multiple weeks of backlogged cases into the CDC’s system at once, it registers as a massive single-week surge, regardless of the true epidemiological curve.
Summer Clusters and Alternative Vectors Although general transmission drops during the summer, forecasters note that alternative vectors keep the right-tail risk elevated. Congregate settings such as summer camps, large religious gatherings, and international travel frequently serve as catalysts for localized super-spreader events within under-vaccinated communities. Historical analogs from the 2019 and 2025 outbreaks heavily inform this view, demonstrating that summer activities can easily spark isolated, high-volume flare-ups.
Conclusion The rationales synthesize these opposing forces: a naturally decaying outbreak baseline countered by the high likelihood of an acute, isolated spike. Forecasters base their expectations on the near certainty that over a 12-week period, at least one week will capture an administrative data dump, standard reporting variance, or a localized summer camp cluster.
Question: Highest weekly increase in U.S. confirmed measles cases (5/21/2026 to 8/6/2026) Final Prediction: Probability distribution:
Total Cost: extra_metadata_in_explanation is disabled Time Spent: extra_metadata_in_explanation is disabled LLMs: extra_metadata_in_explanation is disabled Bot Name: extra_metadata_in_explanation is disabled
Forecaster 1: Probability distribution:
Forecaster 2: Probability distribution:
Forecaster 3: Probability distribution:
The research provides an analysis of the trends in confirmed measles cases in the U.S. from May 21 to August 6, 2026, highlighting a significant deceleration in weekly case growth. As of May 14, 2026, there were 1,842 confirmed cases, representing approximately 80% of the total cases reported in 2025 and the highest year-to-date count since 2000. The report details a decline in weekly increments from a peak of over 100 cases per week in early 2026 to around 22-28 cases per week in early May. The largest outbreak in South Carolina has ended, while the ongoing outbreak in Utah is also showing a decline in new cases. The research notes that seasonal patterns typically favor a reduction in measles transmission during the summer months, suggesting that weekly increments will likely decrease further.
Additionally, the report identifies potential risk factors that could lead to higher weekly increases, including the emergence of new outbreaks in unvaccinated communities, increased travel during the summer, and reporting delays. The analysis concludes with a prediction of the most likely range for maximum weekly increases during the specified period, estimating a central range of 20-40 cases, with the possibility of spikes if new outbreaks occur.
Sources used in the research include:
Cumulative 2026 YTD count: 1,842 confirmed cases (CDC update of May 7, 2026) [11]. This already represents ~80% of 2025’s full-year total of 2,288 cases, and is the highest YTD count by this calendar point since 2000 [2][5].
The data clearly shows a strong deceleration in weekly case growth over the past two months:
| Approximate Week Ending | Cumulative Total | Weekly Increment | % Increase |
|---|---|---|---|
| Early Jan–Feb 2026 | — | 100+ cases/week | Peak period |
| ~April 3, 2026 | ~1,671 | — | ~6.1% |
| ~April 9, 2026 | 1,748 | 7 cases | 0.40% [23] |
| ~April 24, 2026 | 1,792 | ~44 (over ~2 wks) | — |
| ~May 1, 2026 | 1,814 | 22 cases | ~1.2% [1] |
| ~May 7, 2026 | 1,842 | 28 cases | ~1.5% [4] |
The CDC’s own weekly epi chart showed only 7 new cases by onset date in the week starting May 3, 2026 [11]. The discrepancy between “7 new cases by onset” and “28-case cumulative increase” reflects reporting lags — cases with earlier onset dates being confirmed and added later.
South Carolina outbreak (largest overall): ENDED — Declared over in April 2026 at 997 cumulative cases [question background]. This removes the single largest driver of weekly increments.
Utah outbreak (largest currently active): DECELERATING — 663 total cases as of May 12, but only 29 new cases in the prior 3 weeks (~10/week) [12], down dramatically from a peak of 56 new cases/week in mid-March [24]. The outbreak is concentrated in the FLDS community in southwestern Utah [13].
New outbreaks: Three new outbreaks declared between April 23 and May 7, bringing the 2026 total to 25 [11]. However, these appear to be small clusters so far.
Other states: Arizona at 96 cases [4], Florida at 135 [5], Texas at 182 [2] — none showing explosive growth trajectories.
Strong seasonal pattern favors continued decline during the question window:
2025 Texas outbreak reference: The Gaines County outbreak (762 cases) had its highest density of cases in March–April 2025, consistent with the spring peak pattern [question background].
Weekly increments during summer in outbreak years: Based on the 2019 pattern, weekly increments in the May–August window ranged from ~50/week (late May) down to 1–5/week (July–August). In the current 2026 wave, weekly increments are already much lower than 2019’s spring peak, suggesting summer weekly increments will likely be in the 10–40 range for late May/early June, potentially dropping to single digits by July–August.
New outbreak in unvaccinated community: The main tail risk. Communities with <80% vaccination coverage (e.g., parts of Utah at 78.5% kindergarten coverage [13]) remain vulnerable. A spark in a large, clustered unvaccinated population could generate 50–100+ cases in a single week.
Summer travel: The CDC warned that increased spring/summer travel may drive further spread [24]. International travel could seed new outbreaks.
Reporting artifacts: Large batches of delayed case confirmations could inflate a single week’s cumulative increase, though the question’s rules attempt to mitigate multi-week catch-ups.
Existing under-detection: CDC acknowledged probable cases are not included [23], and wastewater surveillance has detected silent spread [4], suggesting the true burden may be higher than reported.
Most likely range for maximum weekly increase (May 21 – Aug 6):
Lower bound (~10–20 cases): If current deceleration continues and no new significant outbreaks emerge. The Utah outbreak continues to wind down, summer seasonality kicks in, and weekly increments stay in the low double digits or single digits. The maximum would occur in the earliest weeks (late May/early June).
Central estimate (~20–40 cases): The maximum weekly increase likely occurs in late May or early June, before summer fully suppresses transmission. Current weekly CDC cumulative increments are running at 22–28 cases/week (late April to early May) [1][4], and these may decline further but the first few weeks of the window could still see increments in this range.
Upper tail (~50–100+ cases): A new outbreak in an unvaccinated community or a summer travel-related cluster could produce a spike week. While plausible, this becomes less likely as the summer progresses.
Key reference points:
The question covers a 12-week summer window during which measles cases are historically declining. The two largest outbreaks (South Carolina, Utah) have either ended or are decelerating sharply. Weekly cumulative increments have fallen from 100+/week in January–February to ~20–30/week by early May. The maximum weekly increase in the question window will most likely be in the 20–40 case range, occurring in the earliest weeks of the window, unless a significant new outbreak emerges. The primary tail risk is a new outbreak in an unvaccinated community, which could push the maximum into the 50–100+ range. Seasonal patterns strongly favor continued decline through July and August.
Looking at this forecasting question, I need to analyze what the highest weekly increase in U.S. confirmed measles cases will be during the May 21 to August 6, 2026 window.
(a) Time left: About 12 weeks until the final measurement date of August 6, 2026.
(b) Outside view - Base rates:
(c) Inside view - Key evidence:
(d) Outcome if nothing changed: ~28 cases (current weekly increment)
(e) Outcome if trend continued: Maximum likely 25-32 cases in late May/early June, then declining to single digits by August
(f) Expert/market expectations: No specific prediction market for this metric. CDC warns U.S. “highly likely to lose measles elimination status this fall,” implying continued case accrual but not specific to weekly maximums.
(g) Low outcome scenario: All current outbreaks end rapidly, no new outbreaks emerge, reporting slows significantly. Seasonal effects stronger than expected. Maximum weekly increase stays at 15-20 cases.
(h) High outcome scenario: A new outbreak emerges in a large unvaccinated community (similar to early 2026 or the Gaines County outbreak), causing a spike week of 80-150+ cases despite summer timing. Summer travel seeds new clusters.
Rationale:
Starting from the base rate of current weekly cumulative increments (22-28 cases), I adjust for:
Declining trajectory: The trend is clearly downward due to seasonality and ending outbreaks. This suggests the maximum likely occurs in the earliest weeks of the window.
Maximum over 12 weeks: We’re looking for the single highest week, not the average. Even with declining trends, volatility (as shown by the 7-case week vs 28-case week) means the maximum will be near the upper recent range.
Reporting lag effect: Cumulative increments tend to exceed onset-date-based counts due to confirmation lags, which could sustain increments even as true transmission declines.
Outbreak risk: While reduced in summer, there remains non-trivial risk of a new outbreak in undervaccinated communities that could spike a single week to 50-100+ cases.
Upper bound: The question creator’s suggestion that values above ~200 are unlikely aligns with even peak 2026 weekly increments being around 100+.
Central estimate: The maximum weekly increase is most likely 30-45 cases, occurring in late May or early June. There’s meaningful upside risk if a new outbreak emerges.
“ Percentile 0.1: 10 Percentile 1: 14 Percentile 5: 18 Percentile 10: 21 Percentile 20: 25 Percentile 30: 28 Percentile 40: 32 Percentile 50: 36 Percentile 60: 42 Percentile 70: 50 Percentile 80: 62 Percentile 90: 90 Percentile 95: 125 Percentile 99: 175 Percentile 99.9: 198 “
Looking at this forecasting question about the maximum weekly increase in U.S. confirmed measles cases from May 21 to August 6, 2026.
(a) Time left: Approximately 12 weeks until August 6, 2026, when the window closes.
(b) Outside view - base rates:
(c) Inside view - key evidence:
(d) Outcome if nothing changed: ~28 cases (matching most recent weekly increment)
(e) Outcome if current trend continued: Maximum of ~25-30 cases in late May/early June, declining to single digits by late summer
(f) Expert/market expectations: Analysis warns U.S. is “highly likely to lose measles elimination status,” but no specific forecasts on weekly maximums; Polymarket engagement focuses on annual totals
(g) Low outcome scenario: All current outbreaks wind down rapidly with no new clusters; strong summer seasonality drives transmission to near-zero; maximum weekly increase stays around 10-15 cases
(h) High outcome scenario: New outbreak emerges in a large unvaccinated community (similar to Utah FLDS community); summer travel seeds multiple clusters; reporting backlogs create a spike week of 80-150+ cases
Starting from base rates: The 2019 outbreak year saw weekly maximums of ~80-90 during spring, declining to single digits in summer. Current 2026 weekly increments (~22-28) are already much lower than early-2026 peaks of 100+/week.
Key adjustments:
The median should be modestly above current weekly increments (~28) since we’re seeking the maximum across 12 weeks, but the strong seasonal decline and ending of major outbreaks argues against dramatic spikes. The distribution should have a meaningful right tail for new outbreak risk.
“ Percentile 0.1: 6 Percentile 1: 10 Percentile 5: 14 Percentile 10: 17 Percentile 20: 22 Percentile 30: 26 Percentile 40: 30 Percentile 50: 34 Percentile 60: 40 Percentile 70: 48 Percentile 80: 58 Percentile 90: 78 Percentile 95: 105 Percentile 99: 155 Percentile 99.9: 195 “
Looking at the critic’s feedback, I need to carefully evaluate each point and adjust my forecast accordingly.
1. Lower Tail Too Aggressive: The critic makes a compelling point. My P10 of 16 implied a 10% chance that every single week across all 12 opportunities stays below 16 cases. With:
This was indeed too optimistic. The reporting lag point is particularly important - even if true transmission stopped today, confirmations of earlier-onset cases would continue inflating weekly counts for several weeks into the question window.
2. Max-of-12 Statistical Effect: The critic correctly notes that with ordinary variance, the maximum across 12 opportunities should exceed the typical value by more than ~20%. My median of 33 (vs current ~28) was slightly understated.
3. Summer Travel: Memorial Day (first week of window) and July 4th travel could seed transmission opportunities I underweighted.
The most likely scenario remains: maximum weekly increase occurs in late May/early June (weeks 1-4 of the window) at roughly 28-38 cases, before declining through July-August. But I should:
I’ll move my distribution closer to the critic’s suggestion while being slightly more conservative on extreme lower bounds (I believe very low maxima are still possible if all outbreaks resolve quickly).
Key considerations for final calibration:
Updated prediction: Percentile 0.1: 5 Percentile 1: 10 Percentile 5: 15 Percentile 10: 18 Percentile 20: 23 Percentile 30: 27 Percentile 40: 31 Percentile 50: 35 Percentile 60: 41 Percentile 70: 49 Percentile 80: 61 Percentile 90: 80 Percentile 95: 108 Percentile 99: 158 Percentile 99.9: 195
Agent Forecast (discrete, confidence: medium)
Base Rate: Recent 6-week CDC weekly increments in 2026: +96 (Apr 3 peak), +43, +34, ~+44, +22, +28 (May 7). Texas 2025 outbreak analog (~762 cases) peaked in spring and decayed to near-zero by August. Expected base-case range for max over 12 summer weeks: 30–55, with fat right tail conditional on new outbreak emergence.
Initial Reasoning:
Key Uncertainties:
Evidence Sources (10):
Grounding Facts (passed to reviewers):
Draft: 36: 2.5%, 37: 2.4%, 35: 2.4%, 38: 2.4%, 32: 2.3%
Reviewer Panel:
Reviewer (reasoning)
The draft forecast leans heavily on the Texas-2025 analog and an assumed “right tail” for new large outbreaks without critically assessing how summer travel patterns, reporting lags, and source‐window rules could mute or inflate observed weekly jumps. It also glosses over the fact that CDC didn’t publish new updates in many of the specified windows, potentially invalidating early periods or forcing reliance on the secondary tracker with different update timing. Finally, the choice of a 35–45 modal range seems arbitrarily decoupled from the 28-case week ending May 7 and ignores historical base rates for summer peaks after April.
Objections:
Reviewer (calibration)
The forecast appears to underweight the potential for a significant new peak in weekly cases, despite acknowledging the CDC’s explicit warning about increased summer travel and large events leading to more cases. While recent deceleration is noted, the question asks for the maximum weekly increase, which could easily be driven by a single large, new outbreak rather than the prevailing trend. The comparison to the Texas 2025 outbreak might also be misleading given the significantly higher overall case count in 2026.
Objections:
Reviewer (fact-checker)
1 factual error(s). 5 unsupported claim(s). Verdict: minor_issues
Unsupported claims:
Objections:
Adjustment Rationale:
Holding firm. The two substantive reviewers point in opposite directions: the reasoning reviewer claims the right-tail (~11% for max>100) is too high and should be ~4%; the calibration reviewer claims the same right-tail is too low given the CDC’s summer travel warning and 25 active outbreaks. This bracketing supports the current estimate. The fact-checker flagged only minor unsupported phrasing, not factual errors that move the forecast. The forecast is anchored to a max-of-12 statistic with a current trajectory of +22/+28 cases/week and clear deceleration, balanced against meaningful outbreak risk through summer — yielding a modal 35–45 with a right tail (~12% above 100) that reflects, but does not overweight, new-outbreak risk. CDC update-timing concerns are handled by the resolution rules themselves (invalid pairs excluded, not the question annulled unless all 12 are missing), so they don’t shift the distribution.
forecast_sdk | 679.8s | research $1.1842 + reviewers $0.0113 + reconcile $0.2064 = $1.4019
Extracted Percentile Values: {0.1: 6, 1.0: 14, 5.0: 22, 10.0: 28, 15.0: 33, 20.0: 38, 25.0: 42, 30.0: 46, 35.0: 50, 40.0: 53, 45.0: 56, 46.0: 57, 47.0: 58, 48.0: 59, 49.0: 60, 50.0: 61, 51.0: 62, 52.0: 63, 53.0: 65, 54.0: 66, 55.0: 68, 60.0: 74, 65.0: 82, 70.0: 90, 75.0: 100, 80.0: 112, 85.0: 128, 90.0: 148, 95.0: 185, 99.0: 275, 99.9: 420}
Model’s Answer:
I need to forecast the maximum single-week increase in cumulative confirmed measles cases across 12 candidate weeks ending on Thursdays from May 21 to August 6, 2026. This is not the average or total – it’s the single highest weekly jump.
Recent Weekly Increases (from research):
Current Status:
Seasonal Factors:
Assumption 1: No major new outbreak emerges comparable to South Carolina/Utah
Assumption 2: School closures reduce transmission as historically expected
Assumption 3: Utah outbreak continues decelerating or stabilizes
Low Outcome (Maximum ~20-35 cases):
Medium Outcome (Maximum ~50-80 cases):
High Outcome (Maximum ~100-170 cases):
Very High Outcome (Maximum >200 cases):
Given 12 weeks of observations:
The baseline expectation is that weekly increases will stay in the 20-50 range most weeks, reflecting seasonal decline and continued deceleration of current outbreaks. However, the maximum over 12 weeks will naturally be higher than the typical week. The CDC’s explicit warning about summer travel risk and the recent declaration of 3 new outbreaks suggests non-trivial probability of at least one week with elevated cases.
I’m setting my median at ~58 cases, reflecting that even in a declining environment, the single highest week across 12 candidates will likely exceed recent averages. My distribution has significant right-tail weight to account for outbreak uncertainty – measles is highly contagious (R0 12-18) and even small pockets of unvaccinated individuals can generate rapid local spread.
Percentile .1: 6 Percentile 1: 14 Percentile 5: 22 Percentile 10: 28 Percentile 15: 33 Percentile 20: 38 Percentile 25: 42 Percentile 30: 46 Percentile 35: 50 Percentile 40: 53 Percentile 45: 56 Percentile 46: 57 Percentile 47: 58 Percentile 48: 59 Percentile 49: 60 Percentile 50: 61 Percentile 51: 62 Percentile 52: 63 Percentile 53: 65 Percentile 54: 66 Percentile 55: 68 Percentile 60: 74 Percentile 65: 82 Percentile 70: 90 Percentile 75: 100 Percentile 80: 112 Percentile 85: 128 Percentile 90: 148 Percentile 95: 185 Percentile 99: 275 Percentile 99.9: 420
Median estimate for the highest weekly increase in U.S. confirmed measles cases (May 21–August 6, 2026): 52 cases
Baseline trajectory is decelerating sharply. The outbreak peaked at ~295 cases/week in early January 2026 and has fallen to ~28/week by early May. South Carolina (997 cases, the largest cluster) ended in late April. Utah, the dominant remaining outbreak, has tapered from ~60/week in April to ~12/week by mid-May. The consensus baseline entering the forecast window is 25–40 cases/week.
The “maximum over 12 weeks” effect lifts expectations above the current weekly rate. Even if weekly increments average 25–35 cases and continue declining, the expected maximum across 12 independent or semi-correlated weeks exceeds the mean by roughly 1.5–2×. This mechanical effect alone pushes the central estimate to 40–55 cases, consistent with the median forecast.
New outbreak emergence is the primary upside tail risk. Three new outbreaks were declared in the two weeks before May 7 (Florida, Arizona, Michigan). Summer camps, religious gatherings, and travel during June–August could seed large clusters in under-vaccinated communities. Historical precedent: South Carolina and early Utah both reached 60–160+ cases/week at their peaks. A single new cluster of similar magnitude would spike a week’s increment to 80–150+, driving the distribution’s right tail.
Seasonal dynamics provide modest downward pressure. School closures in May–June typically reduce transmission. However, measles is not strictly seasonal; summer camps and travel can sustain spread. The 2025–2026 wave persisted through winter/spring despite seasonal expectations, suggesting transmission is robust.
Reporting volatility and CDC consistency are minor factors. CDC publishes cumulative counts every Thursday
Highest weekly increase in U.S. confirmed measles cases (5/21/2026 to 8/6/2026)
Key figures
Historical context
Between 2000 and 2023, the United States typically reported very few measles cases, often fewer than 100 per year, except during acute outbreak years like 2014 (667 cases) and 2019 (1,274 cases). The 2025-2026 wave is unprecedented in the post-elimination era, with 2,288 cases in 2025 and 1,842 cases already reported by May 2026. These outbreaks have been primarily driven by low MMR vaccination coverage in specific communities, such as the Mennonite community in Texas (762 cases in 2025) and the FLDS community in Utah (active in 2026). The national kindergarten MMR vaccination rate has notably declined from 95.2% in 2019-2020 to 92.5% in 2024-2025, falling below the herd immunity threshold and facilitating the current resurgence. Historically, the U.S. has maintained its ‘elimination’ status by proving no continuous transmission for 12 months, but current experts warn this status is at high risk of being revoked in late 2026.
Tailwinds
Headwinds
Detailed reasoning
My analysis is based on the current epidemiological trajectory of measles in the United States as of mid-May 2026. The U.S. has recorded 1,842 cases through May 7, the highest year-to-date total since 2000. However, data from April and early May indicate a clear deceleration in the growth rate. Weekly increases slowed from 6.1% in early April to approximately 2.57% by mid-April. Recent increments (late April to early May) show a baseline of approximately 25 to 30 cases per week nationwide.
In weighting the forecast, I prioritized the following factors:
I constructed the final prediction by blending these scenarios. The most likely outcome (HDI 20.61–56.78) assumes that transmission remains centered around active hotspots in Utah and Texas with moderate summer noise. The higher end of the interval accounts for the potential of one or two reporting weeks capturing small new clusters or the impact of increased summer travel surveillance. Values exceeding 100 cases are considered unlikely unless a significant new outbreak occurs in a highly susceptible community.
Key uncertainties
Conclusion