The CDC's most forward-looking measure of COVID transmission — not hospitalizations, not case counts, but the mathematical signal that tells epidemiologists where transmission is heading before it gets there — shows COVID infections growing or likely growing in three U.S. states as of June 23, 2026.
According to the CDC's CFA Modeling and Forecasting program, the time-varying reproductive number (Rt) indicates: growing or likely growing transmission in 3 states, stable transmission in 12 states, and declining transmission in 34 states. National COVID-19 activity overall remains low — but the tool is specifically designed to detect change before it is visible in hospitals or case counts.
Why This Matters
Most people track COVID through the metrics they encounter in news coverage: hospitalizations, deaths, and occasionally case counts. These are all lagging indicators — by the time they rise, the transmission event that caused them happened one to three weeks earlier.
Rt (the time-varying or effective reproduction number) is different. It estimates the average number of secondary infections currently being caused by each infected person in a population. When Rt is above 1, each infected person is generating more than one new infection, meaning transmission is growing. When it is below 1, each infected person generates fewer than one new infection, and the outbreak is declining.
Rt updates weekly based on testing data, emergency department visits, and other early indicators, providing the earliest available quantitative signal of where transmission is heading — typically two to three weeks before the resulting change in hospitalizations would be apparent.
This matters particularly now because the CDC has specifically flagged that the South and West face elevated risk of a summer COVID increase — and Singapore's NB.1.8.1 variant is currently driving 60 percent weekly growth internationally, with U.S. genomic surveillance showing the variant has established a presence in multiple states.
What We Know So Far
The June 23, 2026 CDC Rt estimates show the nationwide COVID picture as follows:
- Growing or likely growing : 3 states
- Stable : 12 states
- Declining : 34 states (plus territories and D.C.)
The national picture is one of overall low activity, but the three states showing growth represent the type of early signal — detected weeks before any corresponding rise in hospitalizations — that the Rt tool is designed to catch.
The CDC has been explicit that its current assessment of a potential summer increase is based on population immunity patterns, not current case data: the Northeast and Midwest had higher COVID activity during winter 2025-2026, building more recent immunity; the South and West had comparatively lower winter activity, leaving those populations with less recent immunity heading into summer.
The International Context: NB.1.8.1
The variant currently attracting the attention of global COVID surveillance is NB.1.8.1, a descendent of JN.1 that has been driving approximately 60 percent weekly growth in Singapore and has been detected in wastewater and case sequencing from multiple U.S. states. NB.1.8.1 has demonstrated immune-escape properties in laboratory studies that suggest it may be able to partially evade immunity from prior JN.1-lineage infections — though real-world data on severity relative to other recent variants remain limited.
Current COVID vaccines are still expected to provide meaningful protection against severe disease from NB.1.8.1 and other circulating variants, based on the cross-protective immunity the vaccines generate even against variants with modified spike proteins.
How to Read the CDC's Rt Tool for Your State
The CDC's CFA Modeling and Forecasting Rt estimates page (cdc.gov/cfa-modeling-and-forecasting/rt-estimates/index.html) provides state-level Rt estimates updated on a weekly basis. Each state's estimate includes a confidence interval — the range of values within which the true Rt is likely to fall — rather than a single point estimate.
Key interpretive principles:
- Rt above 1 with a confidence interval that does not include 1 = growing transmission with statistical confidence
- Rt above 1 but with a confidence interval that includes 1 = "likely growing" — still a meaningful signal but with more uncertainty
- Rt at or below 1 = stable or declining transmission
The page also shows the trend direction compared to the prior week, providing additional context about whether the signal is strengthening or weakening.
What Doctors and Experts Say
Public health researchers consistently describe Rt modeling as a more actionable early-warning tool than case counts or hospitalizations for planning purposes — particularly for health systems that want to consider staffing adjustments, visitor policies, or protective equipment procurement before a wave is visible in admission data.
At the individual level, Rt data is most useful for people in high-risk groups (older adults, immunocompromised individuals) who are making decisions about boosters, masking, or attendance at large indoor gatherings, and who benefit from having the earliest possible signal about the direction of transmission in their region.
Who Should Pay Attention to This Data Most Closely?
- Adults 65 and older, immunocompromised individuals, and people with chronic conditions — the groups for whom COVID severity risk remains meaningful and who benefit most from advance awareness of transmission trends
- Residents of the South and West — the regions the CDC has specifically flagged as potentially elevated risk this summer
- Health system administrators and hospital capacity planners who benefit from the 2-to-3-week lead time the Rt tool provides relative to admissions data
- People who have not received a COVID booster in the past 6 to 12 months and are deciding whether to get the updated fall 2026 formulation
What You Can Do Now
- Bookmark the CDC's Rt estimates page at cdc.gov/cfa-modeling-and-forecasting/rt-estimates/index.html and check your state's current trend weekly.
- If your state is in the "growing" or "likely growing" category and you are in a high-risk group, consider timing a booster shot now — updated fall 2026-2027 formulations targeting XFG are expected in August-September, but the current 2025-2026 vaccine remains the available protection option right now.
- If you are in a high-risk group and planning large indoor events or gatherings in the coming weeks, factor your state's current Rt into your decision-making.
- Check the CDC's broader respiratory data dashboard at cdc.gov/respiratory-viruses for context on overall COVID and other respiratory illness activity.
What Happens Next
The CDC's Rt estimates update weekly. MedicalDaily will report on any meaningful acceleration in Rt values — particularly if the number of states with growing transmission increases significantly, or if the South or West states the CDC has flagged as elevated-risk begin showing consistent growth signals.
The Bottom Line
Three U.S. states have growing COVID transmission as of June 23, 2026 — not measured in hospitalizations or cases, but in the Rt metric that provides the earliest available warning of where transmission is heading. National activity is low, but the combination of lower recent immunity in the South and West and an internationally rapidly growing variant means the conditions for summer acceleration exist. For high-risk individuals, now is the time to check your state's Rt and make an informed decision about timing and protective measures.
