What Is the Reproduction Number (Rt)? A Plain-English Guide

What Is Rt?

Rt (pronounced “R-t” or “R-sub-t”) is the reproduction number — it measures how many people, on average, each infected person spreads a disease to at a given point in time.

• Rt = 1.0 means each sick person infects exactly one other person. The epidemic is stable.
• Rt > 1.0 means the epidemic is growing. Each case creates more than one new case.
• Rt < 1.0 means the epidemic is declining. Each case creates fewer than one new case.

The “t” stands for “time” — unlike the basic reproduction number (R0), Rt changes week to week as behavior, immunity, and interventions change.

Why Does Rt Matter?

Rt is the single best indicator of whether an epidemic is getting better or worse right now. Case counts tell you what already happened. Rt tells you what’s coming next.

For example, a state might have high flu activity (lots of cases), but if Rt drops below 1.0, those cases will start declining in the coming weeks. Conversely, a state with low case counts but Rt above 1.0 could be at the start of a surge.

How Is Rt Calculated?

The CDC’s Center for Forecasting and Outbreak Analytics (CFA) estimates Rt weekly for every US state using a statistical model called EpiNow2. The model:

1. Takes weekly surveillance data (emergency department visits for flu, COVID, and RSV)
2. Adjusts for reporting delays and incomplete data (called “nowcasting”)
3. Estimates the most likely Rt value with a 95% confidence interval
4. Classifies each state as: Declining, Likely Declining, Stable, Likely Growing, or Growing

What Rt Looks Like Right Now

As of early April 2026, here’s the national picture:

Flu

Most states show flu declining (Rt below 1.0), which is typical for this time of year as flu season winds down. Check the national flu forecast for a state-by-state breakdown.

COVID-19

COVID-19 Rt is mixed — some states declining, some stable. This is consistent with the post-winter pattern. See the COVID-19 dashboard for your state.

RSV

RSV is declining in nearly all states. RSV season peaks in December-January and by spring, most states are well past their peak. View the RSV dashboard.

Check Your State’s Rt

Every flu, RSV, and COVID-19 state page on Local Health Signal now includes the latest CDC Rt estimate. For example:

• Oregon flu forecast — Rt, growth probability, and plain-English explanation
• California COVID-19 forecast
• Texas RSV forecast

Or start from your state’s health dashboard to see all three diseases at once.

Rt vs. R0: What’s the Difference?

R0 (R-naught) is the basic reproduction number — how contagious a disease is in a completely susceptible population with no interventions. Measles has an R0 of 12-18, making it one of the most contagious diseases known. Flu typically has an R0 of 1.2-1.4.

Rt is the effective reproduction number at a specific point in time. It accounts for immunity (from prior infection or vaccination), behavior changes, seasonality, and interventions. Rt is always changing, which is why the CDC re-estimates it every week.

Limitations

Rt estimates have uncertainty — that’s why each estimate comes with a 95% confidence interval. Rt can also be hard to estimate when case counts are very low or reporting is incomplete (which is why some states show “Not Estimated” for certain diseases).

Rt also doesn’t tell you about disease severity — just transmission speed. A disease could have a high Rt but cause mild illness, or a low Rt but cause severe outcomes.

About Our Data

We source Rt estimates from the CDC Epidemic Trends and Rt dataset, updated weekly. See our methods page for how we process and display this data.

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Updated weekly with CDC CFA data. Last update: April 3, 2026.