CDC updates state Rt estimates May 22

The U.S. Centers for Disease Control and Prevention updated its public state-level Rt estimates for COVID-19, influenza and RSV on May 22, adding a fresh weekly readout to one of the agency’s main respiratory surveillance tools. The estimates appear on the CDC’s “Current Epidemic Trends (Based on Rt) for States” page, which maps whether transmission is growing or declining by state. The page says the estimates are based on incident emergency department visits and are intended to show current epidemic trends rather than finalized case counts. The latest CDC charts display estimated Rt values and uncertainty intervals from March 25, 2026 through May 19, 2026. ### What did the CDC update on May 22? The CDC’s May 22 update refreshed state-by-state Rt estimates for three respiratory viruses: COVID-19, influenza and respiratory syncytial virus, or RSV. The agency’s Center for Forecasting and Outbreak Analytics hosts the page and describes Rt as the time-varying reproductive number, a measure of transmission. (cdc.gov) The CDC says the map on that page estimates the probability that Rt is greater than 1 for each state. The same page also provides state and national trend charts with uncertainty intervals, allowing users to view recent movement over time rather than a single-day point estimate. ### What does Rt above 1 mean on the CDC map? (cdc.gov) The CDC says estimated Rt values above 1 indicate epidemic growth. An Rt below 1 indicates transmission is declining, while values around 1 suggest more stable spread, according to the agency’s description of the metric and its respiratory virus season materials. CDC researchers wrote in a 2024 Morbidity and Mortality Weekly Report that epidemic trend categories based on Rt estimates can help detect changes in community transmission before reporting is complete. (cdc.gov) That paper said nowcasting methods can mitigate reporting lags that affect routine surveillance data. ### Where do these estimates come from? (cdc.gov) The CDC says its current epidemic trend estimates are based on data from incident emergency department visits. On a related respiratory virus activity page published May 22, the agency said it uses emergency department visit data to model epidemic trends and determine whether new respiratory infections are growing or declining in a state. (cdc.gov) The agency’s “Behind the Model” page says Rt estimates can be used during an epidemic to assess trend direction and to forecast short-term changes in cases, hospitalizations or deaths. That page also says Rt reflects current population susceptibility, interventions and behavior at the time underlying infections occurred. (cdc.gov) ### How is this different from the CDC’s measles tracker? The CDC’s measles surveillance is posted separately on its “Measles Cases and Outbreaks” page rather than on the Rt dashboard. That page was updated yesterday and serves as the agency’s weekly tracker for confirmed U.S. measles cases and outbreaks. The CDC said 1,952 confirmed measles cases had been reported in the United States in 2026, with 29 new outbreaks reported this year and 93% of confirmed cases linked to outbreaks. (cdc.gov) The page also said nine measles cases were reported among international visitors to the United States. ### Where can readers check the next update? The CDC’s Rt estimates are available on the agency’s Current Epidemic Trends page, and the underlying public dataset is also posted through CDC data tools. (cdc.gov) The measles totals are posted on the CDC’s measles data and research page, which the agency updates weekly. The CDC’s latest Rt charts currently run through May 19, 2026, and the respiratory virus activity page was published May 22, 2026. (cdc.gov) The measles page was published May 22 as well, providing the next official place to watch for updated U.S. case counts and outbreak totals. (cdc.gov)