Evaluating Epidemic Trends (based on Rt)

Highlights

  • ÐÇ¿ÕÓéÀÖ¹ÙÍø and the New Mexico Department of Health assessed models that detect trends in COVID-19 transmission before data reporting is complete.
  • Epidemic trend estimates based on Rt identified increases in COVID-19 before they appeared in surveillance data and confirmed that fewer emergency department visits reflected actual decreases in community transmission.
A graphic depicting Rt estimates in New Mexico.

Partnering for Public Health

In advance of the 2024–25 respiratory virus season, the New Mexico Department of Health (NMDOH) collaborated with CFA to assess state-level performance of epidemic trends for COVID-19. The study's authors published the results in Morbidity and Mortality Weekly Report (MMWR), showing that epidemic trends provided an early signal of the COVID-19 summer wave in New Mexico. At the end of the summer wave, epidemic trends also helped confirm that the wave had peaked and distinguished true decreases in emergency department visits from apparent decreases due to reporting delays.

Importance of Early Insights for Preparedness

Timely data is critical for effective outbreak response. However, most data sources have reporting delays. Analytic methods, like nowcasting and the time-varying reproductive number (Rt), can help account for these data delays and estimate whether the number of infections is currently increasing, decreasing, or not changing.

In the Morbidity and Mortality Weekly Report (MMWR), led by CFA and co-authored with the National Center for Immunization and Respiratory Diseases (NCIRD), the National Syndromic Surveillance Program (NSSP), and NMDOH, ÐÇ¿ÕÓéÀÖ¹ÙÍø analyzed weekly national and state-level Rt estimates for COVID-19. During summer 2024, Rt was an early indicator of increases in COVID-19 community transmission in the United States on the national level and on the state level in New Mexico. Later in September, Rt helped differentiate true decreases in COVID-19 transmission from apparent decreases resulting from delayed reporting.

Rt estimates, when combined with other surveillance data, can offer valuable insights for public health readiness. State-level trends provide local awareness and can assist states in tracking regional disease patterns.

U.S. Map
Figure 1: In summer 2024, the United States experienced a rise in COVID-19 cases, with epidemic trend categories showing growth in May and peaking in late August. By September, transmission began declining nationwide, as indicated by trend categories.

Future Work

In addition to ongoing evaluations of national epidemic trend categories based on Rt, CFA also continues to collaborate with health departments to implement models locally. For example, we are continuing to work with the New Mexico Department of Health to explore Rt estimates and epidemic trend categories at the sub-state level. Other health departments interested in applying this work should contact CFA.

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Modeling and Forecasting
CFA uses data, modeling, and analytics to assess public health threats.
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Current Epidemic Trends (Based on Rt) for States
CFA’s Rt page estimates COVID-19 and influenza epidemic trends for U.S. states.
Keep Reading Detection of Real-Time Changes in Direction of COVID-19 Transmission Using National- and State-Level Epidemic Trends Based on Rt Estimates — United States Overall and New Mexico, April–October 2024

Acknowledgments

  • NCIRD works to prevent disease, disability, and death through immunization and by surveillance and control of respiratory and related diseases.
  • NSSP helps gather and provide syndromic data from collaborations with state and local health departments and others.
  • CFA aims to use data, modeling, and analytics to respond to outbreaks in real time to inform effective decision-making.
  • works with its partners to promote health and well-being and improve health outcomes for all people in New Mexico.
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Content Source:
Center for Forecasting and Outbreak Analytics