Coronavirus Seasonality: What to Expect Each Winter
"Unraveling the trends of common human coronaviruses to better prepare for future respiratory virus seasons."
Human coronaviruses (HCoVs) are a group of viruses known to cause respiratory infections, ranging from the common cold to more severe conditions. Among these, OC43, 229E, NL63, and HKU1 are frequently detected, particularly during winter months. Understanding their behavior and prevalence is crucial for public health preparedness.
Recent advances in diagnostic technologies have improved our ability to identify and track these viruses. Since 2014, voluntary reporting of HCoV data to the National Respiratory and Enteric Virus Surveillance System (NREVSS) has provided valuable insights into their seasonal patterns.
This article explores the trends of HCoV infections based on data reported to NREVSS, focusing on the seasonality, age distribution, and potential impact of different strains. By understanding these patterns, we can better anticipate and manage future respiratory virus seasons.
From Wuhan to Seasonal Coronaviruses
The CDC timeline reports that a cluster of patients in Wuhan began experiencing an atypical pneumonia-like illness on December 12, 2019. On December 31, the WHO Country Office in China was informed of several pneumonia cases of unknown cause in Wuhan. Separately, research identifies four globally endemic seasonal human coronaviruses: 229E, NL63, OC43, and HKU1. The review estimates that they account for 5–30% of human respiratory infections and notes that their epidemiology and evolution remain understudied.
Seasonal Trends of Human Coronaviruses
A comprehensive study reviewed weekly aggregate test results for HCoV OC43, 229E, NL63, and HKU1, voluntarily reported to NREVSS by U.S. hospitals and clinical laboratories from July 1, 2014, to April 30, 2017. The data revealed distinct seasonal patterns and variations in the prevalence of each virus.
- Peak Seasonality: The percentage of positive tests for HCoV generally peaked between December and March, indicating a strong winter seasonality.
- Strain-Specific Patterns: HCoV-OC43 showed distinct annual peaks with variations in magnitude by year. HCoV-HKU1 and NL63 had similar patterns, with notable peaks during winter 2016 compared with 2015 or 2017. HCoV-229E showed a discernable peak in 2017 compared with the previous 2 years.
- Age Distribution: Analysis of individual-level data reported via the Public Health Laboratory Interoperability Project (PHLIP) showed that age distribution differed between HCoV types.
Winter Patterns in Temperate Climates
A study of common human coronaviruses in the United States from 2014 to 2021 describes different seasonal patterns by climate. In temperate climates, these coronaviruses circulate during winter, aligned with cooler ambient temperatures. In tropical regions, their seasonality is more varied and less predictable.
Limits in Seasonal Surveillance
A systematic review found that 19.4% of included studies (n = 39) were from China, while the Southern Hemisphere was underrepresented. The CDC notes that NREVSS is a passive, voluntary surveillance system. Data from participating laboratories vary by season and might not represent all geographic areas or age groups.
Different Measures of Seasonal Coronavirus Activity
In a retrospective study in Stockholm covering 2009–2020, seasonal coronaviruses were detected in 2,130 samples, or 3.9% of samples, and made up 8.1% of all virus detections. In the Flu Watch cohort, the confirmed HCoV infection rate across all seasons was 390 per 100,000 person-weeks (95% CI 338–448). That cohort’s highest reported rate was 674 per 100,000 person-weeks (95% CI 537–835), during the November–March 2008/9 season.
Preparing for Future Coronavirus Seasons
The research emphasizes that peak positivity for HCoV occurs during winter months, with notable annual differences in circulation by HCoV type. This highlights the need for ongoing awareness and proactive measures to mitigate the spread of these viruses.
Seasonality as a Planning Question
For its 2025–2026 respiratory disease season outlook, the CDC consulted CDC and external experts on COVID-19, influenza, RSV, and infectious disease dynamics. A 2024 analysis evaluated respiratory-virus seasonality using time-series analyses. Its authors describe understanding seasonal behaviors as important for preventing infections.
Preparing for Uncertainty and Immunization Decisions
A 2026–2027 respiratory-season evidence review says updated evidence on influenza and COVID-19 vaccine safety, and growing safety evidence for RSV immunizations, was provided to leading medical societies to inform their recommendations. A CDC Respiratory Disease Season Outlook said there was not enough information at the time to assess how the emergence of BA.2.86 might affect the upcoming season. It added that its outlined scenarios likely account for the emergence of a wide range of variants.
Climate, Behavior, and Regional Timing
A multi-year analysis of surveillance data across hemispheres reports seasonal divergence for SARS-CoV-2: consistent summer surges in tropical regions such as India, and winter peaks in temperate climates. A review of respiratory viral infections says the two major contributing factors examined in explanations of seasonality are changes in environmental parameters and human behavior. Together, these sources point to regional differences and multiple factors as important context for interpreting seasonal patterns.
Continued testing and surveillance are crucial for further characterizing circulation trends over time and by geographic region. Such efforts will enhance our understanding of HCoV's contribution to the winter respiratory virus season and inform public health strategies.
By staying informed about these patterns and practicing preventive measures, individuals and communities can better navigate future coronavirus seasons and protect public health.