A highly infectious form of norovirus, commonly known as “winter vomiting disease,” is spreading across the country, including Pennsylvania, according to wastewater analyses at two state surveillance sites.
Norovirus cases typically surge after Christmas and New Year’s celebrations, but recent data from November show a noticeable uptick in new infections. The Centers for Disease Control and Prevention tracks norovirus mainly through voluntary reports from clinics and state health departments, yet the agency acknowledges that this method can be unreliable because many people suffer for a few days and never seek medical care or hospitalization.
What’s happening in Across Pennsylvania can be followed for free with the latest updates from Patch. The CDC does not monitor wastewater directly, but WastewaterSCAN provides relevant data on viral activity in wastewater across various regions.
In Pennsylvania, wastewater monitoring at Chester and Harrisburg indicates the virus is present, with Harrisburg showing the steepest rise: activity there more than doubled in the first week of December and began climbing just before Thanksgiving. Chester also shows a significant increase, though not at Harrisburg’s level, and its rate has roughly doubled since the period just before Thanksgiving.
University Park previously reported notable levels in spring, but has since halted data collection. While the Pennsylvania Department of Health has not released detailed outbreak data for the current season, it has issued warnings and safety recommendations, alongside several counties, in response to rising cases early in January 2025.
Overall, about 2,500 norovirus outbreaks are reported annually in the United States. Some experts worry that the GII.17 variant—responsible for last year’s spike—may have mutated to become more contagious. Historically, the GII.4 strain dominated for over three decades, but last year’s outbreak pattern saw GII.17 account for roughly 75 percent of cases, according to the CDC.
Dr. Robert Atmar, a physician at Baylor College of Medicine, shared with NBC News two possible explanations: the virus could have become more transmissible, or the population could have become more susceptible. He emphasizes that both factors are likely at play, and the big question now is whether this pattern will persist this year.
Symptoms associated with the GII.17 strain include forceful vomiting, severe diarrhea, and stomach cramps, which tend to hit children under five, adults older than 85, and individuals with underlying health conditions hardest. Dehydration remains the primary risk. Since GII.17 is a newer strain, people generally have less immunity against it, potentially leading to more severe illness.
Experts like Lee-Ann Jaykus of North Carolina State University note that emerging strains are a natural part of norovirus dynamics. She explains that new variants typically appear every seven to ten years, and this situation aligns with that pattern.
To curb spread, vigorous hand-washing with hot water and soap, along with good overall hygiene, is essential. Norovirus can spread through direct contact with an infected person, consuming contaminated food or drinks, touching contaminated surfaces, or inadequate hand hygiene.
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