Pandemiology Watch (December 17-23, 2025)
MPOX
Ireland.
A study published in Eurosurveillance on 18 December 2025 reports a mpox clade Ib outbreak in Ireland between August and October 2025, involving four epidemiologically linked cases. The cluster began with a traveller returning from Pakistan and included a confirmed nosocomial infection, acquired in a healthcare setting. Phylogenetic analysis showed genetic similarity to a virus sequence from the Eastern Mediterranean region, suggesting transmission links beyond Ireland. The authors highlight the importance of early detection, rapid testing, and coordinated public health action to prevent further spread of mpox clade Ib in countries where it is not endemic.
U.S.
A report published in NEJM Evidence on December 17, 2025, documents the first evidence of community transmission of mpox clade Ib in the United States. Phylogenetic analysis suggests that four 2025 cases in California likely stemmed from a single introduction of clade Ib, followed by local spread. The infections occurred among gay, bisexual, and other men who have sex with men, with no recent international travel, indicating the virus may be circulating undetected within social networks in Southern California. While genetic data link the cases, direct transmission pathways remain unconfirmed, highlighting the need for enhanced surveillance and clade-specific testing.
MEASLES
US.
According to CIDRAP on December 23, South Carolina reported nine new measles cases, bringing the state total to 156 in 2025, with most infections occurring among unvaccinated individuals. In Utah, measles cases have reached 142, including 101 cases in the Southwest Utah health region and 16 cases in Utah County. Health officials say this represents the highest measles activity in the state in more than 30 years.
As of December 23, 2025, Center for Disease Control reports a total of 2,012 confirmed measles cases in the U.S. in 2025. 11 % of cases have required hospitalization (227 cases of 2,012), and three deaths have been reported.
Canada.
As of December 22, 2025, Canada has reported 5,353 measles cases where 71% occurred in children, according to the Public Health Agency of Canada. Ontario accounts for 45% of the cases, followed by Alberta with 37%. 89% were unvaccinated, and 7% required hospitalization. Genotype D8 remains the most frequently detected strain. 2 deaths have been reported.
H5N1
U.S.
On December 19, 2025, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service reported that whole genome sequencing has confirmed the strain of highly pathogenic avian influenza detected in a Wisconsin dairy herd as H5N1 clade 2.3.4.4b genotype D1.1. The genetic analysis indicates this represents a new spillover event from wildlife into dairy cattle
As of December 23, the Centers for Disease Control and Prevention reports has not reported any cases of person-to-person transmission of H5N1 in the United States. The total remains at 71 confirmed human cases and two deaths.
Marburg virus
On December 22, 2025, Ethiopian health authorities reported 14 laboratory-confirmed Marburg virus cases, including nine deaths, in the country’s South and Sidama regions. Additional suspected cases are under investigation. No Marburg cases linked to this outbreak have been reported outside Ethiopia according to CDC.
Rift Valley Fever
Senegal.
On December 17, Africa CDC reported a total of 543 confirmed Rift Valley Fever cases in Senegal, including 13 new cases since the update on December 10. In addition, 18,151 suspected cases have been reported in total.
POLIO
As of December 17, 2025, the Global Polio Eradication Initiative reported new poliovirus detections in several countries. In Africa, detections included one cVDPV2 case and two cVDPV3 cases in Chad, one cVDPV2 case in Ethiopia, five cVDPV2 cases in Sudan, one cVDPV2-positive environmental sample in Somalia, and two cVDPV2-positive environmental samples in Tanzania. Outside Africa, Pakistan recorded 38 WPV1-positive environmental samples, and Yemen reported four cVDPV2-positive environmental samples.