While MPOX appeared to be under control since the 2022 global outbreak, an unexpected resurgence worries health authorities. Three serious cases of a new variant, clade I, were identified in October 2025 in California, without epidemiological link or international travel. This red flag reveals possible silent community transmission in the United States. A study pre-published on medRxiv, by researchers from the University of California at Berkeley and Kaiser Permanente Southern California, shows that actual infections exceed officially diagnosed cases by 33 times in certain at-risk populations.
In a context of federal budgetary tensions, the responsiveness of the public health system is being tested. Understanding this discreet resumption of the virus, its modes of diffusion and the current limits of surveillance becomes crucial to avoid a new epidemic shift.
A discreet variant, but well established in California
In October 2025, California health authorities announced the identification of three cases of MPOX due to the clade I variant in residents of Los Angeles and Long Beach. None of these patients had recently traveled abroad, nor had any known contact with each other, ruling out the possibility of a single imported outbreak. This lack of epidemiological link constitutes, according to the CDC, a strong indicator of local community transmission.
This clade I, historically limited to certain regions of central Africa, is considered more virulent than clade II. The latter was predominant during the global epidemic of 2022. The emergency hospitalization of the three patients demonstrates the potential severity of this form of the virus. According to the California Department of Public Health (CDPH), this strain appears to circulate primarily within the social networks of men who have sex with men. But data on patient profiles remains limited.
Genomic analysis of the isolated viruses suggests a relationship with a case recorded in August, linked to travel to a risk area. However, the exact chain of transmission could not be traced. This highlights a major weakness. Contact tracing and routine surveillance struggle to detect this type of silent spread.
Dr Rita Nguyen, from CDPH, recalls that immunocompromised people remain particularly vulnerable to severe forms. Faced with a virus that is both more virulent and more discreet, health authorities are calling for increased vigilance in densely populated urban areas.
Massive underdetection of the variant revealed by field data
Beyond clinically identified cases, the work carried out by Joseph A. Lewnard (University of California at Berkeley) and Miguel I. Paredes (University of Washington) reveals a much more alarming reality. In their recently published study, researchers analyzed rectal samples from a cohort of men who have sex with men (MSM) in Los Angeles, alongside clinical follow-up during the summer of 2024.
The results are clear. MPOX infections are 33 times more numerous than officially diagnosed clinical cases. This massive underdetection, corroborated by phylogenetic analyzes and meta-analysis of surveillance data, indicates that the epidemic is spreading largely under the radar of the public health system.
Even more worrying, the authors estimate that 61% to 94% of transmissions could come from asymptomatic or undiagnosed people. In other words, transmission chains escape traditional tracing and isolation measures. Contrary to dominant health messages, the infection often turns out to be subclinical. In other words: without visible symptoms, which makes identifying cases very difficult.
This cryptic circulation calls into question the current objectives of eliminating the virus, which are based on the detection of symptomatic cases. In fact, current surveillance mechanisms prove insufficient to contain a transmission that is mostly invisible. According to Lewnard, the situation requires rethinking the detection strategy, in particular through systematic approaches or targeted sampling in at-risk groups, regardless of apparent symptoms.
Transmission dynamics different from previous epidemics
The epidemiological profile of clade I appears to diverge from that observed with clade IIb during the 2022 global epidemic. At that time, the vast majority of reported cases involved men having sex with men, in the context of festive gatherings or multiple sexual contacts. This plan allowed targeted health communication and community prevention actions.
The clade I variant, on the other hand, displays more diffuse dynamics. In Central Africa, particularly in the Democratic Republic of Congo (DRC), where this variant originates, it affects men and women in equal proportions, mainly aged 25 to 40. According to Dr. Jason Kindrachuk, expert at the Public Health Agency of Canada, cited by The Guardian, this spread is linked to “dense sexual networks”. Multiple partners increase the likelihood of transmission, regardless of gender or sexual orientation.
However, the characteristics of these networks can vary greatly from one country to another. Kindrachuk points out that it is difficult to extrapolate African observations to North American or European contexts. The absence of detailed data in California (gender, sexuality, profession of patients) complicates the understanding of current methods of diffusion.
The other notable distinction concerns severity. Clade I more frequently causes severe forms requiring hospitalization, particularly in immunocompromised people. This accentuates health risks and highlights the need for a more robust and expanded response. Finally, remember that transmission can occur outside of sexual intercourse alone. Close contact or sharing of personal items is also involved. This broadens the spectrum of populations potentially at risk.
A weakened health response in the face of a more discreet variant
As MPOX returns silently, health response capacities are put to the test. In the United States, the partial shutdown of the federal government has paralyzed many public services, including the CDC. They find themselves weakened by a loss of almost a third of their workforce in 2025. This context complicates national coordination and the rapid adaptation of surveillance protocols.
The JYNNEOS vaccine, administered in two doses 28 days apart, remains the main prevention tool. However, according to CDC data, vaccination coverage remains incomplete. In addition, the effectiveness of the vaccine remains primarily preventive against serious forms. It is not enough to stop transmission, especially if the infections turn out to be mild or asymptomatic.
The vaccination strategy is mainly based on the vaccination of at-risk groups: MSM men, people living with HIV, or who have recently frequented places of collective sex. But access to the vaccine is not uniform. Miguel Paredes emphasizes that precarious populations or those without medical coverage encounter obstacles, especially when vaccination centers are not accessible outside of working hours.
Awareness campaigns, which had shown their effectiveness in 2022 thanks to partnerships between public institutions and LGBTQ+ communities, are struggling to relaunch. The associative fabric, although essential to breaking stigma and encouraging screening, lacks resources.
Faced with a less visible virus, Paredes recommends alternative methods, such as analyzing wastewater to spot weak signals. He also calls for field, mobile, accessible and community actions, so as not to leave exposed populations to their own devices.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
