Swiss researchers question thousands of suspected mpox diagnoses in Congo

Nearly 35% of more than 2,700 suspected mpox samples may reflect contamination rather than infection. That finding, from a University of Geneva led analysis, could force health authorities in the Democratic Republic of Congo to revise estimates of the outbreak and reconsider how they deploy testing, isolation and vaccination.

The international research team examined samples collected between April 2024 and April 2026 in Goma, Kamituga, Kinshasa and Uvira. The study appeared in The Lancet Infectious Diseases, according to the University of Geneva and Swissinfo.ch.

The result matters because a positive PCR test can trigger a chain of public health decisions. Patients may receive treatment, be placed in quarantine or be counted in official surveillance data. If environmental viral DNA is mistaken for evidence of an active infection, those measures can reach people who were never infected.

The researchers identified two distinct groups of test results. One showed high viral loads, consistent with infection. The other barely crossed the detection threshold, suggesting residual traces rather than a patient carrying an active case. The finding does not erase the serious health burden caused by mpox in Central Africa. It sharpens the need to distinguish genuine infections from contaminated samples.

Surface testing at two healthcare centres found mpox DNA in the surrounding environment. The researchers say viral traces on surfaces or in the air can remain for several weeks, creating a route for contamination during the collection or handling of later samples.

That evidence helped explain why some results showed only a minimal signal. A test that barely reaches the detection threshold can indicate a genuine infection, but it can also pick up residual material in a clinic or other healthcare setting. The study’s comparison of viral loads gave the team a way to separate the two patterns.

The distinction is especially important in facilities handling large numbers of suspected cases. Frequent contact with patients, equipment and clinical surfaces raises the risk that viral material will travel beyond the person who originally carried it. Contamination does not mean laboratory workers acted improperly. It shows how testing systems must account for the environments in which samples are collected.

The University of Geneva said rapid and accurate identification remains essential for targeting vaccination and organising public health measures. Its researchers are working with the World Health Organization to improve virus detection in several countries.

Around 89% of people classified as false positives had no antibodies against mpox. The serological result gave the researchers a second line of evidence beyond the PCR findings. Antibodies can indicate that a person has previously encountered the virus, so their absence supported the conclusion that many of these people had never been infected.

The team also examined whether other viruses could explain the fever and skin rashes recorded among suspected cases. Most patients in that separate analysis carried other infections that can produce similar symptoms. Only a small proportion were actually infected with mpox, according to the report.

Mpox can cause fever and painful, disfiguring skin lesions, and it can be fatal in some cases. Those symptoms overlap with other illnesses, making laboratory confirmation important. A clinical diagnosis alone may lead health workers to count different diseases under the same outbreak label.

The study therefore points to two linked problems: environmental contamination can distort PCR results, while other infections can mimic mpox. Better testing would give clinicians clearer information and help surveillance teams direct scarce resources toward people who need them.