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.

More than 120,000 mpox cases and around 1,500 deaths were recorded in Africa between 2024 and 2026. The scale of that emergency explains why authorities moved quickly to test suspected patients, isolate them and organise vaccination. It also shows why errors in case counts carry practical consequences.

The World Health Organization declared the mpox outbreak a public health emergency of international concern in August 2024. It lifted that alert in September 2025. Even after the international alert ended, health systems in affected countries continued to manage suspected infections and the risk of transmission.

False diagnoses can impose costs on patients and public services. The University of Geneva said some people received unnecessary treatment and were placed under quarantine even though they were not infected. Misclassification can also draw vaccines, protective equipment and staff time away from confirmed cases.

The researchers’ findings do not establish that every weak positive result was false. They identify a substantial pattern across four locations and support further validation of testing procedures. Authorities will need to compare laboratory signals with symptoms, exposure histories, antibody results and local transmission data before revising official estimates.

The University of Geneva is pressing for changes to PCR interpretation as it works with the WHO. The research team says the PCR positivity threshold needs to be lowered to reduce false positives without substantially increasing false negatives. The recommendation will require careful technical validation because changing a threshold can alter how many cases laboratories detect.

For Switzerland, the study highlights the role of Geneva researchers in international outbreak response. Swiss laboratories and public health institutions regularly contribute expertise to pathogen detection, while Swiss policy decisions also affect access to vaccines and other medical resources in countries facing outbreaks. The findings add pressure for international testing guidance that works in crowded clinics and laboratories with uneven resources.

The next steps include validating the results at additional sites, improving sample collection and handling, and pairing PCR with serological and clinical evidence. Health authorities will also need transparent methods for revising case estimates if a significant share of reported infections proves to be contamination or another illness.

Better diagnosis would help patients avoid unnecessary isolation and treatment. It would also give vaccination planners and epidemiologists a more reliable picture of where mpox is spreading and where support is most urgently needed.