In the early morning of August 16, a 41-year-old man in China’s southwest-central municipality of Chongqing got up and went for a jog along a lake in a local outdoor park—something that should have been a pleasant, if not unremarkable, outing. But what really happened during that 35-minute jaunt has now sparked international alarm and debate, with some scientists doubtful of China’s startling account.

According to the Chinese Center for Disease Control and Prevention, the unmasked man infected 33 unmasked park visitors and two unmasked park workers with the coronavirus omicron subvariant BA.2.76 during his short run. The agency claimed transmission occurred in fleeting outdoor encounters as he trotted past people on a four-meter-wide foot path. Many others were infected without any close encounter. Twenty of the 33 infected park goers became infected by simply visiting outdoor areas of the park the jogger had previously passed through, including an entrance gate. The two infected workers, meanwhile, quickly passed the infection on to four other colleagues, bringing the jogger’s park outbreak total to 39.

To support these unusual conclusions, the CCDC cited case interviews, park surveillance footage, and SARS-CoV-2 genetic data, which reportedly linked the cases but is notably absent from the report.

The report’s claims, if accurate, would suggest a significant update is in order for our current understanding of SARS-CoV-2 transmission risks. Though transmission outdoors is known to be possible, it’s considered far less likely than transmission indoors, where virus particles can hang in stagnant air and build up in enclosed spaces over time. Outdoor encounters that are transient are especially not considered a significant risk, as vast volumes of moving air quickly disperse infectious doses of viral particles. For the same reason, SARS-CoV-2 is not thought to linger in menacing clouds outdoors in an infected person’s wake.

For now, experts outside China are not revising their thinking on transmission risks, citing the report’s missing genetic data and other questionable conclusions.

Missing data

Given China’s strict “zero COVID” strategy, the CCDC dismissed outright the possibility that infections were part of an undetected outbreak in the greater community, calling exposure to the jogger (aka “patient zero”) the “only possible exposure.”

The CCDC claims that genetic data links all the cases together, showing that patient zero was the source of the 39 infections. Specifically, they report that 29 of the 39 cases had “the exact same gene sequencing as Patient Zero; 5 cases had a mutation site added to Patient Zero’s gene sequence; and the other 5 cases could not be sequenced because of unqualified specimens.” But there is no sequencing data included with the report, and it’s unclear what sequencing was actually done to support their claims.

“If they had sequence data that showed 29 cases had identical genomes to ‘patient zero,’ that would suggest that all the cases came from a single source,” virologist Angela Rasmussen told Ars. Rasmussen is a researcher at the Vaccine and Infectious Disease Organization at the University of Saskatchewan and an affiliate at Georgetown University Center for Global Health Science and Security.

“But,” she said, “it’s unclear whether they did whole genome sequencing of all the cases, what sequencing platform they used (Illumina vs Nanopore) etc.” The report only mentions “gene sequencing,” which may suggest only partial genome sequencing, not “whole genome sequencing” that would definitely indicate a direct link between the cases. Without knowing the sequencing data and methods, it’s impossible to confirm if the jogger was the source.

The CCDC also offers a puzzling explanation of how the jogging patient zero came to be infected in the first place.

Patient zero

According to the CCDC, the man became infected from a vague “exposure to contaminated airline environments.” The man had taken a trip from Chongqing to the northern city of Hohhot on August 11, and flew back to Chongqing on August 13—three days before his jog. Neither flight had any known SARS-CoV-2 cases on board that could explain the man’s infection. But, the plane he took for the returning trip had transported four SARS-CoV-2-positive passengers the day before, on August 12.

On August 12, four passengers from Tibet took the plane from Chongqing to Hohhot and later tested positive in Hohhot. The plane, meanwhile, wasn’t disinfected after their flight, and the Chongqing man boarded the next day and sat (in seat 33K) near where three of the infected passengers had been sitting (seats 34A, 34C, 34H). It’s unclear how the man could have become infected this way—SARS-CoV-2 is not known to linger in the air for such long periods, and transmission from contaminated surfaces is rare. Moreover, the report does not indicate that any other passengers on the flight also became infected, including people who actually sat in the same seats as the passengers from Tibet. But patient zero was infected with BA.2.76, which was circulating in Tibet, which led the CCDC to conclude a connection.

“I think it’s also very dubious that ‘patient zero’ was infected on that plane,” Rasmussen said. “I noticed that the prior flight with the passengers that were supposedly the source of the infection came from Chongqing—that could suggest cryptic spread of BA.2.76 in Chongqing, not (just) Tibet as the paper claims. In this case, if a whole bunch of people in Chongqing have BA.2.76, the sequencing data might just point to a much larger outbreak in Chongqing, but you’d need the actual sequencing data to really figure out what’s going on.

“Bottom line: any claims about what the data actually shows depends on actually including the data in the paper,” she said. “Otherwise it’s just speculation.”