A 41-year-old male COVID-19 case (Patient Zero) was reported in Chongqing on August 16, 2022. He flew to Hohhot City on August 11 and flew back to Chongqing on flight CZ2752 on August 13, 2022. On August 12, this plane was from Chongqing to Hohhot (CZ2751), it housed 4 passengers from Tibet who tested positive for SARS-CoV-2 once they arrived at Hohhot. The flight arriving in Hohhot at 20:00 was not disinfected for the following day’s departure for Chongqing (CZ2752) at 09:59 on August 13. Patient Zero took flight CZ2752 and his seat (33K) happened to be situated around the seats of the 3 positive passengers (34A, 34C, 34H) (CZ2751). Case interviewing found that Patient Zero has no epidemiological association with previous cases in Chongqing. All the 40 persons he contacted in Hohhot had negative testing results for SARS-CoV-2 nucleic acid.
The genome sequence analysis conducted by Chongqing CDC showed Patient Zero infected with Omicron BA.2.76. with the same 75 nucleotide mutations as strains from recent local cases in Tibet. The genetic sequences were highly homologous between Patient Zero and the 4 infected passengers in flight CZ2751, which suggested they might belong to a same transmission chain. Patient Zero was infected most likely because of the exposure to contaminated airline environments.
Patient Zero participated in screening for SARS-CoV-2 virus by using community PCR testing sites in Chongqing on August 9, 10, 11, 13, and 14 and results were negative. There was no screening test on August 12 when Patient Zero was in Hohhot. On August 15, his throat swab specimen was taken at 21:39 and sent to a Medical Laboratory. The positive test result was available at 08:00 on August 16 with ORF lab/N gene: 29.19/31.86. At this time, he just came back home after jogging in the park. He was informed to stay at home and resampled at 09:45, and this result was positive with lower Ct value (ORF 1ab/N: 19.23/16.96) tested by local CDC.
Local CDC identified close contacts and at-risk populations by case interviewing, review of surveillance footage and action track positioning. Close contacts were persons who have a distance of less than 1 meter with Patient Zero and without effective prevention measures. At-risk populations were persons who had been to areas that Patient Zero visited while without close contact with Patient Zero. Close contacts were quarantined at hotels for 7 days and at-risk populations were quarantined at home for 3 days. Finally, 256 close contacts and 20,496 at-risk populations were identified. Health personnel took daily throat swab for SARS-CoV-2 nucleic acid PCR tests for them.
Among those close contacts and at-risk populations, 48 were infected. Overall, 9 of the 48 individuals were exposed to Patient Zero on August 15 or 16 before Patient Zero jogged, including Patient Zero’s wife, 4 colleagues, 2 foot massage therapists, 1 breakfast server, and 1 person on the road. The other 39 cases all had the same exposure of being in the park where Patient Zero jogged on August 16, including 33 visitors and 6 park staff (4 cleaners, 1 lawn mower and 1 park officer). None of the 39 cases were exposed to other previous reported cases or traveled to regions with COVID-19 cases. The 39 cases had positive test throat swabs or developed symptoms between August 17 and 22. The epi curve indicated a point source exposure for 33 visitor cases and the first 2 park cleaner cases (Figure 1). Among the 39 cases, 29 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.
The distribution of sampling date for positive specimen in the outbreak of severe acute respiratory syndrome coronavirus 2 Omicron subvariant BA.2.76 in an outdoor park in Chongqing Municipality, China, August 2022.
Note: Two cases developed symptoms earlier than the positive specimen date, their onset dates were used in the epi curve.
The investigation team highly suspected that Patient Zero was the source of this outbreak due to his maskless jogging in the park. Investigators focused on the activities in the park for both Patient Zero and the 39 cases. The park is a cultural park of 42.5 acres. On August 16, Patient Zero entered the park through the east gate at 6:54 am, jogged counterclockwise to the lake and circled the lake 4 times. He left the park along the same roads at the east gate at 07:29. The path width of jogging was about 4 meters. The wind speed was 0.5–3.0 m/s, the temperatures were 33.0℃–42℃ and the air humidity was 44%–48% when Patient Zero was jogging. The east gate of the park is the main gate, with convenient transportation, and a good flow of people. During the jogging time, there were 104 close contacts who have a distance less than 1 meter with Patient Zero and without wearing masks.
Patient Zero felt fatigue on August 15 but had no symptoms of fever or cough. He did not wear a mask during his jogging in the park. He did not use any facilities and interact with others in the park. Among the 33 cases among visitors, 13 were close contacts who had faced with or passed by Patient Zero when they exercised by reviewing surveillance footage in the park, 20 were at risk due to exposure to Patient Zero. Among the 20 at-risk populations, 10 cases had both stayed for a while around the lake and entered the park through the same east gate as Patient Zero, 2 cases stayed around the lake, 1 case entered the park through east gate, the other 7 cases had neither stayed around the lake nor entered the park through east gate, but part of their walking routes in the park overlapped with the route Patient Zero had taken. The 33 visitor cases did not know each other, no COVID-19 cases in the communities where they dwelled and no time-space overlap with previous reported COVID-19 cases before they tested positive.
There were 24 staff in the park. From August 18 to 20, they worked in the park during the day and rested in two big meeting rooms at night until they were transferred to quarantine hotels. During the 3 days, they shared the same toilet. Two park cleaners who worked in the area of the lake tested positive on August 18, then the other 4 park staff subsequently tested positive on August 20 (1 case), 21 (1 case), and 22 (2 cases). It was possible that the first 2 cleaner cases infected the other 4 staff during the 3 days.