Distribution of clusters
On January 16, 2020, the first local COVID-19 cluster was reported in Sichuan. On January 24, 2020, a level-1 public health emergency response was launched, followed by the successive execution of multiple measures with respect to infection sources, routes of transmission and susceptible populations to effectively reduce population movement and gatherings and strengthen the management of close contacts. Main measures and the timeline are shown in Figure 1.
On January 16, 2020, the first local COVID-19 cluster was reported. A total of 31 clusters were reported in January and 36 in February. The daily number of clusters peaked on January 30, 2020 (n=7) and gradually declined after February 06, 2020. A total of 50 (74.63%) clusters were reported as of February 06, 2020. Temporal distribution of cluster reports is shown in Figure 1. Among the clusters, 49 (73.13%) were caused by imported cases, and in 19 the infection source of the first case was unknown.
The 67 clusters had involved 16 prefectures and 48 districts/counties (Figure 2). Chengdu had the largest number of clusters (n=17), followed by Dazhou (n=12), Nanchong (n=7) and Guang’an (n=6). Clusters in the four prefectures accounted for 62.69% (42/67) of the total in the province.
Households were the primary exposure place in the reported clusters, accounting for 68.66% (46/67); living in the same household was the primary form of exposure, accounting for 60.87% (28/46). Clusters caused by exposure in multiple places and in multiple forms accounted for 11.94% (Figure 3). Significantly more clusters were caused by family gatherings in February than in January (p=0.002) (Table 1).
Characteristics of confirmed cases involved in clusters
In the 67 clusters, a total of 226 confirmed cases were reported, accounting for 42.01% (226/538) of the total in the whole province; among these confirmed cases, 164 were local confirmed cases resulting from clusters, accounting for 72.57% (164/226); the male/female ratio was 1:1, the mean age was 47.56 years (1 month-87 years), and patients 30 years of age or above accounted for 87.17% (197/226); involved were 75 (33.19%) G1 cases, 131 (57.96%) G2 cases and 20 (8.85%) G3 cases (Table 2); 23.01% (52/226) had a history of travel or residence in Wuhan, and 25.66% (58/226) had a history of travel or residence in other provinces except Wuhan. The constituent ratios of cases with a history of travel or residence in both Wuhan (p<0.001) and other provinces except Wuhan (p=0.006) were higher in G1 cases were higher in G2 and G3 cases, and the differences were statistically significant (Table 3).
The 226 confirmed cases were clinically typed as follows: 65.04% (147/226) were ordinary, 26.55% (60/226) were mild, and 8.41% (19/226) were severe; no death occurred. With the increase of the transmission generation, the number of ordinary cases gradually decreased, with a significant difference noted overall (p=0.045); a significant difference was found between G1 and G3 cases in the constituent ratio of ordinary cases (p=0.013). No apparent between-generation difference was found in the constituent ratios of mild or severe cases (Figure 4).
Among the 226 cases, clinical information about 221 was complete. 137 cases had fever, accounting for 61.99%, with a median maximum temperature of 38.09℃ (37.3-39.5℃). Other main symptoms included dry cough, [66.52% (147/221)], sputum production [26.24% (58/221)], fatigue [25.34% (56/221)], sore throat [19.46% (43/221)] and headache [17.19% (38/221)]. Gastrointestinal symptoms were occasional, mainly including nausea [10.96% (24/221)] and diarrhea [6.79% (15/221)].The number of cases with fever showed a declining trend overall with the increase of transmission generation (p=0.019), and significant differences were noted between G1 and G2 cases (p=0.009) as well as G3 cases (p=0.027) in terms of the constituent ratio of cases with fever. No statistically significant difference was found between generations in other symptoms (Figure 5).
Among the 67 clusters, G2 cases were identified in 57 (85.07%) and G3 cases in 10 (14.93%); a total of 267 confirmed cases (including 41 asymptomatic carriers) were reported, and on average each cluster resulted in 3.99 (2-17) cases; the average attack rate was 8.54% (1.02-100%), and the average secondary attack rate was 6.11% (0.51-66.67%).
Starting from February 02, 2020, we expanded the time window of close contact tracing to cover three days prior to the appearance of clinical symptoms in primary confirmed cases or prior to the obtainment of a positive nucleic acid test in primary asymptomatic carriers. Attack rates and secondary attack rates before and after this time point were compared, and it was found that, following the expansion of the time window of close contact tracing, both the attack rate and the secondary attack rate increased, with statistical differences identified (Table 4).
Timing of key events
Among the 226 cases, 78.76% (178/226) had been exposed to confirmed cases. In 82.02% (146/178) of such cases, the exposure to the confirmed cases occurred at or after onset of the disease in them. In 10 cases (involved in 9 clusters), the exposure occurred before the confirmed cases they contacted with developed clinical symptoms, and they had been exposed only once while the possibility of exposure to other infection sources could be ruled out. The median time of exposure was 3 d (1-4 d) prior to disease onset in the confirmed cases they contacted with, and the mean was 2.5 d; 90.00% (9/10) of the cases were exposed within 3 d prior to disease onset in the confirmed cases they contacted with.
Of 226 confirmed cases, the median interval between hospital visit and onset was 1 d (0-23 d), the median interval between confirmation and onset was 5 d (0-27 d), and the median interval between isolation and confirmation was -2 d (-19-0 d). 28 cases met the criteria for incubation period determination (involved in 17 clusters). The average incubation period was 10.3 d, the median incubation period was 8.5 d (1-24 d) and the incubation period was >14 d in 21.43% (6/28).
In the 67 clusters, a total of 41 asymptomatic carriers (all were identified during medical observation of close contacts) were reported, including 23 males and 18 females. The mean age was 35.31 years (8 months-81 years); and patients 30 years of age or above accounted for 70.73% (29/41). Involved were 8 (19.51%) G1 cases, 31 (75.61%) G2 cases and 2 (4.88%) G3 cases.
In one cluster, one G1 case (severe) was found to have spread the disease to one G2 case (mild) and one G3 case (asymptomatic). Two clusters were caused by asymptomatic carriers and, in both clusters, only G2 cases developed, including one (asymptomatic) to two (confirmed) transmission in one cluster and one (asymptomatic) to one (confirmed) transmission in another cluster. The possibility of other infections was ruled out in all G2 cases. By the end of the two clusters, the G1 asymptomatic carriers had not presented with clinical symptoms or imaging abnormalities.