A total of 178 clusters were collected between 20th January and 11th February 2020 in Zhejiang province, China. These clusters were classified as follows: family clusters (158, 88.76%), workplace clusters (5, 2.80%), entertainment place clusters (3, 1.69%), clusters because of religious gathering (2, 1.12%), flight cluster (1, 0.56%), banquet clusters (2, 1.12%) and others (7, 3.93%). The 178 clusters covered a total of 543 confirmed cases, including 331 first-generation and 212 secondary cases. Among 331 first-generation cases, 221 (66.77%) cases were citizens ever lived (181, 54.68%) or travelled (40, 12.08%) to Wuhan or its neighbouring regions like Xiaogan city in Hubei province within 14 days before onset; 58 (17.52%) cases had ever contacted confirmed people came from epidemic regions in Hubei province and 17 (5.14%) cases ever contacted confirmed cases from other provinces of China expect Hubei when they were travelling outside Zhejiang within 14 days before onset; 21 (6.34%) cases had unclear source of infection, and the last 14 (4.23%) cases’ epidemic information was insufficient to infer their source of infection.
After examining the epidemic link between 212 secondary cases with their previous cases, we extracted the data from 49 secondary cases with clear epidemic link with their previous cases within 26 clusters; all 49 cases were exposed to a previous generation of patients during the latter’s incubation period. These cases accounted for 23.11% (49/212) of the total secondary cases. There were 22 males and 27 females. The average age of the patients was 50.70 years (SD 15.78) (Table 1). According to available information of 47 of these secondary cases, we found the median incubation period was 5.00 (IQR: 3.00-7.00) days, and 82.98% of patients had incubation shorter than 7 days (Table 1).
Table 1. Demographic characteristics and incubation period of COVID-19 cases infected during incubation period of previous cases between 20 January and 11 February 2020.
Characteristic Number of cases
|
(N=49)
|
%
|
Average age*(SD, range)-yr
|
50.70(15.78,7-96)
|
|
<15 yr
|
1
|
2.17
|
15-44 yr
|
14
|
30.43
|
45-64 yr
|
23
|
50.00
|
>=65 yr
|
8
|
17.39
|
Male sex
|
22
|
44.90
|
Incubation period – days
|
|
|
<3 days
|
8
|
17.02
|
3-7 days
|
31
|
65.96
|
8-13 days
|
8
|
17.02
|
*:Age was not available for three cases, which were not included in the mean calculation for total cases.
The median days from the last exposure of secondary cases to the onset of their previous generation of cases were -2.00 days (IQR: -1.00~-5.00 days), ranging from -0.00 to -12.00 days, with a 90th percentile of -9.00 days. This lag was more than -10 days among 8.16% (4/49) of the secondary cases (Figure 1).
We selected six index and seven secondary cases with clear exposure date and illness onset date from six of the 26 above clusters. For index cases, their median incubation period was 4.00 (range: 2.00 ~ 8.00 days) days, while the median days between the last exposure of the index case and the last exposure of the secondary case was 1.00 day. In further investigation of the exposure history, two of the secondary cases (case 3 and case 7 in Figure 2) lived together with their index cases during the incubation period, and the other five secondary cases had continued exposure to their index cases at least half a day with eating, chatting or shopping together.
Cluster 5 and cluster 6, with clear transmission chains, were further examined to analyze the epidemic links. The index case in cluster 5 had lived in Wuhan and came back to Zhejiang on January 15. She fell ill on January 19, and was confirmed by RT-PCR on January 24. On January 16 and 18, her sister (patient 1 in cluster 5) came to visit her home. They ate and shopped together on both days. Patient 1 went back to her own home in the afternoon of January 18, began to feel ill on January 20, and was confirmed by PCR on January 26. On January 19, a banquet was held at patient 3’s home. Patient 1 (patient 3’s aunt) and patient 2 (patient 3’s brother in law) came to visit. They ate and chatted together till nighttime. Both patient 2 and patient 3 in cluster 4 became ill on January 22, and were confirmed on January 26. For details, see Figure 2.
The index case in cluster 6 had lived in Wuhan and came back to Zhejiang on January 20. Her illness onset occurred on January 21 and was confirmed by RT-PCR on January 27. In the nights of January 22 and January 23, her friend (patient 1 in cluster 6) came to visit. They played cards together till the next morning on both nights. On January 28, patient 1 in cluster 6 was isolated as a close contact of the index patient, and fell ill on January 30 (confirmed on January 31). On January 23, patient 1’s daughter (patient 2 in cluster 6) came back home from school because of winter vacation. During January 23-28, they lived together, till her mother was isolated. She also fell ill on January 30, and was confirmed on January 31 (Figure 2).