Epidemiology of Coronavirus Disease in Gansu Province, China, 2020

Jingchun Fan; Xiaodong Liu; Weimin Pan; Mark W. Douglas; Shisan Bao

Disclosures

Emerging Infectious Diseases. 2020;26(6):1257-1265. 

In This Article

Discussion

Our data suggest that most COVID-19 cases that occurred in the first 6 days (January 23–28, 2020) were imported from Wuhan (the coronavirus epicenter). Most secondary cases occurred in the second 6 days (January 29–February 3, 2020).

In Gansu Province, the patients identified in the early period were younger than those identified in the late period, but overall the 54 patients were younger (median age 38 years) than patients identified in the early stage of the outbreak in Wuhan (median age 59 years).[13] This finding is most likely because Wuhan is where the outbreak originated in China.[17] Patients from the early period were younger and had mainly imported primary cases, possibly because most of them were labor workers in Wuhan who traveled back to Gansu Province; in contrast, a greater proportion of patients in the late period were senior retirees with secondary indigenous cases. The proportions of COVID-19 patients who had visited an epidemic area and the frequencies of transfer were 3 times greater during the early than the late period, suggesting that exposure risk would decrease from epidemic areas and public places as the epidemic spread. The current situation may result from the travel bans from Wuhan or other places as well as most persons' awareness and education about COVID-19,[18] but human transportation had spread the virus before Wuhan was locked down.[19]

In our study, distribution of illness by sex did not differ significantly between the early and late periods, but female patients predominated slightly. In contrast, Chen and colleagues reported more male than female patients in the Wuhan outbreak.[20] One possible explanation is that in Wuhan, men dominate the labor market and probably had close contact with the seafood market in Wuhan (the source of the outbreak), resulting in more male patients in the Wuhan study. In contrast, the COVID-19 outbreak in Gansu was 2 weeks later, during the Chinese New Year festival, in which family members of both sexes participated. Thus, our data further support the need for broad infection control measures in surrounding areas, irrespective of age, sex, place of residence, or mode of transportation.

The first case of secondary infection was identified in a patient who had had close contact with the first case-patient with primary COVID-19 in Gansu Province, after a lag of 4 days. Subsequently, 6 family clusters were identified over 12 days. These data confirm direct human-to-human transmission of the COVID-19 virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), through close contact.[21] This finding supports the government restriction of persons gathering in large groups, to reduce or minimize virus transmission. The 95% percentile range for the incubation period found in our study is consistent with the findings for Wuhan, which almost completely overlap,[13] suggesting that within this short time, SARS-CoV-2 has not mutated sufficiently to affect incubation time.[22]

The proportion of persons who visited a medical center or hospital within 2 days from illness onset was 61% in Gansu Province, compared with 27% in Wuhan.[13] In addition, 68% of patients in Gansu Province were hospitalized within 5 days after illness onset, whereas only 11% of patients in Wuhan were hospitalized in the early stages of the disease.[13] These differences between Gansu Province and Wuhan may partially result from differences in numbers of staff and medical resources between Wuhan and Gansu Province, relative to the number of COVID-19 patients. Another potential factor is advanced warning about COVID-19 in Gansu Province via various channels. Although knowledge about COVID-19 was minimal at the beginning of the outbreak, the seriousness of COVID-19 was well broadcast throughout China, including Gansu Province, after the initial outbreak. Compared with the initial stage of the COVID-19 outbreak in Wuhan, the reduced incidence of COVID-19 in Gansu Province over this period may be associated with reinforcement of mandatory control and prevention interventions, patients voluntarily seeking medical assistance after being educated by the media, or possibly both.

Our study demonstrates a significant spatial heterogeneity of COVID-19 cases in Gansu Province over this 2-week period; cases were mostly concentrated in Lanzhou and surrounding areas. LISA analysis findings are in agreement with the spatial distribution of COVID-19 at the county levels of Gansu Province. This analysis confirms that the distribution of cases was not random: hot spots were mainly restricted to the Chengguan District of Lanzhou, the most densely populated and most developed area.[11] This case aggregation is closely associated with the development characteristics of Gansu Province, which is at the high end of economic, medical, population, and cultural development. Consequently, Chengguan District was the most common pathway for most persons returning from Wuhan and other cities, who end up in or transit through Lanzhou. Land formations in Gansu Province limit travel, particularly in mountainous rural areas, which may have slowed down or restricted the spread of COVID-19 across Gansu Province. For example, cases have been reported in Wuwei, a city in a remote, mountainous, hard-to-reach area in Gansu Province. Our findings are supported by other studies, which demonstrate a close correlation with population and economy in other major cities of China where outbreaks of COVID-19 occurred within 1–2 weeks of the original outbreak in Wuhan.[23–25]

Although case numbers are small, in Gansu Province the severity of COVID-19 illness did not differ between persons with primary and secondary cases, suggesting that the virus does not mutate to decrease virulence during ongoing transmission. This finding is consistent with recent reports from Italy and the United States, which suggest that mutations that reduce transmission do not spontaneously develop in SARS-CoV-2.[2,3] In Gansu Province, we observed fewer severe clinical cases of COVID-19, lower rates of patients in critical condition (13%), and no deaths, compared with Wuhan, where ≈30% patients were admitted to intensive care and 4% died.[26] One possible explanation is insufficient medical staff and resources in Wuhan to deal with the large outbreak and only extremely serious or critical patients being hospitalized, in contrast to Gansu Province, where relatively more medical staff and resources were available. Furthermore, a large proportion of the Wuhan population who left the city before the lockdown were migrant workers, commercial personnel, and college students; the population remaining in Wuhan after the lockdown therefore comprised mostly elderly persons with compromised immunity.[27] The incubation period in Gansu Province was longer than that in Wuhan (3–6 days for Wuhan).[28,29] This finding may be the result of the government's intensive provision of information for the whole nation about controlling and preventing COVID-19.

A limitation of our study is the difficulty of calculating county-level incidence or estimating the risk factors affecting SARS-CoV-2 transmission in Gansu Province because of the relatively small number of cases and the short study period (i.e., 12 days), which may not reflect the entire epidemiology of the virus in Gansu Province and will only become clear over time. Whether the outbreak will be controlled soon, and if so when, will be answered in due course.

In conclusion, our study demonstrates the epidemiology of a relatively small-scale outbreak of COVID-19 outside of Wuhan. Our cohort included primary cases from Wuhan and subsequent secondary cases, including several family clusters. Such information should be useful for other regions and countries to help combat the spread of this lethal disease[30] by informing the development of more effective local infection control policies and recommendations.

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