Background
Coronavirus disease 2019 (COVID-19) is caused by SARS-CoV-2 with typical symptoms of fever, dry cough, and tiredness [
1‐
3]. On average, the incubation period is 5–6 days from the time someone is infected with the virus to the onset of symptoms, with a maximum of 14 days [
3]. Nucleic acid detection and genome sequencing have commonly been conducted with pharyngeal swabs, sputum, alveolar lavage fluid, feces, and other samples from patients to detect SARS-CoV-2 [
4‐
8]. It has been reported that COVID-19 can be transmitted person-to-person, with the main transmission methods being either by air or contact [
9‐
13]. Therefore, persons can be infected by inhaling droplets or aerosols containing the etiologic agent SARS-CoV-2 that are exhaled by someone with the infection, or by contacting virus-contaminated items.
The World Health Organization (WHO) announced that this disease represented a public health emergency of international concern. Due to its diverse transmission routes and strong transmissibility, COVID-19 quickly became pandemic. As of April 8, the number of confirmed cases worldwide reached 1,353,361 and there were 79,235 cumulative deaths [
14]. According to the report of the Chinese Health Commission, as of April 9, a total of 81,865 confirmed cases and a total of 3335 deaths were reported in China [
15]. Since Jilin Province launched the Public Health Events level I emergency response on January 25, the epidemic in Jilin Province has been controlled by implementation of measures to control the non-resident population, such as isolation and observation at home, temperature measurement screening, and wearing masks [
16]. According to data from the Jilin Provincial Center for Disease Control and Prevention, as of March 14, a total of 97 cases with one death were reported [
17]. Although the severity of the domestic epidemic has declined, the problems of imported cases and asymptomatic cases remain serious.
Several studies of COVID-19 transmission models have been conducted to evaluate the transmissibility of the virus and predict the future pandemic situation [
9,
18‐
20]. In COVID-19 transmission models, the influence of asymptomatic infection factors in the transmission process is considered. This study is based on our previous research, with the addition of an asymptomatic infection factor. We use the epidemic data of Jilin Province to re-verify the applicability of the susceptible–exposed–infectious–asymptomatic–recovered/removed (SEIAR) model, and to further discuss the role of asymptomatic infection in the spread of COVID-19 [
21‐
24]. The more important issue at present is to consider asymptomatic infections when designing models. Asymptomatic infection refers to cases who tested positive for COVID-19 in laboratory tests and had no symptoms, but can still potentially transmit the virus to others. In a report in Nature on March 20, 2020, a public health expert from Wuhan Huazhong University of Science and Technology noted that “at least 59% of the infected individuals were out and about, without being tested and potentially infecting others” [
25]. The calculated transmissibility results of a model will differ, depending on whether the model considers asymptomatic infection. That is, ignoring asymptomatic cases will affect the accuracy of the model. At the same time, traditional infectious disease models were built under the condition that the disease is allowed to develop [
2,
9,
18,
19,
26‐
30]. However, China declared a first-level health emergency in the early stage of the outbreak, and, with a strict supervision system and a high degree of cooperation of the people, a series of prevention and control measures were implemented, such as wearing masks, restricting travel, and suspending work and school. In this study, our COVID-19 model was established with thorough consideration of most of the possible comprehensive prevention and control measures that exist. Moreover, there is no domestic province that can be used to construct a dynamic model of the spread of COVID-19 according to the local population characteristics and distribution. Hence, the transmissibility of COVID-19 in Jilin Province remains unclear and the effect of current prevention and control measures on the pandemic still needs to be explored. This study focused on the SEIAR model based on the distribution of outbreaks in Jilin Province. The various parameters in the model were calculated based on the actual cases obtained, to accurately model the real situation. This study explored the goodness of fit between the model and actual data, calculated the transmissibility of COVID-19 in Jilin Province, and evaluated the effectiveness of local health departments’ prevention and control measures. We further predicted the progress of theCOVID-19 pandemic if no measures were taken at the corresponding time point or if intervention measures were implemented at different time points.
Discussion
Based on the epidemic situation of COVID-19 in Jilin Province, we constructed a transmission dynamics model that accorded with the population characteristics of the province. Furthermore, based on the collection of 97 cases as of March 14, the true parameters of Jilin Province were calculated. Using imported cases as the source of infection, the model calculated fitted secondary cases based on local secondary cases. Therefore, the design of the model, the calculation of parameters and the fitting of data were consistent with the actual situation in the province, and the transmissibility index was accurate.
According to the temporal distribution of COVID-19 in Jilin Province (Fig.
3), the imported cases in Jilin reached a peak on January 22, and decreased after January 23. Since January 31, the imported cases have remained at a low level. On January 23, the city of Wuhan was closed. At the same time, Jilin Province implemented measures involving screening and isolation for outsiders. This time coincided with the period of decline in imported cases, indicating that the above interventions had obvious effects. On January 31, 2020, the measures of having a flexible working system and fewer meetings were implemented. Personnel were required to wear masks when entering or leaving public places. From Fig.
3, we can see that since February 1, the number of secondary cases and daily actual incidence has been decreasing. Since January 31, 2020, the implementation time of intervention measures such as reducing travel and wearing masks has been consistent with the incidence decline time. This shows that the above intervention measures were effective during this period.
The National Health and Construction Commission of China analyzed more than 8400 cases that recovered and discharged. The clinical classification of these cases shows that the proportion of mild and normal cases is 90.8%, the proportion of severe cases is 7.2%, and the proportion of critical cases is 2% [
37]. The mild and normal cases in Jilin Province accounted for 89.2% of all clinical classifications, which is consistent with the national clinical classification distribution [
38]. This shows that most cases have mild symptoms and are as easily treated as patients with common influenza. For this reason, it has been difficult to investigate with whom infected persons have had close contact. Therefore, many sources of infection were not effectively isolated in the external environment in the early stage of the disease and in the early stage of the outbreak, which was the main reason for the public response delay in the early stage of the outbreak.
The age of onset of COVID-19 in Jilin was mainly between 20 and 59 years. Among these cases, people aged 30–49 years most commonly had mild and normal cases [
39]. Therefore, among young adults and middle-aged persons, prognosis is better and mortality is low.
In this study, according to the time distribution characteristics of the epidemic curve of COVID-19 in Jilin Province, taking February 1st as the time cut-off point, the data were divided into two sections to fit the secondary cases; the fit was improved (R2 = 0.593, P < 0.001). According to the fit results, Reff of the first stage (before February 1) was 1.64, indicating that the infection source of COVID-19 could infect approximately two people during the infectious period. If intervention had not occurred in time, allowing the disease to progress naturally, COVID-19 in Jilin would have continued to spread widely. The Reff in the second stage (after February 1) was 0.05; that is, the infection source of a new coronavirus could infect 0.05 people during the infectious period, indicating that the epidemic situation had been controlled by this stage. The comprehensive intervention measures in Jilin Province reduced the transmissibility of COVID-19 by 96.95%.
Combined with a series of related measures since the outbreak in Jilin Province, a series of other measures were also launched on January 22, including closing tourist spots, suspending business operations, ensuring good sanitization and ventilation in public places, and banning trade in wild animals. On January 25, 2020, the Jilin provincial government launched a Public Health Events level-I emergency response, strengthened the investigation of non-native people and isolated non-native people at home, strengthened body temperature testing, implemented disinfection and sterilization measures, encouraged wearing of masks, and strengthened the management of large-scale activities. From January 31, 2020, the unit flexible working system was implemented to reduce the number of meetings and personnel input. The above measures were effective in the second stage of COVID-19, and transmissibility was reduced by 96.95%. Additionally, by the deadline (February 19), the actual number of secondary cases had been reduced by 92.31%, so that the pandemic was controlled. If Jilin had not taken measures and had allowed the disease to develop before February 1st, the prevalence of COVID-19 would have continued to spread in the province. The peak would have been reached by October 22, 2020, with the number of cases on that day being 177,011. The pandemic would have continued to be prevalent for 22 months, with a cumulative number of 17,129,367 cases, and an attack rate during the pandemic of 63.66%. Therefore, the series of prevention and control measures formulated and implemented in Jilin Province effectively controlled the progress of the COVID-19 pandemic, and, to the extent possible, helped avoid an interpersonal epidemic.
In the early stage of the outbreak, we developed a Bats-Hosts-Reservoir-People transmission network and assessed the human-to-human transmissibility of COVID-19 in Wuhan as 3.58 [
21]. Studies have been conducted on the transmissibility of COVID-19 in different provinces and cities in China at different time periods, which found that the reproduction number ranged from 1.4 to 6.49, with a median of 2.79 in 12 studies [
40]. Alimohamadi et al. used systematic reviews and meta-analysis to estimate the pooled
R0 as 3.32 (95% CI, 2.81 to 3.82) [
41]. Musa and others estimated that the
R0 of COVID-19 in Africa was 2.37 [
42]. Torres-Roman et al. estimated the overall basic reproductive number in Peru during the outbreak period was 2.97; Lima had a similar outcome, with an
R0 of 2.88. Previous studies found that the transmissibility of COVID-19 in Jilin Province was lower than in other provinces and cities in China. Compared with densely populated cities, such as Wuhan, people living in Jilin Province have less contact with people, and higher social distance. This reduces the possibility of susceptible people contacting the infection; hence, the transmissibility in Jilin Province is lower than that in cities with higher exposure. This also illustrates the importance of isolation and increasing social distance. At the same time, due to geographical factors, the outbreak of COVID-19 was late to reach Jilin Province. The early outbreaks in cities such as Wuhan and Guangdong led to the accumulation of experience in responding to the outbreak by China’s health departments and the people. The people’s prevention and control measures were highly coordinated, and the health system responded quickly. As a result, compared with some European, African, and other countries, transmissibility in Jilin Province remained lower than that of other states. This shows that the earlier the medical system responds, the easier it is to control the spread of the outbreak. In the current study, we also found that most studies used the date of onset of confirmed cases to fit the model. However, because the data collection occurred at the beginning of the outbreak, there were some onset cases that had not been detected and reported. The incompleteness of the epidemic curve may cause
R0 to become higher [
41]. At the same time, the low early disease incidence and uneven quality of case reports may contribute to the difference in
R0 [
43], showing that the more complete the data when estimating the transmissibility of infectious diseases, the more accurate the research results.
Limitations
The parameters in this research model were calculated based on the actual data of Jilin Province; therefore, data quality was high. However, the small number of actual cases would have affected the calculation of the model. There were only four asymptomatic infections in the data obtained, which reduced the reliability of the proportion estimation of asymptomatic infections in Jilin Province. At present, studies have shown that asymptomatic infections also have transmissibility. Such cases are not easy to find and isolate, which promotes the spread of disease and the outbreak. This model considered the effect of asymptomatic infection in the population. Therefore, error in the proportion of asymptomatic infections may have caused the prediction results to deviate from the actual situation.
In this study, the reciprocal of the incubation period calculated using the actual data of the COVID-19 spread in Jilin Province was a parameter in the model; thus, the accuracy of the incubation period calculation can also affect the model’s prediction. The incubation period of COVID-19 is 5–6 days [
44], and the incubation period of the disease calculated in this study was 10 days in Jilin Province. The reason for this discrepancy may be that the time of contact with the first case is uncertain, and there are some cases with unclear contact time, such as repeated or continuous contact. Therefore, it is necessary to clarify the activity trajectory of secondary cases, or how long susceptible persons may infect others after being exposed to the source of infection. This is also a direction for exploration in future research.
In accordance with the epidemic trend of the disease, this study fitted the actual number of secondary cases in two stages. Additionally, the transmissibility of COVID-19 after February 1 was evaluated, and the effectiveness of preventive measures was verified. However, this study evaluated comprehensive prevention and control measures, but did not evaluate specific measures. It is not possible to determine which specific measures produced an effect. To solve this problem, it will be necessary to establish a model that considers individual prevention and control measures. However, the specific implementation time and completion status of each measure are difficult to determine, so this is likewise difficult to achieve.
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