The novel coronavirus disease (Zhu et al., 2020) . Early cases of COVID-19 suggested that it might not be as severe as SARS-CoV and MERS-CoV, but the rapid increase in confirmed cases and the evidence of human-to-human transmission indicated that this coronavirus was more contagious than both SARS-CoV and MERS-CoV Guan et al., 2020; Chan et al., 2020) .
Compared with the outbreak of SARS, the Chinese government mounted responses rapidly to contain COVID-19 by reducing the risk of personal exposure and transmission (Tu et al., 2020) . As the city with the largest number of confirmed cases in Zhejiang province (National Health Commission of the People's Republic of China, 2020a), where it activated the first-level public health emergency response on January 23, 2020, Wenzhou had suspended all inter-provincial and inter-city shuttle buses, and chartered buses, and carried out temperature testing for passengers of buses arriving in Wenzhou as its own policies to deal with the epidemic situation since January 27, 2020 (Wenzhou Municipal People's Government, 2020a). These strict control measures were implemented across Wenzhou, where one family member was assigned to go out for necessity purchase of each household every two days since February 1, 2020 (Wenzhou Municipal People's Government, 2020b). Thus, Wenzhou had adopted the stringent intervention policies, including both traffic blockade inside and outside the city, and the strictly closed management of the community, to intensively prevent and control of the epidemic situation of COVID-19. In addition, an industrial city of Guangdong province, which activated the first-level public health emergency response on January 23, 2020 (Health Commission of Guangdong Province, 2020), Shenzhen adopted closely following the close contact of confirmed cases and isolating them on the same time.
Comparatively, Shenzhen implemented relatively mild but early interventions.
This paper concentrated on the strict intervention policies of COVID-19 that started on February 1, 2020 in Wenzhou and evaluated the treatment effects of those strict interventions, providing insights into the effects of the strict interventions. We also assessed what could have happened in Wenzhou if the stringent interventions were delayed by 0 to 5 days or if mild interventions were implemented instead. We accomplished this through simulation by using the time-varying Susceptible-Infected and infectious without isolation-Hospitalized in isolation-Removed (SIHR) model (Tan et al., 2020) . Finally, we simulated the likely outcomes if relatively mild interventions as used in Shenzhen, China, were implemented in lieu of the stringent interventions.
In the next section, we describe the classical causal effects methods and dynamic transmission models of COVID-19 in detail. The corresponding analysis of epidemic data in Wenzhou are illustrated in Section 3. Section 4 discusses the effects of stringent interventions of COVID-19.
We compiled a total of 504 confirmed cases (Health Commission of Wenzhou, 2000) from January 21, 2020 when the confirmed cases were first reported, to February 25, 2020 when no newly reported confirmed cases had been reported for a week, from the official website of the health commission of Wenzhou, and obtained the residential population of Wenzhou from the 2019 statistical yearbook of the Zhejiang Provincial Bureau of Statistics (Statistics Bureau of Zhejiang Province, 2019). The total confirmed cases of Shenzhen during the same period were collected. The cases per 100,000 individuals were calculated by the prevalence of COVID-19 multiplying 100,000 individuals in Wenzhou. We searched the daily number of cumulative confirmed cases in each city of China using the R package "nCov2019" (Wu et al., 2020) , and collected the number of cumulative confirmed cases from the top 100 cities for the epidemic of COVID-19 (except Hubei province) on February 25, 2020. The corresponding population density and GDP per capita statistics of these 100 cities were obtained from their latest statistical yearbooks.
To identify the treatment effects of the highly stringent interventions implemented on February 1, 2020, we compared Wenzhou with cities without those interventions. The synthetic control method was used to construct a data-driven control-group in the absence of those interventions in Wenzhou (Abadie and Gardeazabal, 2003; Abadie et al., 2010) . The observed cases per 100,000 for Wenzhou (active intervention city) at the time t with interventions were denoted by Y 1 1t , and cases per 100,000 for cities j at the time t without interventions by Y 0 jt . The difference of Y 1 1t − Y 0 1t was the treatment effect of the interventions on the cases per 100,000 for the active intervention city in the post-interventions period. Note that Y 0 1t is counterfactual for the active intervention city in the post-interventions period. Reliable estimates of cases per 100,000 for a synthetic intervention city were used to derive Y 1 1t − Y 0 1t , where a synthetic intervention city resembled the active intervention city by possible preinterventions characteristics. We selected the GDP per-capita , population density (Oto-Peralías, 2020), and three-day cases per 100,000 before the implementation of interventions to construct a resembling city of Wenzhou. The effects of unobserved factors varied over time and were controlled by the linear combination of cases per 100,000 of preinterventions (Abadie et al., 2010) . Thus, the treatment effect of interventions in Wenzhou was formulated as (Abadie et al., 2011) :
To determine J, the number of cities demographically similar to Wenzhou, we selected the top 100 cities of cumulative confirmed cases on February 25, 2020 in China (except Hubei province) and used hierarchical clustering (Ward Jr, 1963) to group those cities based on population density and GDP per capita. A homogenous group including Wenzhou and other J cities were grouped together. Thus, those J cities could be considered as highly resembling to Wenzhou. The sum of weights for each of J cities was 1 and the values of weights in Equation (2.1) were determined by pre-interventions characteristics. The placebo tests were used to identify the magnitude of the treatment effect (i.e. the causal effect of interventions) (Abadie et al., 2010) .
For the strict intervention policies of Wenzhou, we vertically compared the difference in the epidemic situation of COVID-19 before and after the implementation of such intervention policies. Regression Discontinuity (RD) (Thistlethwaite and Campbell, 1960; Hahn et al., 2001; Imbens and Lemieux, 2008 ) was used to evaluate whether the implementation of strict intervention policies of Wenzhou on February 1, 2020 had a significant effect on its own COVID-19 epidemic. The outcome of these stringent interventions was formulated as:
where the logarithmic transformation of the daily number of cumulative confirmed cases as a response variable. The values of t include 1 to 36, representing the number of days starting from January 21, 2020 as a rate variable in Equation (2.2) of RD, for example, the date of February 1 was denoted as 12 of t. h is an indicator variable for the implementation of the strict intervention policies in Wenzhou, given a value of 1 as the implementation of the policies but a value of 0 as no implementation of the policies. This indicates that the values are 0 and 1 before and after February 1, 2020 (including February 1 itself), respectively and the subscript of t indicates the number of days as defined before. c is the number of days corresponding to the cutoff-point whose value is 12, representing the policy implemented on February 1, 2020. The values of k could be denoted as the k th order of interaction effect between rate variable and policy. We compared the k th order of interaction effect item in the Equation (2.2) by AIC.
In RD, to determine whether c is an obvious jump point, the plot of both the rate variable and response variable was used to identify their relationship (Calonico et al., 2015a) .
Different numbers of bins could be used to divide both the left-hand side and right-hand side of the cutoff-point to small intervals. Thus, the scatter plot of both median values of the rate variable and mean values of the response variable could be drawn (Calonico et al., 2015b) . The actual rate variables are weighted uniformly in each bin, and the fitted curves of both the left-hand side and right-hand side are used to examine whether the cutoff-point can be considered as a threshold.
(SIHR) model
The dynamic system of SIHR with four classes: Susceptible (S), infected and infectious without isolation (I), hospitalized in isolation (H), removed (R) (Li, 2018; Hsieh et al., 2004; Tan et al., 2020) was defined as:
where the transmission rate α is the average rate of being infected given contact over unit time, 1/β is the mean of the incubation period, 1/γ is the mean of the hospitalization period.
To consider the effects of interventions, we introduced the time-varying transmission rate, which was defined as (Tan et al., 2020) :
where α 0 denotes the maximum transmission rate of COVID-19 during the early outbreak, Before the strict interventions were implemented on February 1, 2020, the trends of actual cases per 100,000 in both Wenzhou and "synthetic Wenzhou" were highly similar, suggesting that this synthetic city can be used to estimate the "counterfactual" results of Wenzhou.
After 2 days of the implementation of the policies, the growth rate of actual cases per 100,000
in Wenzhou remarkably slower than that of "synthetic Wenzhou". The number of cases per 100,000 in "Synthetic Wenzhou" on February 25, 2020 was 10.32, which is 1.69 times the actual Wenzhou (6.08) (Figure 1) gap between Wenzhou and synthetic Wenzhou.
The values of AIC for each regression discontinuity model for different k th order of interaction effects between rate variable and policy were shown in Table 1 . Table 2) . 
Adjusted R 2 0.994 q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q 
Our simulation projected that the expected cumulative confirmed cases would be 1.84 times of the actual cases for a 2-day delay, 2.45 times for a 3-day delay, 3.26 times for a 4-day delay and 4.30 times for a 5-day delay on February 25, 2020. The corresponding 95% credible interval (CI) for these projected numbers of cases were presented in Table 3 . The full simulation results from January 21 to February 25, 2020 are presented in Figure 2 of the Supplementary Materials. According to q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q 
We used daily reported cumulative confirmed case data from January 21 to February 25, 2020 in Wenzhou. By using the synthetic control method, the trend of COVID-19 cases per 100,000 for the synthetic Wenzhou as a control group of no intervention were compared with those for Wenzhou as a treatment group with interventions. The COVID-19 cases per 100,000 in Wenzhou were significantly lower than those in synthetic Wenzhou after February 3, 2020. This indicated that the implementation of the strict interventions on February 1, 2020 in Wenzhou had a significant effect on controlling the epidemic of COVID-19.
By using regression discontinuity analysis, we also concluded that the implementation of strict intervention policies had a significant treatment effect on the epidemic in Wenzhou.
Moreover, the statistical treatment effects were evaluated. That is, since the intervention policies were implemented in Wenzhou (corresponding coefficient of policy : -0.350), the growth rate of the reported cumulative confirmed cases of COVID-19 were reduced, and this "reduction" effect would increase over time (corresponding coefficient of interaction Tai'an 726.7105 64714 0 q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q 0 500 1000 1500 2000 0 1 / 2 2 0 1 / 2 4 0 1 / 2 6 0 1 / 2 8 0 1 / 3 0 0 2 / 0 1 0 2 / 0 3 0 2 / 0 5 0 2 / 0 7 0 2 / 0 9 0 2 / 1 1 0 2 / 1 3 0 2 / 1 5 0 2 / 1 7 0 2 / 1 9 0 2 / 2 1 0 2 / 2 3 0 2 / 2 5 Date Number of cumulative confirmed cases Delay (Days) 0 1 2 3 4 5 q Actual cumulative confirmed cases 