The COVID-19 outbreak that originated from Wuhan, a major transportation hub of China, had caused more than eight million confirmed cases and 444,563 deaths in the world as of June 17, 2020 [1] . Multiple countries reported having their first cases imported from Wuhan or by a returning visitor to Wuhan [2] [3] [4] [5] [6] [7] . Thus, it is important to understand the infection transmission dynamics within the epicenter where the virus first originated, especially before intervention measures were taken such as issuing a lockdown of the city and other social distancing policies. In spite of the worldwide interest in understanding the initial phase of the pandemic in Wuhan, researchers' efforts have been hampered by lack of transparency including the lack of comprehensive and reliable data from this period.
By the time of the lockdown on January 23,2020, the Chinese official report for the COVID infection in Wuhan is 495 cases. However, media reports from China suggest that the official confirmed cases in Wuhan before and around the lockdown were significantly underreported. This under-reporting seems to be due to multiple factors, such as limitation of testing and surveillance capability [8, 9] ; limited quantity of test kits provided to local hospitals [10] and generally low testing accuracy (low sensitivity, 30-60%) [11] . More importantly, government policies and intervention, such as the different versions of diagnostic criteria, seem to have a heavy influence on the information sharing and preparation for the pandemic [12] .
The current study investigates the early outbreak through search and screening into various types information, including interviews with those who were in direct contact with patients, Chinese media reports and government reports. We found that the key information of human to human transmission had not been reported by the Chinese government for at least three weeks, despite convincing evidence. We provide our estimates of the impact of this delay of three weeks and other epidemiological parameters related to COVID-19 outbreak in Wuhan, before the lockdown and other public quarantine measures on January 23, 2020.
In an effort to reconstruct what happened before and during the lockdown in Wuhan, we screened media reports published in mainland China from January to April 2020, covering the details about various events and measures taken by local hospitals and government on various levels concerning the Wuhan lockdown. We have complied the epidemic relevant events into a timeline so that the scientific community will have information that can potentially impact understanding the dynamics of the COVID-19 virus in Wuhan, China.
We searched and profiled data through the following different information channels. 1). Pubmed search using keyword "COVID-19". Search results were screened for daily incidence by PCR diagnosis from admitted patients. Two references were found [13, 14] . [Appendix A, column B].
2). Screening of published Chinese media reports with recounts from local physicians and hospitals. Search results were screened for daily and total hospitalization cases including the PCR confirmed cases and patients with high suspicion of COVID infection with severe symptoms. We found the following Chinese media reports and government announcements [15] [16] [17] [18] [19] [Appendix A, column C].
strongly correlated to the known COVID pattern in two radiology departments in two hospitals in Wuhan from media reports [9] [Appendix A, column H].
We have created three data categories for our calculations:
A: Hospitalized patients with COVID infection confirmed by PCR in Wuhan citywide.
B: Hospitalized patients due to COVID infection in Wuhan citywide (including highly suspicious clinical cases not yet confirmed by PCR or local diagnosis criteria).
C: Daily symptomatic incidences based on daily cases with COVID patterns identified by radiologists from CT reading rooms from two similarly sized Grade III Level A (tertiary) hospitals.
Timeline of COVID-19 Outbreak Before Lockdown and Chinese Government's Reaction A brief timeline of COVID-19 outbreak, focusing on features of human-to-human transmission, before Wuhan lockdown on January 23, 2020, is listed in Figure 1 were not limited to, prohibiting medical staff from releasing information to public, prohibiting discussions among medical staff on the "pneumonia of unknown etiology", prohibiting staff from wearing PPE (personal protection equipment), prohibiting 3rd party labs from doing viral sequencing, making diagnostic criteria difficult for diagnosis confirmation, deleting reported cases, disciplinary action against "whistle blowers" and closure of the lab which shared viral genome information with the international community.
Wuhan local and the Chinese National CDC has implemented at least seven different versions of diagnosis criteria since the outbreak, thus making it hard to identify and understand the infection number in a consistent manner [12] . If taking infection with mild or no symptoms into account, a study estimated that 86% of infected individuals were not documented in China before January 23, 2020 [22] . Policies and interventions for reporting (holding cases for 12 days) made the confirmed cases artificially low, as reported by mainland Chinese media [15, 16, 23] . press conference almost daily on the epidemic.
Assuming the ratio of symptomatic cases in the total infected population is constant, and the proportion of symptomatic patients seeking medical help remains constant over the time, we use the growth rate of hospitalized people, PCR test positives or CT positives in our dataset as the proxy for the growth rate of the infected population.
R0 represents the average number of secondary cases that result from the introduction of a single infectious case in a totally susceptible population during the infectious period. Wallinga
and Lipsitch have pointed out that the exponential growth rate (r) during the early phase of the outbreak can be used to calculate the R0 by using the formula R0 = 1/M(-r), where M is the moment generating functions of the serial interval (SI) distribution of the disease [24] .
We assumed a gamma distribution for the serial interval, with 7.5 as the mean and 3.4 as standard deviation, based on a prior study of Wuhan data of the initial epidemic stage [13] . R package "R0" is used to provide estimates of the R0 and its 95% confidence interval [25] .
In the Feb. 3 speech, Chinese President Xi told China's most powerful leaders that he had "continuously given verbal and written instructions" since Jan 7, and he had personally ordered the quarantine of about 60 million people in Hubei province later in January. We found PCR reported cases decreased after January 9, 2020, while no countermeasure had been taken by then.
The decrease in reporting could well be driven by political reasons. Thus, we utilize (dataset A)
PCR results from December 1, 2019 to January 8, 2020 (the growing portion which is less subject to political intervention) [ Figure 2 ], R0 is estimated to be 3.12, with 95% confidence interval as (2.69, 3.64) [ II.II. Estimate of incidence in Wuhan based on the SEIR model Susceptible-exposed-infectious-removed (SEIR) model is applied to estimate the incidences. The following epidemiologic parameters are assumed in the estimation [13] . admission rate. The above hospital admission ratio was based on the estimates of infectious cases, many infected but no longer infectious cases needed medical care as well.
The period of the three weeks before the lockdown was a crucial time in the early stage of the epidemic. If countermeasures would have been taken on January 2, 2020, immediately after the authorities were informed about the human-to-human transmission, the epidemic development would have taken a completely different trajectory. If we assume the intervention reduces the daily contacts of the infectious by 2/3, our study estimated that there would have been 10,989 total infected cases, vs 239,875 cases when lockdown was taken place on Jan 23, 2020. The group of people with infectivity (infectious+ latent) would be reduced from 149,170 to 1,916, and the daily new cases of the infectible group would be reduced from 30,922 to 10 on January 23, 2020 [ Table 3 ] [ Figure 3B ]. This demonstrates that if countermeasures would have been taken three weeks earlier, the epidemic would have been largely contained by January 23, 2020.
By the time the authorities locked down Wuhan, about five million people (about 5/14 of the total Wuhan population) had left Wuhan, resulting in a potential outflow of 41,008 exposed cases and 12,267 infectious cases, contributing directly to one of the most serious public health crises of this century.
The above estimations assume the intervention reduces the daily contacts of the infectious by 2/3. After the long delay, Wuhan took a drastic lockdown measure to contain the virus spread, which reduced the daily contacts of the infectious by more than 2/3. If a strict intervention would have been applied on January 2, 2020, we would have seen an even larger decline in the epidemic spread.
Potential impact of the delay of the critical three weeks Based on our media report screening, we did not identify any public quarantine measure taken by the government in Wuhan before January 23, 2020. Instead, communities were encouraged to continue hosting large scale gatherings to celebrate the lunar New Year. On January 18, 2020, over 40 thousand families attended the Ten Thousand Families Banquet at the Baibuting Community, none of the attendees wore masks. Three days before this event, some organizers asked to cancel the banquet, but the request was not approved [34] . In the meantime, the authorities took steps to underreport the incidence and infectivity of COVID-19 to China and the world.
In this study, we did not incorporate the dynamics of the population in Wuhan. About five million people left Wuhan before the lockdown, with a large portion happening during the week before Chinese New Year's Day (January 25, 2020). The infected among this emigrant group got in contact with population outside Wuhan, making the susceptible number larger.
Also, the higher frequency of interpersonal contacts of these emigrants before the Chinese New
Year's Day will result in a higher R0. Hence, the estimation of the infected presented in this paper is likely to be an underestimate. 
To Chinese physicians and all health care professionals, among them are my classmates, and to media reporters, for their courage and professionalism.
We thank Professor Lucia Dunn at the Ohio State University for her help on data analysis and language editing of this manuscript.
None
The authors declare no conflict of interest. Timeline of COVID-19 Outbreak in Wuhan, before Lockdown on Jan 23, 2020 *Executive Summary*  This timeline attempts to be comprehensive, with the main focus on "human-to-human" transmission  By January 1, 2020, there had been convincing evidence of "human-to-human" transmission  On January 20, 2020, Chinese communist party (CCP) government admitted "human-tohuman" transmission including infection of medical staff  Between January 1 and January 20, CCP government had been hiding the epidemic related information, suppressing the spread of epidemic related information and denying "human-to-human" transmission Basically, the "human to human transmission" and "viral genome" are the key clues that we focus this timeline on. which reported to local county CDC. Among these 4 patients, there was a pair of mother-son, another example of human-to-human transmission.
A: Local CDC and City Office arrived in Central hospital, surveyed and sampled the 7 patients. 
A: Wuhan City Health Commission issued 2 emergency policies, the 2nd one stated that "no one or institution is allowed to disclose information about the pneumonia without authorization."
A: Wuhan City Health Commission broadcasted to major hospitals internally that "1, this pneumonia is not SARS, 2 no human to human transmission, 3 does not cause death; 4 no medical staff infection". (see above right) 12/30/2019 B: Wuhan Central Hospital clinical departments shared the message/instruction from Wuhan City Health Commission not to "casually announce the current situation to public or outsider, otherwise they would be seriously punished".
A: Leadership in Wuhan Central Hospital informed all division chiefs to inform their staff that they were not allowed to leak any virus related news and not allowed to wear facial masks.
A: Chinese National Health Commission dispatched a panel of experts to Wuhan for investigation. Chinese CDC chief went to Wuhan on the earliest flight that day.
12/31/2019 B: Zhongnan Hospital in Wuhan initiated a special team for "Pneumonia with unknown etiology". The team leader asked all departments to prepare for infectious disease with human to human transmission potential but need to keep this confidential.
12/31/2019 B: Xiehe hospital in Wuhan established 24 beds in an isolation unit for cares of "Pneumonia with unknown etiology" only, and organized re-distribution of resources including staff.
A: Dr. Li Wenliang was called to hospital administration office for his mistake on spreading "rumors" and was asked to provide written statement for his "mistake".
A: Infectious control staff in PPE came to do cleaning in the Seafood Wet Market with sprayer.
A: Wuhan media received messages that no reports were not allowed. They needed to follow official statements.
A: Wuhan City Health Commission announced at 1:30 pm that recent pneumonia cases seen in some hospitals were related to the Wet Market; so far 27 cases were found, with 7 severe cases, rest cases were stable, and 2 cases improved and would be discharged soon; investigation suggested above were viral pneumonia; so far no obvious findings of "human to human transmission" and no infection of medical staff.
Note: For comparison (or see Table 3 ).
A: China reported to WHO about a series of "Pneumonia with unknown etiology" in Wuhan.
B: Taiwan inquired China and WHO for more information about the outbreak, asking for details about the "Atypical Pneumonia" "patients quarantined" with a strong suspicion of human to human transmission. Tianwan started border infectious screening on direct flights from Wuhan to Taiwan.
Hong Kong media reported the presence of "new SARS" in Wuhan. Hong Kong government had press conference on the epidemic. They started almost daily press conference. Central hospital for her "making rumors", she needed to verbally tell all her 200 staff not to disclose anything about this pneumonia. She was also told she can not use Wechat or text but only by face to face or phone call. Dr. Ai raised the issue of "human to human transmission" and she received no response.
Wuhan Central Hospital leadership informed all staff, discussion of "pneumonia of unknown etiology" was not allowed except at the time of hands-off, any forms of documentation were not allowed.
A: Wuhan Institute of Virology finished sequencing on 1/2/2020 of COVID-19, which was characterized as a "novel" coronavirus, the results were presented to GISAID on 1/11/2020 and publicized on 2/4/2020.
A: CCTV13 aired the news that eight people in Wuhan were punished by "making and spreading rumors". Li Wenliang was about to be fired by the hospital after he was reprimanded by the hospital admin. Hospital admin also issued commands verbally such as "can not tell", "can not wear masks". Hospital admin agreed for the ER, respiratory unit and ICU staff to wear N95 masks but asking other departments not to wear any masks, which lead to massive medical staff infection due to exposure to virus without protection in central hospital: Up to Feb-11, more than 230 medical staff were diagnosed with COVID-19 infection and up to March-09, 4 physicians died, including Dr. Li Wenliang.
A: Wuhan Health Commission : So far 44 cases with 11 severe cases. No evidence of "human to human transmission", no infection of medical staff.
B: Wuhan reported pneumonia through an "online direct reporting system" for the first time to Chinese CDC. Chinese CDC former vice chief Yang Gongchuan stated that she asked the national CDC and confirmed that there was no "direct report" from Wuhan in 2019. Chinese CDC mentioned receiving a direct report from Wuhan from 1/3/2020 -1/10/2020, but reporting stopped suddenly till 1/24/2020. Green cover has 9 files in it while the one with white cover has 10 files. Wuhan Health Commission distributed protocol with white cover with very strict diagnostic criterias. The green protocol was given to Chinese National Health Commission but the white protocol was used for medical practice in Wuhan. The fundamental difference is that "contact with Wuhan seafood wet market or contact history with suspected patients" is a must for diagnosis in the white protocol but optional in the green protocol. Following the white protocol will lead to massive underdiagnosis, under testing and under reporting.
A: Chinese CDC Institute of Virology finished sequencing the whole genomic sequence for the novel coronavirus. This sequencing information were kept as secret.
A: Chinese National Health Commission published "Document on 3" (or Prohibition on 3rd) on 1/3/2020 to prohibit any organization from publishing information about pathogen detection and test results, asking all organizations to destroy virus samples or transfer to designated institutions. Any organization of individuals that did not follow this policy will be legally punished. Commission to warn them that the new virus is in the same origin with SARS and spread from the respiratory tract and recommended disease control measures in public.
A: One physician in the Pulmonary Medicine department has abnormal findings in his lungs in Wuhan Xinhua hospital. The hospital held an internal meeting which stressed that it was not allowed to leak this information out especially "can not tell the media".
A: Wuhan "Two sessions" started. No public media updated the public about the outbreak.
A: Chinese National CDC started 2nd degree emergency response internally.
Chinese President Xi asked that the anti-epidemic measures should not affect normal life.
http://www.qstheory.cn/dukan/qs/2020-02/15/c_1125572832.htm 1/9/2020 A: Insider reported to Caixin Media that Zhongnan Hospital public health department submitted 2 pneumonia cases on 1/9/2020 with direct reporting system to Chinese National CDC and had system confirmation of successful submission. However, they found that the submission on 1/9/2020 was deleted. Wuhan CDC staff confirmed with Caixin Media that the Health Commission and CDC on district, city and province levels can see submission of cases from local hospitals in the system but district and city level do not have the authorization to delete them.
In Hong Kong, experts assumed that the epidemic could have "human to human transmission". 1/10/2020: A: Wuhan "Two sessions" ended, no information of the epidemic was discussed at the conference.
A: Wuhan Health Commission announced its 1st epidemic report. It mentioned that no new cases were identified since 1/3/2020. No evidence for human to human transmission and no medical staff was infected.
A: Dr. Wang Guangfa, expert team member for the National Health Commission, said that the outbreak is controllable when interviewed.
1/10/2020 B: Radiologist, Dr. Li at Wuhan Xinhua hospital identified 3 cases from CT reports with groundglass opacity. Similar cases increase daily with an exponential manner. There were 30 cases on 1/10/2020. Dr. Li believed that situation was severe since he had never seen any virus that grows this fast and multiplied every few days. Dr.Li stopped believing official reports and contacted his peers from other radiology departments, the situations are not optimistic across hospitals.
1/11/2020 B Prof. Zhang Yongzhen's team from the lab of Shanghai Public Health Clinical Center published the genetic sequence of COVID-19 to GISAID and used GISAID、 virological.org and GenBank to share with the world at 9am on 1/11/2020. Chinese National Health Commission announced that China will share the virus genetic sequence with WHO that evening.
A: Dr. Zhong Nanshan confirmed "human -to -human" transmission and infection of health care professionals at the press conference.
Wuhan Health Commission announced the infection of 15 medical staff. 