China has made its best efforts to collect evidence and deepen understanding of the novel coronavirus and released reliable investigative results in a timely way since the outbreak's early stages, senior health officials said on Friday.

The nation has also been fully transparent and open in sharing key information with the rest of the world, never preventing experts of the World Health Organization from visiting laboratories in Wuhan, Hubei province, nor withholding virus samples, they said.

The conclusion that COVID-19 can be transmitted between humans, made public on Jan 20, stemmed from continuous investigative efforts by groups of experts sent to Wuhan, the hardest-hit city in China, and local medical staff, Zeng Yixin, deputy head of the National Health Commission, said at a news conference held by the State Council Information Office.

In response to the allegation that the commission had internally discussed the possibility of COVID-19 developing into a pandemic as early as Jan 14, he said that at the time, the understanding of the virus was still "on the way", with a number of key questions unanswered, including its infectiousness and transmission method.

"On Jan 19, thanks to joint efforts by expert teams and local medical workers, we had gained a more accurate understanding of the virus' transmission ability. On Jan 20, the novel coronavirus was added to the country's contagious disease system and the strictest control measures were ordered to contain the virus," he said.

Looking back, Zeng said China, given limited conditions, took the fastest action to gather evidence, improve its knowledge and enhance control and treatment.

Though many viruses can spread between humans, contagiousness varies greatly, Zeng said.

"At the onset of the outbreak, we were uncertain about the pathogen that caused the disease, its incubation period, transmission route and infectiousness, so we organized multidisciplinary experts to study symptoms of a limited number of cases," he said. "The aim was to provide scientific evidence to inform the country's epidemic control strategies."

The commission learned of a mysterious pneumonia outbreak in Wuhan on the evening of Dec 30 and dispatched personnel in the early hours of the next day to Wuhan. On Jan 1, it set up an epidemic response leading group and sent in more officials and experts. On Jan 3, it had research institutions carry out parallel laboratory testing of samples to identify the pathogen, and ruled out several pathogens on Jan 5.

After the Chinese Center for Disease Control and Prevention isolated the first strain of the virus on Jan 7, the commission on Jan 9 made public that the pathogen that caused the viral pneumonia in Wuhan was a new type of coronavirus. Initial testing kits were developed on Jan 10 and refined the following week. On Jan 12, China shared the genome sequence of the virus with the World Health Organization, alerting the world to the emerging virus.

When Thailand reported the first imported case from Wuhan on Jan 13, the commission held a meeting to instruct Wuhan authorities to strengthen control measures and temperature monitoring at transportation hubs as well as reduce gathering.

Zeng said China has maintained close, multifaceted communication with the United States since the outbreak's early days. "We have no reservation or hesitation in this aspect. We have shared information to the best of our capability to facilitate epidemic control efforts," he said.

A crucial part of China's contribution lies in cooperation with the WHO and steadfast support of the organization's leadership. WHO experts visited China, including Wuhan, twice in January and February. They visited laboratories and other medical institutions and had in-depth exchanges with Chinese experts, said Li Mingzhu, an official at the commission's international cooperation department.

"WHO experts made no request to visit specific laboratories during their time in Wuhan so it's inaccurate to say China refused to take them to certain laboratories," he said at the news conference.

Liu Dengfeng, an official with the commission's science and education department, also pushed back against the accusation that China had asked its laboratories to destroy virus samples on Jan 3, calling it an interpretation taken out of context.

The primary task was to identify the pathogen. The commission immediately instructed laboratories to conduct simultaneous research. "We remained highly vigilant against the infectiousness of the pathogen," he said. "To ensure biosecurity and prevent samples from being leaked, we managed the pathogen… as a Level 2 highly infectious pathogen and gave clear requirements for its collection, transportation, scientific research and destruction." On Jan 3, a directive was issued to prevent biohazards caused by the unknown pathogen in laboratories, he said.