Australian infection expert, part of a team visiting Wuhan, says they were provided only with a summary of data

The WHO team arrive at the Wuhan Institute of Virology earlier in February

There is growing controversy over a World Health Organization investigation into the origins of the Covid-19 pandemic after one of its members said China had refused to hand over key data, and the US national security adviser said he had “deep concerns” about the initial findings.

An international delegation travelled to the Chinese city of Wuhan last month, as part of efforts to understand how the outbreak began. Dominic Dwyer, an Australian infectious disease expert who was part of the team, said they had requested raw patient data but were only given a summary.

Dwyer told Reuters on Saturday that sharing anonymised raw data is “standard practice” for an outbreak investigation. He said raw data was particularly important in efforts to understand Covid-19 as only half of 174 initial cases had exposure to the now-shuttered market where the virus was initially detected.

“That’s why we’ve persisted to ask for that,” Dwyer said. “Why that doesn’t happen, I couldn’t comment. Whether it’s political or time or it’s difficult.”

Wuhan is where the disease was identified, and the first city to be badly hit by a wave of infections, but China has sought to cast doubt on whether it originated in the country, pointing to imported frozen food as a possible conduit.

Last week WHO delegation leader Peter Ben Embarek told a news conference that the virus probably came from animals, but could have taken a “very long and convoluted path involving also movements across borders”. The transmission of the virus via frozen food is a possibility that warrants further study, he added.US national security adviser Jake Sullivan called on China to make data “from the earliest days of the outbreak” available to investigators, and said it was vital that the team’s work was not influenced by pressure from Beijing.

“We have deep concerns about the way in which the early findings of the Covid-19 investigation were communicated and questions about the process used to reach them,” Sullivan said in a statement.

“It is imperative that this report be independent, with expert findings free from intervention or alteration by the Chinese government.”

The international probe had been plagued by controversy even before its members reached Wuhan, with the WHO, in a rare public complaint, accusing China of dragging its heels over arrangements.

The team spent two weeks in quarantine when they arrived in January, but when they emerged their itinerary was limited to visits organised by their Chinese hosts. They were barred from contact with community members because of health restrictions.

Not all of the team agreed that China had been secretive. Peter Daszak, a team member who is the president of the EcoHealth Alliance, said that it “was not my experience”.

“As lead of animal/environment working group I found trust and openness with my China counterparts. We did get access to critical new data throughout,” he said on Twitter.

“New data included environmental and animal carcass testing, names of suppliers to Huanan market, analyses of excess mortality in Hubei, range of Covid-like symptoms for months prior, sequence data linked to early cases and site visits with unvetted live question and answer.”