Covid-19 may have been circulating in China as early as August 2019, a new study from Harvard Medical School (HMS) claims.
The virus, which is widely believed to have originated in a wildlife market in the Chinese city of Wuhan, was first reported to the WHO in late December.
However, an analysis of hospital traffic and search engine data in Wuhan indicates that there was early disease activity in the fall of 2019, according to HMS researchers.
In a study published on Monday on Harvard University's DASH server, analysts used satellite images of parking lots at six hospitals in Wuhan to calculate vehicle counts and estimate hospital occupancy trends. The vehicle numbers were compared to trends seen during other flu-like illness outbreaks.
The research paper — which is not yet peer reviewed — also analyzed data from Chinese search engine Baidu to determine changes in searches for the terms "cough" and "diarrhea" between April 2017 and May 2020.
It found that between 2018 and 2020 there was a general upward trend in hospital occupancy — but there was a steep increase in occupancies from August 2019, which culminated with a peak in December 2019.
Five of the six hospitals included in the analysis showed their highest daily occupancies between September and October 2019, researchers found, which coincided with elevated levels of Baidu search queries for the terms "diarrhea" and "cough."
Search volumes for both terms rose dramatically in the city around three weeks before its spike of confirmed Covid-19 cases in early 2020, the study said.
"Increased hospital traffic and symptom search data in Wuhan preceded the documented start of the pandemic in December 2019," the study's authors said. "While we cannot confirm if the increased volume was directly related to the new virus, our evidence supports other recent work showing that emergence happened before identification at the Huanan seafood market."
The report authors argued that their findings supported theories that Covid-19 was already circulating before the outbreak in Wuhan was first documented, adding that the virus may have even spread internationally before Chinese authorities detected it in late 2019.
"In August, we identify a unique increase in searches for diarrhea which was neither seen in previous flu seasons or mirrored in the cough search data," the research team said.
The U.S. Centers for Disease Control and Prevention (CDC) lists both a cough and diarrhea as potential symptoms of Covid-19. The HMS research team described gastrointestinal symptoms as a "unique feature" of the virus and one that could be the primary complaint for a number of symptomatic patients.
"This symptom search increase is then followed by a rise in hospital parking lot traffic in October and November, as well as a rise in searches for cough," the report's authors added. "While we cannot conclude the reason for this increase, we hypothesize that broad community transmission may have led to more acute cases requiring medical attention, resulting in higher viral loads and worse symptoms."
Hua Chunying, a spokesperson for China's Foreign Ministry, criticized the Harvard research at a press briefing on Tuesday.
"I think it's outrageously absurd if anyone comes to such a conclusion only based on superficial elements like traffic patterns," she said, noting that she had not seen the study first hand.
"I believe we should respect science … The correct response is to leave this to scientists and hear their research conclusion based on facts."
The Harvard researchers noted that there had been some limitations in using satellite and Baidu search data, such as impaired visibility from poor weather, acquiring data from Chinese satellite companies and not knowing the intention of individual web searches.
China has faced criticism over its initial handling of the outbreak, which has included accusations that it withheld critical information from the WHO and delayed reporting the new strain of coronavirus to the organization.
In April, Wuhan authorities revised the city's death toll from Covid-19 up by 50% following a "a city-wide investigation."
Although Chinese leader Xi Jinping has argued the country acted with transparency throughout the crisis, China is resisting complying with a WHO investigation into the global handling of the pandemic.
The Chinese ambassador to the U.K. told Sky News last month that China would allow an investigation into the outbreak — "but not now."