The search for an effective vaccine against Covid-19 appears to have reached a breakthrough, but questions remain about when and where the virus that crippled the world started.
Working with China, the World Health Organisation this month has been given permission to send international experts to the country as part of the organisation's inquiry into the origins of the virus.
Ground zero is Wuhan, a city of more than 11 million people and a natural crossroads for transport, in central China on the banks of the Yangtze river.
The WHO-led investigation will look for answers to some questions that in reality may never be answered. Where did Covid-19 begin and who contracted it first? The so-called Patient Zero.
There were unconfirmed reports in Chinese media of a case on November 17, 2019, while the first reliable confirmation of cases was on December 1, 2019.
According to research by Chinese scientists published in The Lancet in late January, that date was when symptoms in the first 41 patients were recorded in a laboratory.
More details were given in a BBC Chinese Service report in February. After talking to doctors involved in the case, the possible first patient was identified as a man in his seventies, already suffering from Alzheimer's disease and "in very bad condition" when he was admitted to hospital, Dr Wu Wenjuan, director of the intensive care unit at Jinyintan Hospital in Wuhan told the BBC.
The man’s fate was not recorded, but the research is clear that he and his family had no connection with the Huanan Seafood Wholesale Market in Wuhan, often identified as a probable source of Covid-19, along with the theory that it crossed into humans from animals sold there.
The next three cases to show symptoms in Wuhan also had no obvious link to the market, Dr Wu Wenjuan said.
Covid-19 vaccines: everything you need to know
As it turned out, the old man lived some distance from the market, which he had not visited and, in any case, because of Alzheimer's disease, he never went outdoors. To deepen the mystery, none of his family developed Covid-19.
When asked by the BBC about where he had possibly become infected, Dr Wu Wenjuan avoided the question, saying only "what you asked is the direction of our next research".
Of the 41 Covid-19 cases admitted to hospital last December that formed The Lancet paper, 27 had direct exposure to the seafood market, including the first fatal case, a trader whose wife also contracted the disease even though she had not been to the market.
By January 2, 2020, six of those patients – or about 15 per cent – had died. Yet on January 8, the WHO tweeted that the outbreak had caused no deaths and said six days later that, according to China, there was “no clear evidence of human-to-human transmission".
It is discrepancies such as these that raised questions about the origins of Covid-19.
The BBC Chinese Service report was among the first to point the finger of suspicion at the Wuhan Institute of Virology, which faced claims that the disease may have been created there and escaped as a result of poor biosecurity.
This suggestion is strongly denied by China, while other international experts also defended the institute, noting there is no evidence the virus has been artificially modified.
In recent weeks, there was another twist.
More research claimed to have found traces of Covid-19 in Europe months before it was first reported in Wuhan.
An Italian study that later found antibodies to the virus in blood samples taken during a cancer survey last September has been quoted by Chinese authorities as evidence the virus may have several origins.
Other research detected Covid-19 in France on December 27 last year, while a study of wastewater at the University of Barcelona claimed it found traces of the virus in samples taken in March.
These examples led Dr Tom Jefferson, of the Centre for Evidence-Based Medicine at the University of Oxford, to suggest the virus may have been dormant in many countries, simply emerging when the conditions were right.
"They are always here and something ignites them, maybe human density or environmental conditions, and this is what we should look for," Dr Jefferson told the Daily Telegraph in July.
Other experts have disputed the reliability of these tests, saying some are likely the result of laboratory cross contamination.
Only six out of 111 of the Italian samples had enough antibodies to kill Covid-19, while using the same test as a control on different blood samples known to contain viruses failed to detect any antibodies.
Part of the problem is that Covid-19 is frequently asymptomatic, meaning people could have carried the disease undetected before the first hospital cases were observed.
The current WHO inquiry states that its work is “not bound to any location and may evolve geographically as evidence is being generated, and hypotheses evolve".
But it will begin by looking at Wuhan and the now closed seafood market, which at the time sold a variety of farmed and wild animals.
“Research … has shown that a range of animals – including wild and farmed species – are susceptible to infection, but when and where Covid-19 spilled over to humans, and from which animal, remains unknown," the WHO reported on November 5.
It also found no trace of Covid-19 on any animal samples taken from the market but did find it in 69 of 842 environmental samples from drains and sewage on the site.
The WHO admits “very little is currently known about how, where and when the virus started circulation in Wuhan".
The route from a house-bound old man with Alzheimer's disease to 63 million cases worldwide and 1.47 million dead, so far, is no clearer than it was a year ago.
