COVID-19 could have been spreading in Los Angeles in December last year, weeks before the first case on American soil was declared, researchers have found.
Experts at UCLA who analysed patient records found a significant spike in hospital patients with coughs and acute respiratory failure a month before the first official coronavirus infection in the US.
They searched through millions of records to find a 50% rise in cases of coughs and lung failure which lasted until February 2020.
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The number of patients seen in emergency departments for severe coughs and of patients with acute respiratory failure was also higher.
COVID-19 has been suggested as the cause. But UCLA researchers cannot say for certain that coronavirus was behind the trends, as it is impossible to rule out other potential factors, like e-cigarette use, or the flu.
The first confirmed US case was recorded in a traveller who returned to Washington from Wuhan – the Chinese city thought to be the epicentre of the pandemic – on 15 January.
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Then a month later, on 26 and 28 February, the first cases of community spread were detected, according to the US Centres for Disease Control and Prevention.
The findings have added to mounting suspicions that China could have covered up the virus.
Beijing said the disease was first recorded in a Wuhan market just before Christmas. But it has been hit by claims the government knew about it earlier.
The country reported it to the World Health Organisation (WHO) on 31 December.
On Wednesday, a British family released a coroner’s report which said their relative, 84-year-old Peter Attwood, died of coronavirus on 30 January.
It is thought Mr Attwood is the first known death from COVID-19 outside China, coming just 19 days after the first reported fatality from the disease in Wuhan.
The first coronavirus death in the UK was previously thought to have occurred on 5 March.
In the US, there have been 6.2 million recorded cases of COVID-19 and 192,000 deaths.
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The authors of the UCLA study, which appeared in the peer-reviewed Journal of Medical Internet Research, said their work showed how vital it is to monitor health records for trends.
Lead author Dr Joann Elmore said: “We may never truly know if these excess patients represented early and undetected COVID-19 cases in our area.
“But the lessons learned from this pandemic, paired with health care analytics that enable real-time surveillance of disease and symptoms, can potentially help us identify and track emerging outbreaks and future epidemics.”
Meanwhile, coronavirus cases are increasing in many countries across the world.
On Wednesday, the WHO said 106,000 new cases had been reported globally in the last 24 hours – the highest number in a single day since the pandemic began.
Weekly cases in the UK are at the highest levels since May.