Scientists still cannot agree on how deadly Covid-19 is — with fresh data from Italy suggesting it may kill up to 7.4 per cent of patients.
Experts have been baffled over the true mortality rate of the disease since the start of the pandemic in December.
World Health Organization chiefs first claimed the infection-fatality rate was 3.4 per cent but this week changed their estimate to 0.6 per cent.
But experts who analysed coronavirus data from a small town in the Italian region of Lombardy found that around one in 13 people who get infected die.
One epidemiologist — who has reviewed evidence on death rates — claimed it was ‘by orders of magnitude the highest I’ve seen’.
He also pointed to data from a separate study in Qatar that suggested it may be as low as 0.01 per cent — the equivalent of one death for every 10,000 cases.
Gideon Meyerowitz-Katz, of the University of Wollongong in Australia, said it was ‘by a very long way the lowest I’ve seen’.
Similar antibody surveillance studies have produced wildly different results across the world, ranging from as low as 0.25 per cent to 1.4 per cent.
A top WHO official revealed this week that the agency’s best guess on the infection-fatality rate (IFR) was 0.6 per cent.
Dr Maria Van Kerkhove admitted the estimate, based on data from several studies, ‘may not sound like a lot but it is quite high’.
The estimate suggests the coronavirus kills around one in 167 patients, making it six times deadlier than seasonal flu.
The IFR is different to the case-fatality ratio, another measures of how many people die of a disease which is based only on confirmed cases.
But it is not entirely reliable given so many infected people have went untested for the virus during the course of the pandemic.
For example, the CFR ratio in Britain is around 15 per cent — which would mean the disease is 30 times’ deadlier in the UK than what the WHO believes.
The IFR estimates how many people die from the disease overall, including patients who don’t show any symptoms and may never get tested.
To measure IFR accurately, scientists must know exactly how many people have had the coronavirus — technically called SARS-CoV-2.
Millions would have had the disease and not been tested due to a lack of capacity in the early days of the crisis.
The only way academics can currently accurately estimate this is through looking at how many people have developed antibodies to the virus.
Infected patients make antibodies — proteins made by the immune system to fight off pathogens in the future — weeks after their battle with the disease.
But not everyone who has Covid-19 develops antibodies if other parts of their immune system clears the virus, and studies have shown antibodies can wane over time, making them undetectable with testing.
The Italian study was based on screening the blood of thousands of people living in Castiglione d’Adda for antibodies.
Almost a quarter of the population tested positive for antibodies (22.6 per cent) — 1,028 of the 4,550 inhabitants.
But only 184 cases had been officially reported in the town as of June 21 — a week before the study by Gabriele Pagani and colleagues was published.
The town had also recorded 76 Covid-19 deaths by June 21, which out of the 1,028 cases estimated, equates to a death rate of 7.39 per cent.
It would make it almost as deadly as SARS, a related coronavirus which killed one in ten people during an outbreak between 2003 and 2004.
The researchers did not explain why so many people in the town had coronavirus. But they noted Castiglione d’Adda was one of the first places in Italy to be hit, and therefore may not have been prepared.
They also did not say when the study was conducted, as this would have an impact on antibody prevalence.
But researchers have often noted that Italy has an older population, which may to blame for higher cases.
Around one fifth of Castiglione d’Adda’s population is over the age of 65 compared to around 13 per cent in the UK.
When the same death rate is applied to the UK, it suggests the true number of cases is only double what has been detected through testing.
Some 46,300 Britons have officially died after testing positive for the virus, meaning 625,000 people would have been infected if the IFR of 7.4 per cent was true.
But the true size of the epidemic is believed to be in the millions based on antibody surveillance testing ran by the UK government.
The Italian research — released as a pre-print on Medrxiv — was not published in a journal, meaning it has not been through a review process by other scientists.
However, the aim of the research was not to figure out the IFR. Instead, the experts wanted to see how infection rates differed between ages.
The IFR was pointed out by Mr Meyerowitz-Katz on Twitter, who also revealed the very low rate found in the study from Qatar.
Experts at Cornell University in Doha used a mathematical model to get an idea of how many people in Qatar had been infected over the pandemic.
The country of 2.8million people has had only 177 deaths and 111,538 cases up until August 5, according to official data.
The model was based on data about how the virus naturally spreads in a population and the results on results of mass swab and antibody tests.
Experts led by Dr Laith Abu Raddad found that the IFR was 0.01 per cent, which was ‘remarkable’, the researchers said.
They wrote in their pre-print paper: ‘Remarkably, while widespread, the infection has been characterized by relatively low case and infection severity and fatality rates, which were not well above those of a severe seasonal influenza epidemic.’
But the estimate — that Covid-19 only kills 0.01 per cent of people it infects — cannot physically be true for the UK.
It would mean the UK has had 426million cases. For comparison, the population of the UK is around 66million.
Dr Meyerowitz-Katz’s own estimation of the IFR is 0.75 per cent but could be as low as low as 0.49 and as high as 1.01.
He and his colleague Dr Lea Merone, of James Cook University, searched online for IFR studies from around the world.
Meanwhile, a review of antibody surveillance studies by Stanford University experts has suggested the coronavirus has a mortality rate of 0.25 per cent.
Harvard University took a different approach, and studied the Covid-19 outbreak on the cruise ship the Diamond Princess.
Cruise ships are deemed an ideal environment to study because there is complete data available for everyone on board at the time.
The team found the fatality rate was 1.8 per cent — 13 deaths out of 712 cases — but the rate was adjusted to 0.5 per cent to reflect the entire population.
Another German study which honed in on the small town of Gangelt put the rate at around 0.37 per cent.
By comparison, the seasonal flu kills around 0.1 per cent of those it infects. Ebola kills around 50 per cent of all cases.