The death rate for coronavirus patients in English hospitals has fallen to a quarter of the level at the peak of the outbreak, which may mean that doctors are getting better at treating it.
Researchers said it was also possible that the data had a less optimistic explanation, possibly reflecting changes in those being admitted to hospital.
At the beginning of April, when there were 15,000 people in hospital with Covid-19, about 6 per cent died. Since then, the number in hospital has fallen by 2.4 per cent a day, meaning numbers have halved every 29 days.
At the same time the number of deaths has reduced by 4.3 per cent a day, meaning that it has halved every 16 days. As a consequence, in the latest figures the hospital death rate has fallen to 1.5 per cent.
Statisticians are struggling to explain the findings, which imply that patients are more likely to survive today than they were three months ago.
Jason Oke, from the University of Oxford, is one of the statisticians behind the UK analysis. He said that they had initially held off from releasing the figures.
“We sat on it. We had a good discussion about it to try and work out all the different ways we could be wrong,” he said. “Then we thought we should put it out there — it’s what we’ve observed. The caveat is, we don’t really understand why this is happening. But it’s happening.”
While one explanation is better treatment, another is that the patients are different. At the beginning of the outbreak, there is evidence that hospitals were more selective about who was admitted. “Maybe early on the pandemic, when we thought we would be overrun, we took only the severest cases.” This would lead to an apparent improvement in the hospital death rate, even if there was no difference in the actual death rate.
Another way in which the make up of coronavirus wards differs is that at the start many of those people infected caught the virus in the hospital itself, meaning they were already sick and vulnerable.
Whether these explain all the findings, said Dr Oke, is impossible to say at the moment. To add to the mystery, the trend matches that in other countries. The US appears to have a falling death rate, while in Italy a study found a significant rise in the likelihood of patients surviving hospital treatment, even after taking account of their age and previous illnesses.
The author of that research was sceptical of the idea that the virus itself had weakened, but suggested that in the unproven cocktail of drugs given to patients might be the basis for an effective treatment.
Dr Oke said the discovery that a commonly-used steroid, dexamethasone, reduced the death rate in severe cases supported the idea that treatment had improved. But, he said, he did not think it could be the whole explanation.
Even so, he added, we should still take hope from the figures. “It would be a lot worse if it was the other way round, and we were having to find ways to explain a trend towards a higher death rate.”
