August 21, 2020

expert comments about current case rate, hospitalisation rate and death rate

There have been questions form journalists about the UK’s current COVID-19 case rate, hospitalisation rate and death rate.

Dr Jeffrey Barrett, Senior Scientific Consultant, Covid-19 Genome Project, Wellcome Sanger Institute, said:

“This is a question that is on everyone’s mind right now, as the same pattern is being observed in many places around the world: case numbers are increasing again, but relative death rate seems to be much lower than during the first wave of the pandemic. I don’t think there is one easy explanation for this, but there are a few contributing factors that have started to become clear, including:

1. More testing. Most countries, including the UK, have hugely increased testing capacity since the Spring, and this means that the official numbers of new cases per day now are closer to the true total (which is the number who have got a test, plus the number who never get tested and are “hidden” from the data) than it was earlier in the pandemic. One way to see this is in the rate of positive tests, which is currently about 1 in 1000, whereas it was more like 1 in 10 back in April. What we’d really like to know is the relative number of deaths to true cases, but since true cases are hard to calculate exactly, we often use the ratio of deaths to reported cases, which is easier.

2. Better treatment. Medical practice has improved in how to treat the most severely ill Covid-19 patients. For example, the widely available medicine dexamethasone reduces death in very ill patients, and now that we know that there’s much more opportunity to save lives that might have been lost earlier on. It’s important to remember that these people are still often very sick, and may have long lasting complications, even though they are now less likely to die.

3. Different age profile. There does seem to be data that there are more infected younger individuals now than earlier. That may be because older people are taking more care to shield, or it may be related to the testing point above, because young people with mild symptoms can now get tested, whereas they couldn’t earlier on when tests were limited.

4. Things may change! There is a time lag between infections rising and deaths rising. This was observed early on, and continues to be seen more recently. For instance, in Florida the second wave of cases peaked in mid-July, whereas the peak in deaths is just happening now (or indeed may not yet have peaked).  So it’s important to be watchful, as deaths in the UK may begin to increase again in the coming weeks.

“Finally, one thing which absolutely hasn’t changed is the potency of the virus. It is still the same as it was in January, and it’s important that we don’t let down our guard against this deadly infection.”

Prof Brendan Wren, Professor of Microbial Pathogenesis, London School of Hygiene & Tropical Medicine, said:

“The drop in death rates due to Covid-19 infection could be attributed to many factors such as (i) improved awareness and improved treatment of the disease, (ii) the increased infection numbers is specifically in young people whereas the more suspectable population are securely isolated. (iii) a more robust human population as the most susceptible die, and (iv) changes in human practices such as improved hygiene and physical distancing would often make the infectious dose lower than in the pre-lockdown scenario, resulting in less severe disease.

“Like all infectious agents the virus mutates as it circulates within the human population. But without undertaking direct comparative analyses between the mutant and the parent strain in appropriate infection models, it is not possible to correlate the emergence mutants with less (or more) disease severity.”

Prof Duncan Young, Professor of Intensive Care Medicine, University of Oxford, said:

“There are at least three possibilities for the rising “test-positive” case rate compared with hospitalisations:

1. More people overall are being tested, but the proportion of the population (prevalence) is steady, so it is simply that more cases are being detected.

2. The proportion of tested individuals that are positive is rising (ie there are really more cases).

3. The tracing system has caused more patients who are at higher risk to be tested (because of exposure) meaning there were more positive tests in those tested but maybe not in the population.

“It could also be a mix of all three.

“If you look at the test and trace stats (https://www.gov.uk/government/publications/nhs-test-and-trace-england-and-coronavirus-testing-uk-statistics-6-august-to-12-august-2020) the number of people tested has increased from the low point on the graph. In the wb 9/7 there were 361,644 people tested, in the last week for which data are available (wb 6/8) there were 431,268. The positive rate went from 1.12% to 1.54% over the same period. Thus the test numbers have gone up by nearly 20% whilst the positive rate went up only slightly. It is therefore very possible that the increase in cases is mostly related to increased testing, but will a small additional effect from the increased prevalence.

“The contact tracing system was involved in 1-2% of tested cases so I don’t think this is driving the increased positive cases.

“There are no data on ages so I can’t comment on that.

“I think the figures above are enough to explain the increased test-positive cases without invoking a mutation of the virus.”

Prof Paul Hunter, Professor in Medicine, The Norwich School of Medicine, University of East Anglia:

“There are four possible explanations of the declining low case fatality rate now compared to a few months back.

1. Increased testing mean more people are being found positive that would not previously have been identified – and generally these are people who would not have been that ill and so much less likely to die. – DEFINATE

2. The main burden of infection has moved into younger people who would not be expected to die – DEFINATE

3. Doctors are better at keeping patients alive now, partly from experience of handling the new illness but also because of the use of dexamethasone in the most critically ill people which reduces death rate by about 20% – DEFINATE.

4. The possibility that some strains of the virus that are now circulating more widely (i.e. more infectious) are less likely to kill people – POSSIBLE

“Actually it is a combination of at least the first three and maybe all four explanations.”

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

Declared interests

None received.