Measuring mortality during Covid-19: a Q&A

Sarah Scobie answers some key questions on how mortality figures are collected and measured during the Covid-19 crisis. How do the numbers relate to the daily figures reported, and are all the extra deaths due to the coronavirus? This explainer was originally published on 5 May, and was updated on 26 May in light of the latest data.

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Published: 05/05/2020

Updated: 26/05/20

For the UK and for other health systems, the response to the Covid-19 pandemic has needed to balance actions aimed at reducing deaths from the virus with the negative impact of restricting economic and social activity and reduced access to other health and public services. Understanding how many deaths have resulted from the pandemic is therefore critical – this is the yardstick against which we can assess the success or otherwise of the measures we are taking. 

In this Q&A, we set out some of the key questions about how we measure mortality, both now and in the longer term.

How many extra deaths have there been as a result of the pandemic? 

We can estimate the overall impact of the pandemic by comparing the number of deaths each week with those we would expect to see based on the average for the last five years. Between 14 March and 15 May 2020, there were 146,220 deaths registered in England and Wales. That is 54,032 above the expected number of 92,188, which indicates there were 76% more deaths registered over these two months. For deaths registered the week ending 17 April, at the peak of the epidemic, there were more than double the expected number of deaths.

UK weekly deaths 26/05/2020

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Where does this data come from?

When a death occurs, a medical practitioner completes a death certificate that enables the person’s family to register the death. Death registrations are collated and the numbers reported by the Office for National Statistics (ONS) on a weekly basis.

There is a time lag between when the death occurred and when the data is published. For example, data published on 26 May is for registrations that occurred during the week ending 15 May, with those deaths having occurred approximately five days prior to registration. Registrations can be delayed when there is a bank holiday – which we can see in the higher number of registrations in the week ending 15 May than the previous week, which included the Friday 8 May bank holiday.

The death certificate will include the immediate cause of death and the underlying disease or injury that led up to the death. Contributory causes of death can also be recorded, where they may have affected the outcome, but illnesses should not be included on the death certificate if they were present but did not contribute to the death.  

How significant is this increase in deaths?

Death registrations vary on a week-by-week basis each year. This is due to seasonal factors such as winter flu and cold weather, and also as a result of reporting changes, such as a greater time lag in registering deaths over bank holidays.

However, the number of extra deaths we have seen in the last few weeks is among the highest on record, even with lockdown measures. Previous peaks in deaths have occurred over winters where there were bad outbreaks of flu. During the winter of 1999/2000, there were 18,000 deaths recorded in one week – which would be equivalent to over 21,000 deaths given today’s larger population.

Over the past 20 years, death rates have declined overall, so the increases we are seeing now are very significant. 

Are the extra deaths all related to Covid-19, or are some of them the result of increases in mortality for other reasons?

During the peak of the pandemic, there was a gap between the estimated extra deaths and the number of deaths where Covid-19 is mentioned on the death certificate. For example, in the week ending 17 April there were 8,758 Covid-19 related deaths registered, approximately three quarters of the number of extra deaths.

The difference could reflect a number of different factors – both clinical and in the reporting.

The range of clinical presentations of Covid-19 is extremely varied, and it may not be apparent that the patient had Covid-19, particularly for older people who can present with different symptoms.

For patients who haven’t been tested, and more particularly if the patient has tested negatively for Covid-19, doctors may be reluctant to include the diagnosis on the death certificate, even though the clinical pattern suggests Covid-19 is a factor.

In terms of reporting, the ONS data includes any death where Covid-19 is mentioned on the death certificate – either directly, as an underlying cause or as a contributory cause. However, the guidance states that only illnesses that contributed to the death should be listed.

For patients who were already close to the end of their life, doctors may judge that the Covid-19 infection was not a significant factor. Even though the patient died with the disease, they didn’t die from it.

However, it remains the case that some of the extra deaths may be a result of people not seeking help for other illnesses, or because the NHS has stopped planned treatment – which includes care for life-threatening conditions such as cancer, as well as routine check-ups.

In addition, the lockdown response to the pandemic could be affecting the risk of death from other causes – from road traffic accidents through to falls and injuries, drug and alcohol related deaths, and suicide. The impact of these wider changes won’t be clear for some time.

How do the weekly deaths reported by the ONS relate to the daily reported deaths?

Daily reported deaths are collated from reports by hospitals and – from 29 April – care homes.

These figures only include deaths that have been identified at the time as being related to Covid-19. They don’t therefore include the apparently large number of excess deaths in the ONS weekly data that have not been linked to Covid-19 on the death certificate. 

The ONS have shown, by analysing the data retrospectively, that the number of daily hospital deaths matches closely the number of daily deaths occurring that are registered with a mention of Covid-19.

On the day that deaths are published, however, the count will include deaths that actually occurred in previous days. The hospital needs to validate that the death was Covid-19 related, and also inform relatives. This process can take a number of days, and is slower over the weekend.

As a result, death counts are sometimes lower on Saturday and Sunday, and more deaths will be reported midweek once recording catches up.

Where are Covid-19 deaths occurring?

Deaths increased in all settings, but the proportion of deaths in care homes is much higher than usual, accounting for 30% of deaths compared with 21% over the past five years. Alongside the increase in deaths overall, this equates to over double the usual number of deaths in care homes. The number of deaths at home is 60% more than expected.

Place of death, deaths registered mid-March to mid-May 26/05/2020

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It’s possible that part of the increase for both care home and home deaths could be people not being transferred to hospital shortly before death, as might have happened previously, and the effect of people being discharged from hospital in the early stages of the pandemic, to free up capacity in hospitals. In these cases, the death would still have occurred, but in the hospital rather than at home or in a care home.  

In relation to care homes, we know that mortality rate from Covid-19 is much higher in older age groups, and is higher still among older people in care homes than those of the same age in the general population. However, we need to take into account that residents are more likely to be frail or have multiple underlying health conditions than those living independently. It is not yet clear to what extent care home residents have been at higher risk than those at home.

Notably, fewer of the extra care home and home deaths are recorded as being Covid-19 related – suggesting greater levels of under-recognition of Covid-19 in these settings.

Will there be a long-term impact on mortality rates?

We won’t know for some time what the long-term effect will be. That will depend on how quickly deaths from Covid-19 decline, what the wider impact of the pandemic will be on other illnesses, and whether there will be short-term reductions in mortality among older age groups  Approximately 10% of people aged 80 and over are expected to die each year, and the peak in Covid-19 deaths may be followed by lower numbers of deaths from other causes.

So far in 2020, there have been 49,138 more deaths than expected. That is slightly lower than the sum of the differences in the last two months – reflecting lower mortality than expected in January and February this year. 

However, what is clear is that analysis of just those deaths recorded as being Covid-19 related will not give the full picture. As well as having a better understanding of how the recording of cause of death differs by setting, more detailed analysis will need to consider the overall number of extra deaths and the causes recorded for these, along with other factors such as age, gender and place of death.

*This explainer was originally published on 5 May, and was updated on 26 May in light of the latest data.

*If you would like to find out more about Covid-19, Coronavirus: The science explained from UK Research and Innovation includes evidence and facts about the virus.

Suggested citation

Scobie S (2020) “Measuring mortality during Covid-19: a Q&A”, Nuffield Trust comment.

Updated 26/05/2020

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