MRC Biostatistics UnitEvery day the UK Government publishes a figure for the number of deaths within 28 days of a positive coronavirus test. https://coronavirus.data.gov.uk/. This figure, which comes from Public Health England (PHE), represents the number of deaths that have been reported in the preceding 24 hours, rather than the number of deaths that occurred within the previous 24 hours. The notification of a COVID-19 death to PHE is subject to a reporting delay, which is typically several days, but can be as long as several weeks. Consequently, the deaths reported in a 24-hour period are a mixture of deaths that occurred that same day, that occurred the preceding day, two days earlier, a week earlier, and even deaths that occurred over a month earlier. This makes the published daily number difficult to interpret as an indicator of trends in CoVID-19 mortality. For example, the published number tends to be lower on Sundays and Mondays, even though just as many people die on Sundays and Mondays as on other days of the week.
A more useful indicator of how the epidemic is progressing is the numbers of deaths that actually occurred in each 24-hour period. However, these numbers are unknown, because not all deaths have yet been reported. This is particularly true of the most recent days. The process of estimating the number of deaths that occurred on each day from the numbers of deaths that have so far been reported is referred to as `nowcasting’. For details of our approach to such a nowcast see Seaman S et al. 2020.
This method involves using information on the time between date of death and date of report to estimate the pattern of reporting delay and how this pattern changes over calendar time. We use this knowledge to correct the reported death figures to include the deaths that have not yet reported. By doing this, we provide timely estimates of the number of deaths that have occurred on each day and, therefore, of the trends in CoVID-19 mortality, by age group and regions. Uncertainty in the numbers of deaths that occurred on each day is quantified by credible intervals. Note that, in addition to the three sources of death reports described in our paper, we are now using death reports from the Office for National Statistics (ONS) and death reports from Pillar Two testing.
We regularly estimate, for each of five age strata, the number of people who died in England on each day within 28 days and 60 days of a positive antigen test.