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Lag time

August 9, 2021 - 10:45 -- Admin

Some idiots have demonstrated their ignorance about the Covid-19 pandemic in stating that because there are fewer deaths in the current outbreaks, then the Delta variant has a lower mortality rate. The only time you can determine the death rate is after the event, or if you look at the number of deaths today and compare that with the number of cases in the past. That is because there is a lag time between infection of a person and their possible death from the virus. Also, to cope with the vagaries of testing and detection, which make the daily numbers of new infections jump around considerably, I tend to stick to the seven-day moving average (SDMA) of cases. This is where the average for today is the total number of cases detected today and in the previous 6 days, divided by 7. It makes for a much smoother curve and will give a peak infection rate which is clear. Relatively, Australia has had so few cases of Covid-19 that even the SDMA does not smooth the data sufficiently to give accurate estimates of peaks. Here are a few examples.

Example 1: The UK had its third wave of Covid-19 beginning in early December 2020 and it peaked at 59,417 cases per day (SDMA) on January 9th. Many people died during this wave, with the maximum number of deaths being 1251 (SDMA) being reached on January 23rd. That is a lag time of 14 days.

Example 2: The US had its second wave of Covid-19 beginning in the middle of June, 2020, and it peaked at 70,608 new cases per day (SDMA) on July 19th. The peak in deaths was on August 4th with an SDMA of 1,185 per day. That is a lag time of 16 days.

Example 3: France had its second wave of Covid-19 beginning in October 2020, and it peaked at 52,907 new cases per day (SDMA) on November 7th. Deaths peaked at an SDMA of 667 per day on November 19th. That is a lag time of 12 days.

So the lag time between the peak in cases and the peak in deaths is about two weeks. There is another interesting feature of the lag time. It seems to be a feature of everything that occurs during this pandemic. The UK’s fourth wave really started to kick off in May 2021, and it peaked on July 21st at an SDMA of 47,101 new cases per day, two days after Boris Johnson’s ‘Freedom Day’ on July 19th1,4. It then started to decline and that was looked upon by some as vindication of the decision to relax many of the Covid-19 restrictions. While it is true that the number of new cases per day has continued to decline, down to 25,760 (SDMA) on August 3rd, 15 days after the peak. Now, however, that decline has stopped and has reversed. Daily new cases have now climbed to 26,615 (August 7th). Since Johnson’s ‘Freedom Day’, when daily deaths were at 42 per day, they have climbed to 90 per day (August 7th)1. The turnaround in the infection rate does not augur well, especially when you consider that a place like Iceland, which has 75% of its entire population fully vaccinated (the UK is 57%)5, is now having a wave of Delta variant infection, which is higher than the rate of infection in their first two waves in March 2020 and October 20206.

Sources

  1. https://www.worldometers.info/coronavirus/country/uk/
  2. https://www.worldometers.info/coronavirus/country/us/
  3. https://www.worldometers.info/coronavirus/country/france/
  4. https://www.abc.net.au/news/2021-07-20/how-uk-freedom-day-marks-end-of-covid-19-restrictions/100294668
  5. https://ourworldindata.org/covid-vaccinations?country=AUS~AUT~BHR~BEL~BRA~CAN~CHL~CZE~DNK~EST~FIN~FRA~DEU~GRC~ISL~IND~IDN~IRL~ISR~ITA~JPN~MEX~NLD~NOR~POL~PRT~RUS~KOR~ESP~SWE~CHE~GBR~ARE~USA~URY
  6. https://www.worldometers.info/coronavirus/country/iceland/