The first peak of the Covid19 pandemic has been a bit difficult to call because it was so bumpy. Certainly by April 15, 2020 there was a suspicion of a peak – the daily reading was 1 i.e. that day’s reading was the same as the day before.
It bounced around for awhile until the 95% confidence upper limit dropped below the growth line on April 23, 2020. I suppose, at least in terms of hospital Covid19 deaths, we could call that the first peak in the UK.
The grey area and ‘line of best fit’ are a non-parametric regression called loess. Once the data got below the growth line (each successive reading was lower than the day before), the rate of decline in deaths slowed considerably. We still have declines day after day (black dots).
This plot has some similarities with the R0 number but it takes several weeks for a patient to contract Covid19 and then become unwell enough to be admitted. So daily hospital death rate is too much of a lag indicator.