Background: A mathematical investigation of the reasons for the fifth wave's quick expansion and reduction in Tokyo, Japan, is required to avoid the spread of subsequent COVID-19 infections.
Methods: Using the simple IR theory underlying the susceptible-infectious-removed (SIR) hypothesis of infectious disease epidemics, infected persons (I), infection rate, and testing/isolation rate are determined from accessible data of daily positive cases (R) and testing numbers.
Results: The rapid spread of illness from late July to mid-August was owing to a drop in the number of people tested to half that of weekdays during the Olympic Games' four and three-day vacations. The maximum number of daily positives would have been lowered to two-fifths of the actual positives in early August if the number of weekday tests had been maintained during these holidays and would have fallen monotonically thereafter. The infection rates mean value fell steadily from 0.65 in late August to around 0.25 by the end of September. The significant increase in vaccination rates is mostly to blame for the fall in infection rates. In Tokyo, the impact of mRNA-based vaccines on infection prevention and increased vaccination rates could reduce the infection rate to 1/2 on September 10 and 1/3 by the end of October.
Conclusion: According to the findings, a new infection like the delta variant can be suppressed to less than the fifth wave by increasing vaccination rates, eliminating three consecutive holidays, and implementing a precautionary testing system that maintains the same number of tests on weekends as on weekdays in the event of a rapid spread of infection in an emergency.