In order to come up with effective infectious disease countermeasures, we need a method to evaluate the effectiveness of the countermeasures. As the most reliable epidemic diagnosis, we propose the PCR test positive rate. Examining the COVID-19 epidemic in Tokyo shows that the positive rate follows the epidemic accurately, without being affected by medical institution closures or the psychology of people. By comparing with the change of the positive rate, we argue that the psychology of people was the cause of the observed asymmetry and shallow valleys in the number of confirmed cases. We discuss that the positive rate as large as 1-5% in the valleys of the epidemic implies that many people are already infected and immune. The observed fact that the positive rate followed the number of positives accurately and the value lower than those reported in the test-negative design studies of influenza are attributable to the government policy of finding as many patients as possible. We mention three possibilities as the causes of the repeated COVID-19 epidemic in Japan.
PCR test positive rate revealing the real infection epidemic status examined in the COVID-19 epidemic in Tokyo, Japan