Estimation of the Total Number of Infected Cases in the 5th Wave of COVID-19 in Hong Kong

The total number of infected cases in a region in an epidemic is an important measure of the severity of the disease. With the increase in the number of infected people, the number of susceptible people will be reduced, and the recovery number is increased. The present study attempts to estimate the total number of infected cases in the 5th wave of COVID-19 in Hong Kong based on the daily additional cases supplied by the government based on two test schemes. Scheme 1 covers citizens suspected to be infected as referred by medical professionals, or requested or reported by citizens themselves, those returning from overseas, and those in close contact with the infected persons. Scheme 2 covers residents in buildings with a high concentration of virus in sewage. Polymerase chain reaction (PCR) test and then rapid antigen test (RAT) after 26 February 2022 were accepted by the Hong Kong Special Administrative Government in confirming infected cases. The number of infected cases in these two schemes were compared. A prediction model on infection case was proposed based on the transient daily infection curves. The averaged recovery number was estimated by assuming a 10-day infection period, including an incubation period of 5 days, and another 5 days for recovery. The transient number of infected, susceptible, recovered people were then presented. An adjustment factor to extend the scenario to the whole population of 7 million in Hong Kong was estimated and applied to study infection number in Hong Kong. Further, it appears that the infection number at the later stage of the 5th wave is weak around end July 2022. However, the number stayed at a constant value in comparing with rapid rise at the early stage in February 2022, even though the gathering activities were kept normal.