The current epidemic was mainly caused by Omicron variant BA.2, which has a short incubation period, strong transmissibility, short serial interval, and a large ability of immune escape (4-5). It was found that the basic regeneration number (R0) of Omicron variant was about 9.5, and its maximum incubation period was about 9 days (4-5). The median incubation period was about 3 days, which was significantly shorter than that of the Delta variant (4.3 days) and other variants (5.0 days) (4-5). Its median serial interval was about 2.8 days (4-5).
It is reported that the proportion of asymptomatic infections of Omicron variant was relatively high (4). There are some reasons for this phenomenon. First, the characteristics of Omicron variant caused a higher proportion of asymptomatic infections than that of other variants. Second, some people did not develop symptoms even after being infected because of the coronavirus disease 2019 (COVID-19) vaccination. Third, early detection can find infections at early stages when symptoms have not yet appeared. In addition, the training of medical staff to improve their ability to correctly conduct diagnosis and treatment, scientifically and reasonably determinate the asymptomatic and confirmed cases, also needed to be strengthened. According to the Statistics on the 5th Wave of COVID-19 in Hong Kong, the population-wide mortality rate caused by Omicron variant was 799 per million and for people over 80 years old was 10,408 per million ((6-7). According to the real-world data in Hong Kong, the fatality rate of the Omicron variant (0.76%) was significantly higher than that of influenza (0.1%), and it reached 10.4% among people over 80 years old (6-7). The elderly, people with underlying diseases, and those who had not been vaccinated were at high risk of severe illness and death. Of the 8,973 patients who died (0–112 years old) in Hong Kong, 96% were the elderly, and 88% were not fully vaccinated. Fortunately, a large real-world study in Hong Kong showed that three doses of either vaccination against COVID-19 offered very high levels of protection against severe illness and death caused by the Omicron variant (vaccine effectiveness 98.1%, 95% confidence interval: 97.1%, 98.8%) (8). Although the total vaccine coverage in the mainland of China is high, compared with adults, the two-dose or booster vaccination rate of the elderly and children was relatively low. In Shanghai, for example, the two-dose vaccination rate for people over 80 years old is only about 15% (9). There are still a large proportion of susceptible people.