This study, for the first time, categorizes different variants into different grades and further analyzes the secondary situations of different variants based on the grade classification and exposure situations. The results show that the secondary attack rate of the Omicron strain is higher than that of the Delta strain, consistent with studies in Spain, Norway, South Korea, and other countries (6–8). Through analysis, this study found that the secondary attack rate of the Delta strain is 0.85%, while that of the Omicron strain is 1.69%. In studies from Spain, the secondary attack rate of Omicron was 39%, and that of Delta was 26%; in Norway, the secondary attack rate of Omicron was 25.1%, and that of Delta was 19.4%; in South Korea, the secondary attack rate of Omicron was 31%, and that of Delta was 7%. The secondary attack rate in this study is much lower than that in other relevant studies in other countries, which is related to the prevention and control policies of each country. Strengthening personnel control and social control are beneficial for the prevention and control of newly emerging serious infectious diseases. In the Omicron strain, the BA.5.2 strain has a much higher secondary attack rate than the Delta strain and other Omicron subbranches, indicating that with the mutation of the coronavirus, its infectivity is constantly increasing. By further classifying close contacts, the distribution of secondary situation of Delta and Omicron strains, as well as different Omicron strains, is similar, with mostly core close contact, followed by general close contacts, and no cases were found in secondary contact. Therefore, it is recommended to scientifically classify the types of contacts and implement graded management during the high prevalence period, especially when dealing with epidemic situations in special places. There are differences in the proportion of cases among different exposure types between the Delta and Omicron strains, and different Omicron strains. The secondary cases of Delta strain infection were mostly co-living, while the proportion of secondary cases infected with the Omicron strain during dining and in the same space is significantly higher. This also indicates that the Omicron strain is more covert than the Delta strain.
From the analysis of basic demographic characteristics, in terms of age, individuals infected with the Delta and Omicron strains are mainly concentrated in the 18–59 age group, predominantly young and middle-aged adults, which may be related to the high mobility of this population. The BA.2.1 outbreak mainly occurred in the 3–17 age group, primarily associated with index cases originating from schools. At the time, epidemic prevention and control measures were implemented effectively, and the outbreak was mainly contained within schools, hence the majority of infected individuals were students. In terms of gender, females are more affected by the Delta and Omicron strains, consistent with conclusions drawn from localized outbreaks in Shanghai, Beijing, Gansu, and other areas (9–11). Regarding occupational distribution, individuals infected with the Delta and Omicron strains have a wide range of occupations. Cases infected with the Delta strain are mainly in commercial services and household or unemployed, while cases infected with the Omicron strain are mostly farmers. This is related to the high transmissibility of the Omicron strain and the presence of weak control measures in rural areas. Therefore, it is necessary to enhance epidemic monitoring capabilities in rural areas to prevent localized outbreaks on a large scale within a short period of time.
From the clinical classification perspective, the proportion of asymptomatic cases among Omicron strain infections is higher than that of the Delta strain, with the BA.5.2 strain within the Omicron strain having the highest proportion, even exceeding 90%. This is closely related to the intensity of epidemic prevention and control measures at that time, with a large proportion of core close contacts being identified early and receiving intervention treatment in isolation facilities, without experiencing further illness. Overall, in terms of the severity of illness and the proportion of asymptomatic cases, clinical symptoms in individuals infected with the Omicron strain are milder than those infected with the Delta strain. Therefore, the covert nature of the Omicron strain is stronger, posing greater challenges to epidemic prevention and control. In the local epidemic studied, the age group with more severe clinical symptoms among infected individuals was between 18 and 59 years old, suggesting that besides key populations such as young children, school-age children, and the elderly, attention should also be paid to the clinical diagnosis, treatment, and prevention of middle-aged adults.
From the perspective of the incubation period, this study analyzed that the median incubation period of the Delta strain is 3 days, which is consistent with the median incubation period reported domestically and internationally for the Delta strain (12). The median incubation period of the Omicron strain is comparable to that of the Delta strain and other Omicron strains, ranging from 2 to 3 days. The study indicates that a short incubation period affects the duration of transmission. Therefore, the Delta and Omicron strains are prone to causing outbreaks and widespread transmission, leading to multiple generations of cases in a short period, with far-reaching effects. It is crucial to closely monitor the virus mutations and changes in its characteristics. Special attention should be paid to epidemic surveillance in schools, childcare institutions, nursing homes, and other high-risk gathering places. Once an outbreak is detected, timely warnings should be issued to reduce the risk of epidemic outbreaks.