[Epidemiological characteristics and serum antibody detection of a COVID-19 aggregated outbreak in vaccinated population].

An epidemiological investigation was conducted on a cluster epidemic of COVID-19 in the vaccinated population in Beijing in 2022, and serum samples were collected from 21 infected cases and 61 close contacts (including 20 cases with positive nucleic acid in the isolation observation period). The results of antibody detection showed that the IgM antibody of two infected persons was positive, and the IgG antibody positive rates of patients who were converted, not converted to positive and infected persons were 36.84% (7/19), 63.41% (26/41) and 71.43% (15/21), respectively. About 98.78% of patients had been vaccinated with the SARS-CoV-2 inactivated vaccine. The positive rate of IgG antibody in patients immunized with three doses of vaccine was 86.00% (43/50), which was higher than that in patients with one or two doses [16.12% (5/31)]. The antibody level of M (Q1, Q3) in patients immunized with three doses was 4.255 (2.303, 7.0375), which was higher than that in patients with one or two doses [0.500 (0.500, 0.500)] (all P values<0.001). The antibody level of patients who were vaccinated less than three months [7.335 (1.909, 7.858)] was higher than that of patients vaccinated more than three months after the last vaccination [2.125 (0.500, 4.418)] (P=0.007). The positive rate and level of IgG antibody in patients who were converted to positive after three doses were 77.78% (7/9) and 4.207 (2.216, 7.099), respectively, which were higher than those in patients who were converted after one or two doses [0 and 0.500 (0.500, 0.500)] (all P values<0.05).

[Analysis on infection sources and transmission chains of three outbreaks caused by 2019-nCoV Omicron variant in Beijing, China].

Objective: To investigate the infection sources and the transmission chains of three outbreaks caused by 2019-nCoV Omicron variant possibly spread through cross-border logistics in Beijing. Methods: Epidemiological investigation and big data were used to identify the exposure points of the cases. Close contacts were traced from the exposure points, and the cases' and environmental samples were collected for nucleic acid tests. Positive samples were analyzed by gene sequencing. Results: The Omicron variant causing 3 outbreaks in Beijing from January to April, 2022 belonged to BA.1, BA.1.1 and BA.2. The outbreaks lasted for 8, 12 and 8 days respectively, and 6, 42 and 32 cases infected with 2019-nCoV were reported respectively. International mail might be the infection source for 1 outbreak, and imported clothes might be the infection sources for another 2 outbreaks. The interval between the shipment start time of the imported goods and the infection time of the index case was 3-4 days. The mean incubation period (Q1, Q3) was 3 (2,4) days and the mean serial interval (Q1, Q3) was 3 (2,4)days. Conclusions: The 3 outbreaks highlighted the risk of infection by Omicron variant from international logistics-related imported goods at normal temperature. Omicron variant has stronger transmissibility, indicating that rapid epidemiological investigation and strict management are needed.

[Study of transmissibility of 2019-nCoV Omicron variant in Beijing].

Objective: To evaluate the transmissibility of 2019-nCoV Omicron variant under the current prevention and control strategy in Beijing, and provide evidence for the prevention and control of COVID-19. Methods: The information of 78 Omicron variant infection cases involved in clear transmission chains in Beijing during 7-25 March, 2022 were collected, the incubation period and serial interval of the disease were fitted by using Gamma and Weibull distribution. Markov Chain Monte Carlo approach was used to estimate the time-varying reproduction number (Rt). Results: The median of the incubation period (Q1, Q3) of Omicron variant infection was 4.0 (3.0, 6.0) days, and the serial interval was 3.0 (2.0, 5.0) days. The median of the serial interval (Q1, Q3) was 2.0 (1.0, 4.0) days in unvaccinated cases and 4.0 (2.0, 6.0) days in vaccinated cases (Z=-2.12, P=0.034), and 2.0 (1.5, 3.0) days in children and 4.0 (2.0, 6.0) days in adults, respectively (Z=-2.02,P=0.044), the differences were significant. The mean of Rt was estimated to be 4.98 (95%CI: 2.22-9.04) for Omicron variant in this epidemic. Conclusion: Omicron variant has stronger transmissibility compared with Delta variant. It is necessary to strengthen the routine prevention and control COVID-19, promote the vaccination and pay close attention to susceptible population, such as children.

[Survey on source of infection of the first local outbreak caused by SARS-CoV-2 Alpha variant in China].

Objective: To investigate the epidemiological characteristics and the chain of infection of a local outbreak, which was the first outbreak caused by severe acute respiratory syndrome corona virus 2 Alpha variant in China and occurred in Daxing district, Beijing. Methods: Epidemiological investigation and big data technology were used to verify the exposure points of the cases. Close contacts were traced from the exposure points, and their human and environmental samples were collected for nucleic acid tests. Serum samples were collected from key persons for antibody detection. Results: A total of 33 corona virus disease 2019(COVID-19) cases were reported in the local outbreak, from January 17, 2021 to January 29, 2021 in Daxing district, Beijing, and there was epidemiological association in 32 cases. Except for one case who was infected in the workplace, other cases were all infected in the community and family. All cases involved 14 families, of which 6 families were all infected. The attack rate of all family members was 69%(33/48), and the secondary attack rate was 56%(19/34). There was no obvious source of infection found after the investigation of entry-exit personnel and goods. Conclusion: The first outbreak caused by severe acute respiratory syndrome corona virus 2 Alpha variant in China is found and handled in time, and thus the scope of influence is limited, but the family clustering characteristics are more obvious than previous outbreaks.

[Relationship between respiratory viral load of cases of COVID-19 and secondary attack risk in close contacts].

Objective: To analyze the relationship between the cycle threshold (Ct) values of N gene of COVID-19 cases and the secondary attack risk in close contacts, and identify the relationship between respiratory viral load and infectivity in COVID-19 cases. Methods: The COVID-19 cases with records of Ct values of N gene within 0-7 days of onset were selected, and their close contacts were recruited as the subjects of study. We collected the information of close contacts, including name, gender, age, isolation mode, exposure mode and outcome (whether they were infected with SARS-CoV-2 or not) and other variables. Multivariate logistic regression models were used to identify the relationship between Ct values of N gene of COVID-19 cases and secondary attack risk in close contacts. Results: A total of 1 618 close contacts were recruited, in whom 77 were confirmed as symptomatic or asymptomatic COVID-19 patients with overall secondary attack rate of 4.8%. The multivariate logistic regression analysis indicated that eating together (OR=2.741, P=0.054), living together (OR=9.721, P<0.001), non-centralized isolation (OR=18.437, P<0.001) and COVID-19 case's values of N gene within 0-7 days of onset being <20 (OR=8.998, P=0.004) or 20-25 (OR=3.547, P=0.032) were significantly associated with higher likelihood of being infected with SARS-CoV-2 in close contacts. Conclusion: The results indicated that positive relationship exists between respiratory viral load and infectivity in COVID-19 cases, suggesting that Ct values of N gene can be used as an early warning indicator for the management of close contacts of COVID-19 cases.