Objective: To investigate the reference level of surface radiation dose of digital radiography (DR) among adult examinees in Beilun District, Ningbo City, so as to provide insights into the optimization of the parameters of DR systems and rational management of the radiation dose. Methods: Based on the cloud imaging system covering 14 DR systems in 11 public hospitals in Beilun District, the diagnosis of adults receiving DR and DR systems check were collected from May 2020 to April 2021. The air kerma-area product ( PKA ) was selected as a measure, and the local diagnostic reference level ( LDRL ), the regional median dose ( RMD ), the regional lowest dose predicted level ( LDPL ) and the typical dose in the DR imaging room were defined using the quartile method according to the examination site, projection direction and radiography positions. The PKA value was transformed into incident air kerma ( Ka, e ), and compared with the recommended value in the Requirements for Radiological Protection in Diagnostic Radiology ( GBZ 130-2020 ). Results: DR was performed among 133 065 person-times from May 2020 through April 2021 in Beilun District. The greatest RMD was found on abdomen AP DR in the supine position ( 120 μGy·m2 ), and the lowest RMD was seen on chest PA DR in the standing position ( 17 μGy·m2 ). The maximums of typical dose in the DR imaging room on chest PA ( 33 vs. 30 μGy·m2 ), abdomen AP ( 161 vs. 153 μGy·m2 ) and pelvis AP DR ( 164 vs. 162 μGy·m2 ) were greater than LDRL in the supine position, and the minimums of typical dose on chest PA ( 10 vs. 11 μGy·m2 ) and chest LAT DR ( 33 vs. 34 μGy·m2 ) were lower than LDPL in the standing position. Following transformation into Ka, e, the LDRL ranged from 0.27 to 3.80 mGy, and the maximums of typical dose ranged from 0.25 to 3.55 mGy, which was much lower than the recommended dose proposed in the national criteria. Conclusions The reference level of surface radiation dose of DR is established among adults in Beilun District, which is lower than the recommended dose in the national criteria of China. Such a reference level may be employed by medical institutions for optimization of the parameters of DR systems. Key words： digital radiography radiation dose diagnostic reference level air kerma-area product

Objective: To investigate the origin of infection and risk factors of a case with SARS-CoV-2 infection associated with overseas countries in the Ningbo-Zhoushan Port, Zhejiang Province, so as to provide the evidence for improving the COVID-19 control measures at ports. Methods: Ningbo Center for Disease Control and Prevention ( CDC ) and Beilun CDC conducted case finding and epidemiological surveys immediately after being informed. The general information, history of vaccination and the travel during the latest 14 days were collected from the positive case, and all close contacts were tracked. Saliva samples were collected for SARS-CoV-2 nucleic acid testing and whole-genome sequencing, and the sequencing results were aligned with the GISAID's EpiCoV database. The origin of infection and transmission route of the positive case was investigated. Results: A case was identified positive for SARS-CoV-2 nucleic acid during company M's routine screening in the Ningbo-Zhoushan Port on August 10, 2021, and was confirmed positive for SARS-CoV-2 nucleic acid by Beilun CDC and Ningbo CDC on August 11. Whole-genome sequencing showed SARS-CoV-2 B.1.617.2 ( Delta ) variant, which shared the highest homology with the virus sequence uploaded by Russia on June, 2021 ( Russia/MOW-RII-MH27356S/2021 ). The case was a bundling worker for overseas container ships, and reported communicated with foreign boatmen and contacted materials without protected interventions on the SINOKOR AKITA Container Ship between August 4 and 5, 2021. This ship anchored at Vladivostok, Russia from July 27 to 29, anchored at Ningbo Harbor on August 4, and departed on August 5. Then, 11 boatmen from this ship were tested positive for SARS-CoV-2 nucleic acid on August 8. One asymptomatic case was reported in this epidemic; 254 close contacts and 617 secondary close contacts were identified, and all were tested negative for SARS-CoV-2 nucleic acid. No new cases with SARS-CoV-2 infections were detected until August 25, 2021, and the emergency response was therefore terminated. Conclusions The infection was a sporadic COVID-19 epidemic associated with overseas countries, which was caused by Delta variant infection through contacts with foreign boatmen or materials by a bundling worker in Ningbo-Zhoushan Port; fortunately, no epidemic spread occurred. Intensified closed-loop management and increased frequency of SARS-CoV-2 nucleic acid test among high-risk populations, and improving the precision and rapid emergency treatment of COVID-19 epidemics are required for the containment of COVID-19 at ports.