In this study, we collected data on the first outbreak of Delta variant COVID-19 in a factory, analyzed the natural history parameters of Delta variant in environments where people congregate, such as incubation period and time to diagnosis, and we calculated the intergenerational relationship of Delta to estimate the transmissibility of Delta variant in specific sites. In addition, cycle thresholds for confirmed patients were analyzed to assess the differences in Ct values under different conditions and to provide a theoretical basis for diagnostic criteria and timing.
The first case of this outbreak had a history of sojourn in a COVID-19 infected area (Putian City, Fujian Province) from September 5 to September 8, 2021, while the specificity of his occupation (factory worker) led to the outbreak of this aggregated outbreak. The median incubation period calculated for confirmed patients in this study was 4 days, which is similar to the incubation period of the outbreak of the Delta variant in Guangzhou, but higher than the currently prevalent Omicron variant[16, 17]. During the outbreak, positive patients were found in Xiamen mainly by three ways: community screening, active consultation and centralized isolation, and our study found that there was no statistical difference in the time interval between onset and reporting of patients in these three discovery methods, which should be combined to detect and dispose of cases as early as possible in the process of outbreak prevention and control. In addition, we calculated that the median diagnosis time of confirmed cases was 14.4h (interquartile range, 12h to 17h). In terms of time to onset, the time to diagnosis for confirmed cases continued to decrease in the early part of the outbreak (September 9-September 19), suggesting that during the outbreak, as the number of cases continues to increase, the diagnosis time could be continuously reduced by enhanced nucleic acid testing, centralized isolation and other measures. However, due to limited data, we were unable to assess the effectiveness of each intervention.
Therefore, we used the last contact time of cases to calculate GT and found that the average time was 3.6 ± 2.6 days, which was more in line with the exponential distribution (Exponential). To calculate TG, it is necessary to clarify the time when two generations of cases appear contagious. In this study, the mean time to TG in the epidemic was found to be 1.67 ± 2.11 days, which was more in line with the exponential distribution. This result is similar to related studies. TG is affected by specimen collection time, frequency, method and detection sensitivity of the test, therefore is more dependent on laboratory testing, and its value in field epidemiology still needs to be further explored. Since this outbreak was a cluster outbreak in factories, most of the cases were found at the centralized isolation points, and the time of positive tracing was more accurate. In this epidemiological investigation of the outbreak, the time of symptom onset of each infected person was relatively easy to obtain, so SI is a common indicator of intergenerational relationship in field epidemiology. The mean intergenerational time for SI was calculated to be 1.7 ± 3.0 days, which is more in line with the Logistic distribution. The average TG time of this epidemic is 1.67 days, which was shorter than 4 days in other places, indicating that the transmission time of this epidemic was much shorter than in other areas. The clustered outbreak of factories caused by the Delta variant is caused by the high human-to-human contact. During the next outbreak prevention and control, we need to focus on places with a high concentration of people . Meanwhile, the time difference between SI and GT is not significant. Even if the basic reproduction number of this epidemic is estimated according to the intergenerational time of symptoms, it can more accurately reflect the development trend and transmission intensity of this epidemic, and thus better prevention and control measures can be taken.
Ct values as one of the diagnostic methods for COVID-19 were found to be significantly different in terms of genotype, age, occupation and interval between exposure and testing. From the perspective of testing reagents and genotypes, we tested for both N and O genotypes. The analysis revealed no statistical difference in Ct values between the testing reagents of different companies (P > 0.05), but not between genotypes. The variation within the same species of assay (P < 0.001, P < 0.05) also prompted us to be cautious when analyzing a single Ct value, as it may be related to the genes and assay products analyzed. Similar to some studies, Ct values were not significantly different between males and females, but our study found that Ct values were generally higher in male patients than in females. The Ct values of diagnosed patients decreased progressively with age at different ages, but were higher when age exceeded 60 years. This result is further evidence that adults are at higher risk of contracting neo-coronary pneumonia and need to be focused on. Also, we found some differences in Ct values among patients with different occupations (p < 0.05). The Ct values would be relatively higher in the population of children in nurseries, in self-employed, retired, unemployed, and waiter occupations. It has also been reported that the transmission of novel coronaviruses is stronger in populations where aggregation often occurs in occupational settings. Our study found that Ct values (mean values below 30) showed fluctuating changes after exposure. trends in Ct values were similar to those simulated by an agent-based model of one study[25, 26]. When the time interval from exposure to diagnosis is within one week, the in vivo Ct values are relatively stable with a mean value of 20–30. 7 days after exposure, the in vivo Ct values drop to the lowest level. In terms of vaccination, Ct values are higher in unvaccinated patients than in vaccinated patients, but Ct values are lower in patients with one dose than in patients with two doses, suggesting that completion of vaccination reduces transmission of novel coronaviruses, but further validation is needed to verify that transmission is greater in patients who are vaccinated halfway . It has been shown that the Ct values in reported cases are lower than 40[28, 29], which is also the same as our results, which may range from 23–31 in patients with the common type, but are higher in mild patients, mostly above 27, and in severe patients, mostly below 25. In addition, the Ct values of patients diagnosed in different testing methods differed. Patients identified by centralized isolation and community screening had Ct values between 23 and 32, which were higher than those of confirmed patients who actively sought medical care (P < 0.05). Patients who were actively seeking medical care all had Ct values below 25, and they were in the clinical symptom stage at the time of active medical care. At this time, the "detoxification" capacity was relatively significant and the viral load was higher than in the other two groups. Therefore, we recommend symptomatic patients to visit the fever clinic promptly. Seek medical attention to improve the ability to detect cases early, to cut off the transmission chain in time and to control the epidemic.