Background: The Delta variant of SARS-CoV-2 has become predominant globally. We evaluated the transmission dynamics and epidemiological characteristics of the Delta variant in an outbreak in southern China. Methods: Data on confirmed cases and their close contacts were retrospectively collected from the outbreak that occurred in Guangdong, China in May-June 2021. Key epidemiological parameters, temporal trend of viral loads and secondary attack rates were estimated and compared between the Delta variant and the wild-type SARS-CoV-2 virus. We also evaluated the association of vaccination with viral load and transmission. Results: We identified 167 patients infected with the Delta variant in the Guangdong outbreak. The mean estimates of the latent period and the incubation period were 4.0 days and 5.8 days, respectively. A relatively higher viral load was observed in Delta cases than in wild-type infections. The secondary attack rate among close contacts of Delta cases was 1.4%, and 73.9% (95% confidence interval: 67.2%, 81.3%) of the transmissions occurred before onset. Index cases without vaccination (OR: 2.84, 95% confidence interval: 1.19, 8.45) or with one dose of vaccination (OR: 6.02, 95% confidence interval: 2.45, 18.16) were more likely to transmit infection to their contacts than those who had received 2 doses of vaccination. Discussion: Patients infected with the Delta variant had more rapid symptom onset. The shorter and time-varying serial interval should be accounted in estimation of reproductive numbers. The higher viral load and higher risk of pre-symptomatic transmission indicated the challenges in control of infections with the Delta variant.

Background The Delta variant of SARS-CoV-2 had become predominant globally by November 2021. Aim We evaluated transmission dynamics and epidemiological characteristics of the Delta variant in an outbreak in southern China. Methods Data on confirmed COVID-19 cases and their close contacts were retrospectively collected from the outbreak that occurred in Guangdong, China in May and June 2021. Key epidemiological parameters, temporal trend of viral loads and secondary attack rates were estimated. We also evaluated the association of vaccination with viral load and transmission. Results We identified 167 patients infected with the Delta variant in the Guangdong outbreak. Mean estimates of latent and incubation period were 3.9 days and 5.8 days, respectively. Relatively higher viral load was observed in infections with Delta than in infections with wild-type SARS-CoV-2. Secondary attack rate among close contacts of cases with Delta was 1.4%, and 73.1% (95% credible interval (CrI): 32.9–91.4) of the transmissions occurred before onset. Index cases without vaccination (adjusted odds ratio (aOR): 2.84; 95% CI: 1.19–8.45) or with an incomplete vaccination series (aOR: 6.02; 95% CI: 2.45–18.16) were more likely to transmit infection to their contacts than those who had received the complete primary vaccination series. Discussion Patients infected with the Delta variant had more rapid symptom onset compared with the wild type. The time-varying serial interval should be accounted for in estimation of reproduction numbers. The higher viral load and higher risk of pre-symptomatic transmission indicated the challenges in control of infections with the Delta variant.

Using detailed exposure information on COVID-19 cases, we estimated the mean latent period to be 5.5 days (95% confidence interval: 5.1-5.9 days), shorter than the mean incubation period (6.9 days). Laboratory testing may allow shorter quarantines since 95% of COVID-19 cases shed virus within 10.6 days (95%CI: 9.6-11.6) of infection.

Incubation period is an important parameter to inform quarantine period and to study transmission dynamics of infectious diseases. We conducted a systematic review and meta-analysis on published estimates of the incubation period distribution of COVID-19, and showed that the pooled median of the point estimates of the mean, median and 95 th percentile for incubation period are 6.3 days (range: 1.8 to 11.9 days), 5.4 days (range: 2.0 to 17.9 days) and 13.1 days (range: 3.2 to 17.8 days) respectively. Estimates of the mean and 95 th percentile of the incubation period distribution were considerably shorter before the epidemic peak in China compared to after the peak, and variation was also noticed for different choices of methodological approach in estimation. Our findings implied that corrections may be needed before directly applying estimates of incubation period into control of or further studies on emerging infectious diseases.

S crub typhus is a life-threatening disease caused by Orientia tsutsugamushi, an obligate intracellular bacterium transmitted by the larvae of trombiculid mites (1). Only biting larvae of Asian scrub typhus chiggers (Leptotrombidium spp.) can transmit the disease. After the bite of an infective mite, a characteristic necrotic inoculation lesion (an eschar) can develop. The microorganism then spreads through the lymphatic fluid and blood, causing manifestations including fever, headache, rash, lymphadenopathy, and mental changes (1). Without appropriate treatment with specific antimicrobial drugs (e.g., tetracycline, chloramphenicol, doxycycline, or azithromycin), >6% of infected patients will die (2,3). There is no licensed human vaccine to prevent scrub typhus infection. Globally, scrub typhus is traditionally regarded as a disease endemic to a region called the Asia-Pacific tsutsugamushi triangle, which extends from Pakistan in the west to far eastern Russia in the east to northern Australia in the south. In some countries of Southeast Asia, scrub typhus is a leading cause of treatable nonmalarial febrile illness (4-6). It is estimated that scrub typhus threatens >1 billion persons, causes at least 1 million clinical cases per year, and is associated with substantial mortality rates globally (1,7). The 2010s saw a widespread reemergence of scrub typhus in endemic regions such as India, Korea, Laos, and the Maldives (3,6,8). The recent emergence of scrub typhus in the Arabian Peninsula, Chile, and possibly Kenya suggests wider global distribution of this disease in tropical and subtropical regions, far from the tsutsugamushi triangle (9-12). Although scrub typhus poses the greatest threat to residents of eastern and southern Asia as well as tourists traveling to these regions, it remains a neglected disease globally. The lack of both research and a nationwide surveillance system within many endemic regions have resulted in poorly understood epidemiologic characteristics and disease burden of scrub typhus at global, national, and subnational levels (7). The Global Burden of Disease Study publishes estimates for 333 diseases and injuries, but currently bundles scrub typhus with other neglected tropical diseases, rather than providing specific burden estimates (13). In China, scrub typhus cases were recorded in the early 1950s, and a disease surveillance system for scrub typhus was established in 1952 (14,15). Leptotrombidium deliense and L. scutellare mites are the 2 principal vectors transmitting the disease in the country (16). L. deliense mites inhabit southern China and emerge in April, peaking in June-August, and decreasing

Summary Tracing and isolation of close contacts is used to control outbreaks of coronavirus disease 2019 (COVID‐19) in China. However, risk factors associated with occurrence of COVID‐19 among close contacts have not been well described. 106 household contacts were included in this study, of whom 19 were developed into COVID‐19 cases and the secondary attack rate was 17.9%. Multivariable analysis showed increasing risk of occurrence of COVID‐19 among household contacts associated with female of index patients (Adjusted Hazard Ratio [aHR] = 3.84, 95%CI = 1.07‐13.78), critical disease of index patients ([aHR] = 7.58, 95%CI = 1.66‐34.66), effective contact duration with index patients >2 days ([aHR] = 4.21, 95%CI = 1.29‐13.73), and effective contact duration >11 days ([aHR] = 17.88, 95%CI = 3.26‐98.01).The sex and disease severity of index COVID‐19 patients, and longer effective contact duration with COVID‐19 confirmed cases could help epidemiologists to identify potential COVID‐19 case among household contacts at an early stage.

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