As the third highly pathogenic coronavirus, SARS-CoV-2 is thought to be able to be transmitted to others at an earlier stage than SARS-CoV and MERS-CoV. Studies have previously pointed out that 30% -59% of SARS-CoV-2 infections are asymptomatic, which poses a huge challenge to the prevention and control of the new crown epidemic.
On May 26th local time, the "Emerging Infectious Diseases" published by the US Centers for Disease Control and Prevention (CDC) was published online by the Guangzhou CDC A study titled "Secondary Transmission of Coronavirus Disease from Presymptomatic Persons, China". The research team explored the secondary infection rate (SAR) of different types of contacts of COVID-19 patients before the onset of the disease by analyzing the follow-up monitoring data of the contacts of newly diagnosed patients in Guangzhou.
Corresponding authors of the study were Zhang Zhoubin, deputy director of the Guangzhou Center for Disease Control and Prevention, and Qin Pengzhe, the center.
The COVID-19 epidemic caused by SARS-CoV-2 is spreading rapidly around the world. Some previous case reports and modeling studies have also shown that the asymptomatic carriage of SARS-CoV-2 plays a role in transmission. The study pointed out that in order to control asymptomatic infections, China has implemented active case monitoring and strengthened social isolation measures, including contact tracking, isolation of key populations, medical observation, and reduction of social activities. However, more information about the pre-symptomatic transmission characteristics is needed to develop targeted control and prevention guidelines.
The researchers analyzed contact tracking data collected from January 28 to March 15, 2020. The detection of asymptomatic COVID-19 cases is mainly through close contact screening, clustered epidemic investigations, follow-up investigations of infection sources, and active monitoring of key populations with travel or residence history in areas where COVID-19 continues to spread at home and abroad.
The paper mentioned that as of March 15th, a total of 359 cases of new coronary pneumonia have been diagnosed in Guangzhou. Of these, 83 (23%) were asymptomatic at the time of diagnosis, and 71 (86%) of these 83 people subsequently developed symptoms. Of these cases where symptoms later appeared, 38 of them had close contact with more than 1 person (range 1-90, median 4).
The study finally identified and included 369 close contacts. The median age of close contacts was 35 years (range 0-93 years), 23.8% of them were family members of the indicated cases, and 12 cases were finally diagnosed with new crown infection by nucleic acid testing. Of the 12 close contacts, 8 showed symptoms, and 4 were asymptomatic in this study.
Studies have shown that the overall secondary infection rate is 3.3% (95% CI: 1.9% -5.6%). In terms of classification, the secondary infection rate of family contacts is 16.1%, that of ordinary social contacts is 1.1%, and that of workplace contacts is 0.
Compared with other age groups, the older secondary contacts had the highest secondary infection rate; the secondary infection rate for the group over 60 years old was 8.0%, and the secondary infection rate for the group under 60 years old was 1.4% -5.6%.
The asymptomatic index cases had the lowest secondary infection rate of 0.8%. In contrast, the secondary infection rate was 3.5% for mild symptoms, 5.7% for moderate symptoms, and 4.5% for severe symptoms.
In addition, compared with general contacts with friends or relatives, the risk of infection of close contacts who live with the indicated case is 12 times higher, while those with frequent contact with the indicated case are 29 times higher within 2 days before the indicated case is diagnosed .
The researchers claim that the results of this study confirm some previous reports that SARS-CoV-2 can be transmitted during asymptomatic periods. If the contact person and the diagnosed patient are in the same living environment or have frequent contact with the patient, their possibility of infection is greatly increased, which emphasizes the need for immediate contact-based monitoring and maintaining social distance.
The results also indicate that most secondary infections occur in family clusters, and groups older than 60 years seem to be more susceptible to infections. These results are consistent with previous reports of epidemiological characteristics, suggesting that asymptomatic suspected cases should be carefully implemented at home isolation.
The researchers also noticed that asymptomatic people seem to be less efficient at spreading the virus. However, this finding should not hinder isolation and monitoring. The basic infection number R0 of this study cohort was 0.3 (95% CI: 0.2-0.5), which was much smaller than the overall reported R0 (2.2).
The researchers believe that this low level of transmission may be due to the implementation of active surveillance, centralized isolation and a strong social distance policy in Guangzhou.
However, the researchers also mentioned that the results of this study should be interpreted with caution, "some limitations have affected our estimate of the rate of secondary infections in the new crown." First, the number of close contacts is limited, including only those People who have been in contact and may have missed asymptomatic infections. Second, the study excluded close contacts who had contacted more than 2 confirmed cases of COVID-19. Third, the definition of the period of pre-symptomatic transmission is not yet clear.
However, despite these limitations, the researchers believe that the analysis still provides valuable information about the secondary infectious transmission of different types of contacts of COVID-19 patients before onset.
In the future, researchers will need further evidence to determine the characteristics of the population, the period of infection, and the duration and duration of viral discharge in asymptomatic patients.
Surging news reporter He Liping