In a large-scale study, American scientists compared the risks of secondary transmission of coronavirus infection in three categories of COVID-19 patients: asymptomatic, symptomatic and presymptomatic — those who have not yet shown symptoms. The results of the study are published on the preprint server

Most systematic reviews of COVID-19 indicate that the proportion of asymptomatic carriers is 30-45 percent of all cases, but what their contribution to the spread of infection was still unclear. In addition, most studies did not separate those who have the disease completely asymptomatic, and those who have it at the initial stage and the symptoms have not yet appeared.

Epidemiologists from the University of Notre Dame in Indiana decided to fill this gap. In the fall of 2020, they conducted a study on the campus of Midwestern University in Illinois, which involved about 13,000 students.

In August, when students began to return to the university campus, the administration took a number of measures to reduce the risk of spreading the virus. It included mass testing for the virus, contact tracking, isolation of patients and contact quarantine, daily assessment of well-being, as well as monitoring the spread of infection.

The testing was carried out using an express antigen test, which was then confirmed by a PCR test. If the result was positive, or the student had symptoms indicating a coronavirus infection, or he had a history of contact with a COVID-19 patient, the participant was placed in 14-day quarantine. If the tests for the fourth and seventh gave a negative result, the quarantine was stopped.

In total, as a result of mass monitoring, about 1,600 confirmed cases of COVID-19 were detected, including presymtomatic and asymptomatic ones. The latter accounted for about a third of all cases. Moreover, in contrast to the well-established opinion that asymptomatic carriers are not as dangerous as symptomatic ones, the results showed that the difference is not so great. Among the former, the secondary transmission coefficient was about 20 percent, and among the latter-about 30.

During the first four days after infection, the coefficient was highest in symptomatic patients, lower in presymtomatic patients, and lowest in asymptomatic patients. After seven days, this indicator decreased in all categories, and on the 14th day after contact with the virus, it was 25 percent in symptomatic and 19 percent in asymptomatic patients.

The asymptomatic group included those who had no symptoms at the time of receiving a positive test result, but manifested themselves later. Observations of this group allowed the authors to determine the average time of occurrence of certain symptoms. According to scientists, loss of taste and smell appears by the fourth day after a positive test result, and nasal congestion, headache and dry cough-by the fifth.

The authors note that, due to the fact that the number of asymptomatic carriers is very large and the coefficient of secondary transmission of the virus is also high among them, until everyone is vaccinated, it is necessary to continue to perform standard sanitary safety measures — wear masks, disinfect hands and observe social distance.