Background: Corona virus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2, declared as “once-in-a-century” pandemic, has affected about entire human population. Till now, the most promising control strategy is to block the transmission which will never be effective without completely knowing the transmission patterns.
Methods: This was a retrospective case to case study aiming to estimate and calculate the transmission rate of COVID-19 through pre-symptomatic and asymptomatic patients. Contact history of included positive cases during the period of 24 hours before appearance of symptoms and infection confirmation for symptomatic and asymptomatic cases respectively was traced for disease transmission.
Results: Overall, a total of 201 cases had contacted with 7168 people in last 24 hours with an average of 35.66 contacts per patient ranging from a minimum of 4 to maximum of 87 contacts (Meetings). Out of 7168 meeting persons, the infection was traced in 64 (0.89%) persons. For 155 symptomatic patients, a total of 5611 contacting persons were traced before appearance of symptoms (pre-symptomatic) in last 24 hours with an average of 36.20 meetings per patient. The infection was transmitted in 63 (1.12%) people and remaining 5548 (98.88%) were remained uninfected. Out of the 63 transmissions, 62 (98.4%) were traced within last 6 hours before the onset of symptoms while only 1 was traced from 6-12 hours before symptoms. A total of 1557 persons were traced having meeting/contacts with asymptomatic cases in last 24 hours before their infection confirmation. Out of these 1557 persons, only 1 was found to be infected and the infection rate was calculated to be 0.06%.
Conclusion: The study concludes that the risk of pre-symptomatic transmission of infection is low (1.12%) and it becomes very rare in contacts made longer than 6 hours before onset of symptoms. The infection transmission is traced as long as about 9 hours before the appearance of clear symptoms in the patients but the incidence was as low as about 0.02% of the total contacts in that period. Transmission frequency by asymptomatic patients is also rare (0.06%) in the studied population.