From January 21st to March 6th 2020, there were 157 symptomatic cases and 30 asymptomatic cases in the Ningbo Covid-19 data. These infected cases resulted in 2147 close contacts with them, among which 2001 exposures were caused by the symptomatic cases and 146 by the asymptomatic cases. The average number of close contacts by the symptomatic cases is 13 and that by the asymptomatic cases is 5, and the difference is statistically significant with a p-value of 6×10^-6
Fisher’s exact tests are used to investigate the difference in the transmission rates between the symptomatic and asymptomatic patients. We consider two scenarios: One combines the numbers of symptomatic and asymptomatic cases as the total number of infected patients, leading to a 2×2 table; and the other separates them, leading to a 2×3 table.