Although people are thought to be the most contagious when they show the most symptoms, spread may be possible before people show symptoms. 

References:

1.https://www.cdc.gov/coronavirus/2019-ncov/prepare/transmission.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fcoronavirus%2F2019-ncov%2Fabout%2Ftransmission.html