The COVID-19 vaccine works to reduce three things: the likelihood of getting the disease, the shedding period, the amount of shedding virus. The shedding of a virus is a period in which reproduced viral particles are expelled from the host's body to infect new hosts.   This period for COVID-19 is believed to extend for weeks and begins before symptoms may appear but is much shorter and weaker with vaccination. By reducing these three factors, the likelihood of contracting coronavirus declines.