“Threats to Internal Validity”

We conduct an experiment where we randomly assign 500 people to treatment and control groups. Both groups have the same number of participants. The treatment consists of giving a COVID-vaccine to the treatment group and after that they just do their normal lives. The control group does not receive the vaccine and therefore continue to do their normal lives. After 6 months we analyze if people got COVID and we see that a significant larger percentage of people from the control group got COVID in comparison with the treatment group. Can we be certain that the COVID vaccine reduces the likelihood of getting COVID? Why or why not? How can threats to internal validity affect these results?