I have a pretty good idea who the person who tested positive works with and is in contact with the most. Can I send the co-worker notification to them only?

As a manager or supervisor, you are likely in the best position to have a sense of who was and was not in close physical contact with an employee who tests positive.  While we recommend erring on the side of caution and including the employee(s) in the distribution of the message, if you have a clearer sense of those who need to receive a notification, you are welcome to send the co-worker notification to a more localized group of individuals.