Expected behavior is that new alerts are sent to the slack channel automatically.
Using the latest cloudformation template and following the setup guide automatic alert notification does not get sent to the slack channel.
Updates to the slack channel must be initiated from the page https://console.aws.amazon.com/securityhub/home?region=us-east-1#/insights/arn:aws:securityhub:::insight/securityhub/default/ and after all messages are selected the Action drop down menu selected and the slack bot name clicked on.