Go to Administration - Templates and create a new Appointment Reminder Template

Call it "Appointment Reminder SMS Template - COVID-19"  to differentiate between your standard template.

In the Content field, enter the expression:

concat(expr:if(expr:var('patient.name') != '', concat(expr:var('patient.name'), "'s"), 'Your'), ' appointment at ' , $location.name,' is confirmed for ', date:formatDate($appointment.startTime, 'short'), ' @ ', date:formatTime($appointment.startTime, 'short'), $nl, 
'Call us on ', party:getTelephone($location), ' if you need to change the appointment.', $nl,
'Insert COVID-19 message here')

Replace the "Insert COVID-19..." with your actual text.

Verify that it works by selecting a Customer and Patient and the Sample SMS area.

Once complete, go to Administration - Organisation, and edit your Practice. Change the Appointment SMS field to your new "Appointment Reminder SMS Template - COVID-19".  Log out and back in again.

You can verify it will work as expected by going to Workflow - Scheduling, selecting an appointment for a customer with SMS enabled, and clicking Remind. A Send SMS window will be displayed with your new SMS text.