There are various things to be formalized with respect to periods of 'lockdown' in the present circumstances.
Leave Policy During Lockdown
In the interim, it would be fair enough that the 15 days of 'unpaid' leave may be treated as 'leave without pay (LWP)' for those who didn't work, likely without a break in service. This period of 'no work, no pay' can also be considered as special casual leave or special leave on medical grounds as per your policy, if any.
However, a few essential staff might have worked the full month, like security personnel who have to be paid for the full month.