Barring one patient in Aizawl, who tested positive coming in contact with a positive patient, the rest 27 patients have travel history.
The infected patients have been placed under quarantine since their arrival in the state, a statement by the state information and public relations department said.
Of the 28 cases, 19 were reported from Aizawl district, 6 from Lunglei district, 2 from the Serchhip district and 1 case was reported from Kolasib district, it said.
The 18 infected Assam Rifles jawans are from the Assam Rifles quarantine facility at Zokhawsang on the outskirts of Aizawl while 6 BSF jawans were lodged at a quarantine facility in Lunglei district.
There are 585 active cases in the state while 949 people have recovered.
According to the health department, as many as 57,076 samples have been tested for COVID19 till Thursday.