DUBAI—The MERS virus has caused nearly 50% more deaths in Saudi Arabia than has been officially reported, a review of medical data by Saudi health authorities shows.

The survey by the Ministry of Health revealed that 282 Saudis and non-Saudis have died in the kingdom from MERS, or Middle East Respiratory Syndrome, since the disease was first detected in September 2012—a 48% increase over the previously announced toll of 190.

There...