Last year’s Hurricane Maria killed more than 4,600 people in Puerto Rico, 70 times the official toll, according to estimates in a Harvard University study.
A third of deaths after September's hurricane were due to interruptions in medical care caused by power outages and broken road links, researchers say.
Interviews conducted in Puerto Rico suggest a 60 per cent increase in mortality in the three months after the storm.
The official death toll currently stands at 64, reports BBC.
But experts say an accurate count was complicated by the widespread devastation wreaked by the storm.
The Harvard researchers contacted more than 3,000 randomly selected households between January and March this year and asked about displacement, infrastructure loss and causes of death.
They then compared their results with the official mortality rates for the same period in 2016, more than a year before the hurricane struck the island.
The Caribbean island is home to 3.4 million US citizens.