The novel coronavirus has already claimed the lives of more than 61,000 Americans. But experts fear that number could be far higher at this point in the outbreak — perhaps by tens of thousands — once the pandemic subsides enough for officials to go back and make a true reckoning of the dead.

Experts are urging leaders to take measures right now to preserve data and medical specimens so that science has the chance to determine the precise number of people who succumbed during one of the most severe global pandemics in memory.

“Under-counting deaths in this particular epidemic is happening all over,” said Dr. Daniel Lopez-Acuna, an epidemiologist and former top World Health Organization official, who spent 30 years at the organization. “It’s almost inevitable.”