(CNN)In new guidance for mathematical modelers and public health officials, the US Centers for Disease Control and Prevention is estimating that about a third of coronavirus infections are asymptomatic.

The CDC also says its "best estimate" is that 0.4% of people who show symptoms and have Covid-19 will die, and the agency estimates that 40% of coronavirus transmission is occurring before people feel sick.

The agency cautions that those numbers are subject to change as more is learned about Covid-19, and it warns that the information is intended for planning purposes. Still, the agency says its estimates are based on real data collected by the agency before April 29.

The numbers are part of five planning scenarios that "are being used by mathematical modelers throughout the federal government," according to the CDC. Four of those scenarios represent "the lower and upper bounds of disease severity and viral transmissibility."

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