The US Centers for Disease Control and Prevention has updated guidance meant for public health officials and mathematical modelers, which now estimates that 40% of people infected with Covid-19 show no symptoms. 

Back in May, the CDC created five “Covid-19 Pandemic Planning Scenarios,” using data to provide a range of possible situations for Covid-19 in the US. The updated scenarios are based on new data the agency received through June 29. The agency also underscores these estimates are subject to change as more information becomes available, and exact numbers are uncertain. 

Under the CDC’s “current best estimate,” 40% of people with Covid-19 are asymptomatic. This number is up from the 35% the agency estimated on May 20. The percent of asymptomatic cases remains uncertain, the CDC emphasized.

The CDC is now including an infection fatality ratio, which takes into account both symptomatic and asymptomatic cases (in the estimates published in May, the agency only included fatality ratio for symptomatic cases). Under the “best estimate” scenario, the infection fatality ratio is 0.65%, meaning that 0.65% of people infected with Covid-19 are thought to die.

More insight: About half of Covid-19 transmissions happen before people get sick; this is up from the 40% estimate in May.

Under the CDC’s current “best estimate,” the transmissibility of the virus from asymptomatic people — in comparison to people with no symptoms — is now 75%, down from 100%. However, the agency said this “remains highly uncertain as asymptomatic cases are difficult to identify and transmission is difficult to observe and quantify.”

The agency warns this information is “intended to support public health preparedness and planning.”