40% of coronavirus patients have no symptoms

About 40% of people infected with Covid-19 show no symptoms of the disease, according to new guidance from the US Centers for Disease Control and Prevention (CDC), reports CNN.

In May, the CDC created five "Covid-19 Pandemic Planning Scenarios," using data to provide a range of possible situations for the coronavirus in the US.

Those scenarios have now been updated based on new data the agency received in June. 

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 number of asymptomatic cases remains uncertain, the CDC emphasized.

The CDC is also 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 the 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 will die.

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 says this new information is "intended to support public health preparedness and planning."