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Most coronavirus cases are spread by people without symptoms, CDC now says

It's one of the main reasons mask use is so important, the CDC said.

Posted: Nov 20, 2020 8:23 PM

(CNN) -- Most coronavirus infections are spread by people who have no symptoms, the US Centers for Disease Control and Prevention said in newly updated guidance.

It's one of the main reasons mask use is so important, the CDC said.

"Most SARS-CoV-2 infections are spread by people without symptoms," the agency said in a section of its website devoted to explaining the science of how to use masks to control the spread of the virus.

"CDC and others estimate that more than 50% of all infections are transmitted from people who are not exhibiting symptoms," it added in the guidance posted Friday.

"This means at least half of new infections come from people likely unaware they are infectious to others."

According to the CDC, 24% of people who transmit the virus to others never develop symptoms and another 35% were pre-symptomatic. It also said 41% infected others while experiencing symptoms.

Peak infectiousness comes five days after infection, the agency said on the website. "With these assumptions, 59% of infections would be transmitted when no symptoms are present but could range (from) 51%-70% if the fraction of asymptomatic infections were 24%-30% and peak infectiousness ranged 4-6 days."

It cited a study published in July in the Proceedings of the National Academy of Sciences and as-yet unpublished CDC data. Previously, the health organization has not characterized what percentage of infections are spread by symptomless people.

"Infection is spread primarily through exposure to respiratory droplets exhaled by infected people when they breathe, talk, cough, sneeze, or sing," the agency added.

It also said people spew more droplets when they talk or sing louder or when they are exerting themselves more.

The CDC said studies have shown 40-45% of infected people never develop symptoms.

"Among people who do develop symptomatic illness, transmission risk peaks in the days just before symptom onset (presymptomatic infection) and for a few days thereafter."

"Accordingly, the number of infections transmitted peaks when virus levels peak," the agency noted.

So, people are spreading the virus when they have no idea they are infected.

Masks can prevent this spread.

Cloth masks can stop people from exhaling the little globs of mucus or saliva that carry the virus. Those particles can fly directly onto others, fall onto surfaces to be picked up, or linger in the air -- especially in closed rooms.

The CDC also now said masks can help prevent people from inhaling the droplets, large and small, that carry the virus.

California Coronavirus Cases

Data is updated nightly.

Cases: 1200624

Reported Deaths: 19116
CountyCasesDeaths
Los Angeles3908917623
San Bernardino921641129
Riverside857691437
San Diego80018997
Orange767611577
Kern40632448
Fresno37994481
Sacramento36579566
Santa Clara33732476
Alameda29116511
San Joaquin25741502
Contra Costa23571261
Stanislaus21585424
Tulare20634308
Ventura19379175
Imperial15960356
San Francisco15377160
Monterey14753116
San Mateo14148170
Sonoma12049157
Merced11550179
Santa Barbara11340135
Kings1068787
Solano1004481
Marin7704129
Placer635468
San Luis Obispo612936
Madera605085
Yolo473775
Shasta471847
Santa Cruz435828
Butte389459
Sutter314415
Napa298817
El Dorado23154
Yuba198110
San Benito186516
Lassen17733
Tehama164925
Mendocino148522
Nevada11849
Tuolumne9818
Lake90919
Glenn8918
Humboldt8509
Colusa6946
Siskiyou6242
Amador55416
Mono5453
Calaveras47822
Del Norte3131
Inyo29016
Plumas2050
Trinity1580
Modoc1310
Mariposa1302
Alpine500
Sierra190
Unassigned00
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