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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It cited a study published in July in the Proceedings of the National Academy of Sciences and as-yet unpublished CDC data.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: 3015644

Reported Deaths: 33724
CountyCasesDeaths
Los Angeles102419013936
San Bernardino2540941560
Riverside2422062517
Orange2123892367
San Diego2117872103
Santa Clara935571076
Kern85362565
Fresno82039932
Sacramento801541080
Alameda67375764
Ventura61257412
San Joaquin57094748
Contra Costa52146449
Stanislaus42548749
Tulare41016501
Monterey35418250
San Mateo32186294
San Francisco29139262
Imperial25253463
Solano25216105
Santa Barbara24481231
Sonoma23871240
Merced23800296
Kings19254133
Placer17194173
San Luis Obispo15658128
Madera13328130
Santa Cruz12196113
Marin11569154
Yolo10519131
Shasta9594117
Butte9411121
El Dorado765944
Sutter760477
Napa752139
Lassen509413
San Benito494143
Yuba481827
Tehama397142
Tuolumne330438
Nevada317673
Mendocino311432
Amador300129
Lake257228
Humboldt232224
Glenn192419
Colusa16919
Calaveras160823
Siskiyou140813
Mono10914
Inyo92429
Del Norte8572
Plumas5735
Modoc3793
Mariposa3464
Trinity3024
Alpine730
Sierra700
Unassigned00
Chico
Partly Cloudy
50° wxIcon
Hi: 66° Lo: 47°
Feels Like: 50°
Oroville
Clear
55° wxIcon
Hi: 66° Lo: 50°
Feels Like: 55°
Chico
Clear
50° wxIcon
Hi: 59° Lo: 45°
Feels Like: 50°
Red Bluff
Clear
52° wxIcon
Hi: 39° Lo: 26°
Feels Like: 52°
Red Bluff
Clear
52° wxIcon
Hi: 63° Lo: 48°
Feels Like: 52°
Chico
Clear
50° wxIcon
Hi: 67° Lo: 50°
Feels Like: 50°
Another very windy day for northern California as an area of low pressure continues its southward trek over our state. Thankfully we'll have much quieter weather on Wednesday, with more rain and snow chances coming.
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