I think the CDC have changed their testing guidelines in order to prevent the virus being spread by people with a negative test result. This is because people who test negative could go on to test positive in the next few days. This is important as people are most infectious in the day or two immediately before and after the onset of symptoms.
Proportion of asymptomatic coronavirus disease 2019: A systematic review and meta‐analysis (Journal of Medical Virology, 21 July)
Meta Analysis
N = 50, 155
41 studies
Asymptomatic, confirmed COVID‐19
Overall pooled data
15.6% are asymptomatic
84.4 are symptomatic
Asymptomatics in children
N = 1, 152
11 studies
27.7% asymptomatic
72.3 symptomatic
Nearly half of the patients with no symptoms at detection time will develop symptoms later
48.9% of asymptomatic go on to become symptomatic
Asymptomatic COVID‐19 patients could have abnormal laboratory and radiational manifestations
Which can be used as screening strategies to identify asymptomatic infection.
CT results in asymptomatics
N = 36
4 studies
63% had an abnormal CT
Reduced white blood cell count, 27%
Increases lymphocyte count
Increased lactate dehydrogenase, 42.3%
Increased C‐reactive protein, 11.5%
Clinical Course and Molecular Viral Shedding Among Asymptomatic and Symptomatic Patients With SARS-CoV-2 Infection in a Community Treatment Center in the Republic of Korea
Aim
To quantitatively describe SARS-CoV-2 molecular viral shedding in asymptomatic and symptomatic patients
Method
Cohort study
n = 303
Isolated in a community treatment center in Korea
Results
110 (36.3%) were asymptomatic at the time of isolation
21 of these (19.1%) developed symptoms during isolation
Time from detection of SARS-CoV-2 to symptom onset in presymptomatic patients was 15 (13-20) days
Time from diagnosis to the first negative conversion
Asymptomatic, 17 days
Symptomatic, 19.5 days
Viral loads, same
Meaning
Many individuals with SARS-CoV-2 infection remained asymptomatic for a prolonged period
Viral load was similar to that in symptomatic patients
Isolation of infected persons should be performed regardless of symptoms
Temporal dynamics in viral shedding and transmissibility of COVID-19 (Nature medicine, 17th August)
94 patients with laboratory-confirmed COVID-19
Estimated that 44% of secondary cases were infected during the index cases’ presymptomatic stage
Disease control measures should be adjusted to account for probable substantial presymptomatic transmission.
The implications of silent transmission for the control of COVID-19 outbreaks (PNAS – US, 28th July)
If 17.9% of infections are asymptomatic
Presymptomatic transmission, 48% of transmission
Asymptomatic transmission, 3.4% of transmission
Even immediate isolation of all symptomatic cases is insufficient to achieve control
Urgent need to scale up testing of suspected cases
Supplemented by rapid contact-based surveillance to identify exposed individuals
prior to their infectious period
Overview of Testing for SARS-CoV-2 (COVID-19)
Considerations for COVID-19 Diagnostic (Molecular or Antigen) Testing
Mild symptoms of COVID-19
Your health care provider may advise a test.
If you test positive or do not get tested, self-isolate for at least 10 days after symptom onset
And at least 24 hours after the resolution of any fever (without the use of fever-reducing medications)
If you have been in close contact (within 6 feet) of a person with a COVID-19 infection for at least 15 minutes but do not have symptoms
You do not necessarily need a test, unless in a vulnerable group
Or
Health care provider or State or local public health officials recommend you take one
A negative test does not mean you will not develop an infection from the close contact or contract an infection at a later time
You should monitor yourself for symptoms
If you do not have COVID-19 symptoms and have not been in close contact with someone known to have a COVID-19 infection:
You do not need a test.
If you are in a high COVID-19 transmission area and have attended a public or private gathering of more than 10 people (without widespread mask wearing or physical distancing)
You do not necessarily need a test
Live or work in a care facility
You will need to be tested
Critical infrastructure worker, health care worker, or first responder
You may need to get a test, according to your employer’s guidelines.
C.D.C. Now Says People Without Covid-19 Symptoms Do Not Need Testing