COVID-19 Incubation and Transmission

Incubation
How long before I show signs and symptoms?
 
Available evidence supports that half of those infected will display symptoms of the illness five days following exposure, with 97.5% developing signs and symptoms of the illness within 12 days of exposure to COVID-19. 
 
Data culled from 181 individuals who tested positive for COVID-19, whose exposure to an individual with COVID-19 had been confirmed and whose onset of symptoms was documented, provided the evidence for these conclusions about incubation.  Although some developed symptoms between 12-14 days following exposure, only 2.5% of the test group (approximately four individuals) fell within this parameter.  Results of this study can be found here.
 
Clinical signs and symptoms of mild-to-moderate COVID-19 can be found here.  If you have symptoms that are more significant than those reported here, seek treatment from your physician.
 

Transmission
How long am I contagious?
 
Little data is available that provides an adequate answer to this question as there is insufficient data to understand for how long COVID-19 makes a person contagious.  However, looking to the timeline of other viruses does add some perspective to this question. 
 
Clarifying terminology is initially important.  When discussing the period during which a disease can be transmitted from one person to another, the terms “contagious” or “infectious” are routinely used.  “Viral shedding” is another term used by scientists and medical professionals to define this period of contagion.  During a viral shedding period, the viral host can actively shed the disease to others when the illness is present in the host. 
 
A study performed in Nicaragua to determine the transmission of influenza in children and adults can be used to understand the length of time a person is infectious or can “shed” the virus.  Between 2012 and 2014, the period over which four influenza variants had been detected in hosts, 144 cases and 112 household contacts were enrolled in the study.  Over a period of 10-14 days after testing positive via the QuickVue Influenza A+B rapid test (a qualitative nasal/oral swab test), five home visits were conducted during which additional respiratory samples were collected from the infected individuals, and those in the same household with the infected individual.
 
Results of this study revealed the following:

• Before displaying any symptoms, those infected with the virus could transmit the virus to others, with children (0-16 years) displaying pre-symptomatic viral shedding before adults; viral shedding occurred approximate.
• From illness onset to the point when the patient could no longer transmit the virus to another (cessation of viral shedding), all age groups stopped being infectious by the fifth day after symptoms were detected.

 
Why should this influenza investigation be used as a guide to understand COVID-19?

1. Influenza transmission can occur via airborne particles, as is the case with COVID-19.
2. Testing for influenza occurred using a method similar to that used with COVID-19 (a nasal/oral swab test to determine the presence or absence of nucleic acid associated with the virus, which is a qualitative test for the illness).

 
What does this influenza study reveal about COVID-19?

1. A person can transmit the virus before he or she displays symptoms, and for some period after symptoms arise.
2. Understanding the period during which a person is infectious requires time to collect samples from the infected person while the person is fighting the illness, and from others in the household to determine if the virus was transmitted. 
3. If children can transmit the virus for a longer period before symptoms are identified, or if the infected children viral shed and never show symptoms (which is the presumption here due to the low incidence of pediatric patients with COVID-19 symptoms), children may transmit the disease to adults unknowingly. 

 
Although we still do not know how long viral shedding occurs before a person displays symptoms of COVID-19, or for how long he or she is infectious after symptoms are detected, the fact that a person can be contagious for this period explains the need for social distancing, hand washing and other sanitary measures. 

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