Jump to navigation

  • 24 hour numbers
  • Urgent Care
    • 617-253-1311
  • Urgent Mental Health Concerns
    • 617-253-2916
  • Violence Prevention & Response
    • 617-253-2300
  • All contact numbers
  • Close

 

MIT Medical logo
24-hour Numbers
Log in to HealthELife

 

Search

MIT Logo
  • Find Patient Services Care, claims, and more
  • Find a Provider By name, specialty, or location
  • Make an Appointment Online or over the phone
  • Learn about Health Plans Choices & coverage
  • Stay Healthy at MIT For the whole community
  • Start
    Here
    Info & resources just for you
    • Information For
    • Information for New Students
    • Information for Parents
    • Information for Undergraduate Students
    • Information for Graduate Students
    • Information for Campus Employees
    • Information for Lexington Employees
    • Information for New Employees
    • Information for Internationals
    • Information for Retirees
    • Information for Affiliates

COVID-19 testing | COVID-19 updates | Covid Pass testing results | Covid Pass testing hours

Covid-19 Updates

How long before symptom onset is a person contagious?

May 20, 2020 | Kim Schive

May 20: MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer. 

How long before COVID-19 symptoms appear is a person contagious? In other words, based on the day a person actually becomes ill, how far back should contact tracing go?

Line graph showing levels for how sick you feel and how contagious you are over 4 days with a row of illustrations of people in various social situations, the last being in a medical office

While the incubation period for the virus can be as long as 14 days, research suggests that people who are infected with SARS-CoV-2, the virus that causes the COVID-19 illness, may become infectious to others several days before they start to feel ill. 

A study of 94 patients in China showed that viral load peaked shortly after the onset of symptoms, indicating that people may actually be more infectious in the days before they become ill and before the immune system has a chance to kick in. Another study, which looked at 77 pairs of individuals in which one person infected the other, found that contagiousness both began and peaked before the first symptoms of illness — 2.3 days and 0.7 days respectively. Those researchers concluded that about 44 percent of COVID-19 infections spread from person to person before symptom onset.*

For this reason, the Centers for Disease Control and Prevention (CDC) considers contact tracing to be crucial in protecting communities from further spread of the virus. “Time is of the essence,” they state. “If communities are unable to effectively isolate patients and ensure contacts can separate themselves from others, rapid community spread of COVID-19 is likely to increase to the point that strict mitigation strategies will again be needed to contain the virus.” 

The CDC’s contract-tracing protocol involves identifying and contacting close contacts of individuals who are diagnosed with COVID-19. The CDC identifies a “close contact” as “someone who was within 6 feet of an infected person for at least 15 minutes starting from 48 hours before illness onset until the time the patient is isolated.” Anyone who has been informed that they have had that level of contact with someone who has been positively diagnosed with COVID-19 should stay home, maintain social distancing, and self-monitor until 14 days from the last date of exposure to the infected individual.

*After discovering a syntactical error in their original code, the authors of this study reanalyzed their data and published a correction in August 2020. Though this reanalysis did not change their estimate of the proportion of presymptomatic transmission (44%), it did show that infectiousness peaked at symptom onset rather than slightly earlier, as originally observed. They also found that infectiousness started to rise, on average, earlier than noted in their original paper — at about three days prior to symptom onset, with approximately 9% of viral transmission occurring even earlier. (13 October 2020)

This news story has not been updated since the date shown. Information contained in this story may be outdated. For current information about MIT Medical’s services, please see relevant areas of the MIT Medical website.

MEMBERS OF THE MIT COMMUNITY ONLY:
If you are experiencing symptoms of COVID-19, call MIT Medical’s COVID-19 hotline at 617-253-4865.

COVID-19 FAQ
COVID-19 Vaccine information
Testing for COVID-19 FAQ
Positive tests: Isolation, quarantine, and re-testing FAQ
Managing prescriptions FAQ
Student/Affiliate Extended Insurance Plan FAQ

SHARE THIS

this block intentionally left blank by CSS

Cambridge
MIT Medical
Building E23
25 Carleton Street
Cambridge, MA 02142
 
M-F 8 a.m.-5 p.m.
617-253-4481
617-258-0656 (TTY)
Lexington
MIT Lincoln Laboratory
Annex V
244 Wood Street
Lexington, MA 02421
 
M-F 8 a.m.-5 p.m.
781-981-7080
781-981-6600 (TTY)
Urgent Care
MIT Medical
Building E23
25 Carleton Street
Cambridge, MA 02142
 
M–F 8 a.m. – 8 p.m.
Sat, Sun 10 a.m.– 4 p.m.
617-253-1311
617-258-0656 (TTY)

 

FAST FIND

  • About Us
  • Contact Us
  • Follow Us
  • News & Events
  • HealthELife
  • FAQs
  • Forms & Documents
  • Patient Rights
  • Privacy
  • Accessibility
  • Accreditation
  • Advisory Committees
  • Mission & Vision
  • Jobs

INFORMATION FOR:

  • Information For
  • Information for New Students
  • Information for Parents
  • Information for Undergraduate Students
  • Information for Graduate Students
  • Information for Campus Employees
  • Information for Lexington Employees
  • Information for New Employees
  • Information for Internationals
  • Information for Affiliates
  • Information for Retirees
Follow Us Facebook iconTwitter icon
Massachusetts Institute of Technology
© 2021 MIT Medical All rights reserved