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Epi 103: Outbreaks in
Communities
Agenda
How do we track an outbreak?
Exponential growth
Basic reproduction number
Herd Immunity
Attack Rate
Incubation Period
Outbreak investigations and surveillance
Expect: ~40 minutes of short lectures on different topics interspersed with
examples and discussions, followed by Q&A for the remainder of the time
Understanding the Spread of Disease in
Communities
“Life can only be understood backwards, but it must be lived forwards.”—
Soren Kierkegaard
What is exponential growth?
https://www.worldometers.info/coronavirus/coronavirus-cases/
Basic Reproductive Number (R0)
Expected number of secondary cases produced by a single infection in a
completely susceptible population
Example:
https://blossoms.mit.edu/legacy/FluGames/simulation01.html
Smieszek, T., Salathé, M. A low-cost method to assess the epidemiological importance of individuals in controlling infectious disease outbreaks. BMC Med 11, 35 (2013). https://doi.org/10.1186/1741-7015-11-35
Herd Immunity
Herd immunity is the resistance to an attack by a disease to which a large
proportion of the population are immune.
Example:
https://blossoms.mit.edu/legacy/FluGames/simulation05.html
Other Key Terms for Outbreak
Investigations
Endemic
Epidemic
Pandemic
Attack Rate
An attack rate is a measure of the speed of spread of a disease in the
population often used during outbreaks.
Attack rate = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑎𝑡 𝑟𝑖𝑠𝑘 𝑤ℎ𝑜 𝑑𝑒𝑣𝑒𝑙𝑜𝑝 𝑎 𝑐𝑒𝑟𝑡𝑎𝑖𝑛 𝑑𝑖𝑠𝑒𝑎𝑠𝑒
𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑎𝑡 𝑟𝑖𝑠𝑘
Incubation Period
Lauer SA, Grantz KH, Bi Q, et al. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application. Ann Intern Med. 2020; [Epub ahead of print 10 March 2020]. doi: https://doi.org/10.7326/M20-0504
Steps of an
Outbreak
Investigation
Establish a case definition
Enhance Surveillance
Describe occurrence of cases according to time, place, and person
Develop hypotheses about the nature of
the exposure
Conduct analytic studies, if
appropriate
Implement disease-control interventions
Communicate results of the investigation
Koepsell & Weiss, Epidemiological Methods: Studying the Occurrence of Disease
Final Word
As a final note I want to leave you with this thought:
“All models are wrong, but some are useful.”—George Box (statistician, 1919–)
We are in an unprecedented time of massive information sharing. Be
thoughtful, be patient, and be suspicious of anyone who says something is
GOING to happen or that they have certainty about some aspect of COVID-
19. Its new and we don’t know much yet.
Questions and answers
What questions do YOU have?
Please feel free to ask questions by:
Asking them now
Posting them in the chat window
Emailing them to [email protected] – I will get back to you when I can
Evaluation
Link to anonymous four question survey:
https://www.surveymonkey.com/r/6T5DMFK
This is an ongoing class that I want to develop and make better. Your
feedback helps!
If you don’t want to click the link please feel free to leave feedback in the
chat window or by emailing me at [email protected]
Thanks for tuning in!
Next Classes
Next classes:
Epi 101: Friday 2pm PST [5pm EST, 11pm GMT+2 (Cape Town)]
Epi 102: Saturday 2pm PST [5pm EST, 11pm GMT+2 (Cape Town)]
Epi 103: Sunday Friday 2pm PST [5pm EST, 11pm GMT+2 (Cape Town)]
Additional resources for more reading & learning can be found at:
http://karabensley.org/epidemiology-for-our-new-reality