Epidemiology 101: Understanding Infections in Communities

“We owe all the great advances in knowledge to those who endeavor to find out how much there is of anything.”

- James Maxwell (physicist)

Agenda 1. How do we define disease?

2. How to we measure disease occurrence?

Prevalence Incidence

3. Methodological issues

→ Expect: 20-30 minutes of a short lecture, 10-15 minutes of practice, rest of time: discussion/Q&A

“If you wish to converse with me, define your terms.”

- Francois Voltaire

Case Definition

Clinical Diagnosis

Population

Testing Accuracy • Case definition relies on

confirmed laboratory cases

• How do we measure the accuracy of testing?

Measures of Disease Frequency

What is the Prevalence of a disease? Count of prevalent cases = number of persons in the population who have a disease

Prevalence = Defined as the number of affected persons present in the population at a specific time divided by the number of persons in the population at that time

At a specific time,

Prevalence = # 𝑜𝑓 𝑐𝑎𝑠𝑒𝑠 𝑜𝑓 𝑎 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑝𝑟𝑒𝑠𝑒𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛

# 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛

What is Incidence of a disease? Incidence is the number of new cases of a disease that occur during a specified period of time in a population at risk.

Count of Incident Cases = Number of new cases during a specific time period

Incidence Rate = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑐𝑎𝑠𝑒𝑠

𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑎𝑡−𝑟𝑖𝑠𝑘 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒→ Denominator is person-time

https://coronavirus.jhu.edu/map.html

https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section9.html

Methodological Challenges • Numerator issues

• Denominator issues

Examples

Example from WA State DOH

Which is the incident case count?

Example from WA State DOH

Which is the prevalent case count?

How do we interpret this?

https://twitter.com/amdar1ing

How do we interpret this?

What does

this number

represent?

What is the sensitivity and specificity of the following test?

Results

of test

Disease No

Disease

Total

Positive 80 100 180

Negative 20 800 820

Total 100 900 1,000

Sensitivity = 80

100= 80%

Specificity = 800

900= 89%

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 kmbensley@gmail.com – 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 kmbensley@gmail.com

• Thanks for tuning in!

Next Classes• Next classes:

Epi 102: Tuesday 9am PST [noon EST, 6pm GMT+2 (Cape Town)]

Epi 103: Wednesday 9am PST [noon EST, 6pm GMT+2 (Cape Town)]

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