5 Summary

This week, we introduced some key concepts in epidemiology including:

• R₀, the number of people infected by a single infective in a totally susceptible population.
• R_E, the number of people infected by a single infective, in real circumstances.
• R_T, the number of people infected by a single infective at some defined time-point.
• The variables imply that when R >1 an epidemic is spreading, but if R<1 it will eventually die out.

The measurement of these variables assumes homogeneous mixing in the population and that all individuals are either completely susceptible or completely resistant to infection – conditions which do not generally occur in the real world. Nevertheless the variables are useful for determining how an infection will spread in the population as a whole.

The herd immunity threshold (HIT) and critical immunisation threshold (qc) define what proportion of the population must be resistant to infection, for it to stop it spreading. These variables can be calculated if R₀ is known. The value varies for each disease. The larger the value of R₀, the greater the proportion of people must be vaccinated to contain a disease.

In your laboratory investigation, you identified one recently infected individual, using the ELISA to measure different classes of S-antibody.

Now go to Week 7.