Watch Professor Judith Glynn explain what influences the spread of coronavirus COVID-19 infection.
In this mini-lecture Professor Judith Glynn talks about spread of infection, how this is measured and what we know about transmission (recorded 2nd March 2020).
This lecture focusses on R0, as well as explaining secondary attack rates and how we measure them, and describing the transmissibility of SARS-CoV-2 . As you watch the lecture, consider whether transmissions per case are increasing or decreasing in your context. What is it about the context, or interventions, that are influencing this?
R0 is defined as the average number of successful transmissions per case when everyone in the population is susceptible. However in the course of an outbreak – once you have a population in which not everyone is susceptible (survivors with immunity, or after vaccination) you can no longer talk about R0. The reproduction number in that situation is known as R or Rt. It is related to R0:
R = R0 x the proportion susceptible
So as the proportion susceptible goes down (= the proportion immune goes up), R (the number of new cases per case) decreases. For example, if R0 is 2, and half the population is immune, so half is still susceptible, R would be 2 x 0.5 = 1. In this situation each case would on average give rise to one more case and the disease would become stable. The proportion of the population that needs to be immune to bring R down to 1 is known as the herd immunity threshold
. When the proportion immune increases further, R will be less than 1 and the incidence of new cases will decrease.
The secondary attack rate
is defined in particular situations as the proportion of those exposed to the primary case that develop disease as a result of that exposure. For example as the proportion of people in a household who get COVID-19 from exposure to the first person in the household to get it. A paper from China with frequent testing of contacts estimated that the secondary attack rate in households was 13% (90 infected from 699 household contacts). In that situation cases were isolated once diagnosed which may have reduced the transmission in the household.
You may find the FAQs for this Step helpful – they cover a lot of areas. For those looking for additional technical detail, there are also specific papers shared in the See Also section and a link to a global summary of countries with the latest reproduction number estimates, which show whether cases are expected to be increasing or decreasing at present.