Skip to 0 minutes and 13 seconds JUDITH GLYNN: What do we mean by infectiousness of an infectious disease? What does it depend on? And how do we measure it?
Skip to 0 minutes and 23 seconds Infectiousness is a measure of how well an infection spreads. And this depends partly on the organism, but also on the environment. Different infectious organisms have different risks of transmission if there is a contact. And for different diseases, people will be infectious for different lengths of time. How well it spreads also depends on the environment because that determines the probability of a contact and the type and number of contacts. And these will vary from one population to another. And it will also depends on if there any prevention measures in place. There are two main measures we use to measure how well an infection spreads. The secondary attack rate, and the case reproduction number. And we shall look at each of these.
Skip to 1 minute and 5 seconds The secondary attack rate is used in the study of spread of infectious diseases in defined conditions in small communities in schools in households. It’s only relevant for person to person transmission and it’s the proportion of those exposed to the primary case that develop disease as a result of the exposure. So an example, in the first Ebola outbreak in 1976, there were 498 family members exposed to cases of Ebola. And of those people, 38 developed Ebola. So the secondary attack rate is 38 divided by 498, or 7.6%. 7.6% of people in the families exposed the cases developed Ebola. The secondary attack rate is context specific and it depends on a number of things. It depends on the closeness of contact.
Skip to 1 minute and 56 seconds So again, in that first outbreak, although the secondary attack rate overall for family was 7.6%, if we restrict it to the close family, to spouses and children, it was 27%. It was more likely that they would develop Ebola. It also depends on the type of contact. For example, for HIV, transmission by sharing needles gives you a higher secondary attack rate than sexual content. And for Ebola, extensive contact with body fluids as you might get, for example, in preparing a body for burial, would give a much higher secondary attack rate than simple skin contact. And it will also depend on the stage of illness.
Skip to 2 minutes and 35 seconds For example, for Ebola, it’s more infectious as the disease progresses as it gets more severe and as the viral load is higher.
Skip to 2 minutes and 44 seconds Imagine an infection moving through the population. It might start with one person infecting two and each of those infects two more. And it would rapidly increase. How do we describe the spread? This is the case reproduction number. The number of secondary cases per case. The basic case reproduction number which is known as R 0 is the reproduction number at time zero when all people in the population are susceptible. The net case reproduction number, or R, is the reproduction number at a certain time, time t. It’s changeable. It varies with the proportion still susceptible in the population. So R0 the basic case reproduction number is the average number of secondary cases per case in a totally susceptible population.
Skip to 3 minutes and 33 seconds In some situations, it’s possible to measure it directly if all of the population is susceptible. And that would be the case with new diseases such as SARS or diseases such as Ebola which are new to a particular population. For other diseases, much of the population may already be immune and therefore we can’t measure it directly. The R0 depends on three factors– the duration of infectiousness, the probability of infection being transmitted during contact between a susceptible and infected individual, and the average rate of contact between susceptible and infected individuals. The duration of infectiousness and the probability of infection being transmitted during the contact are largely characteristics of the infectious organism.
Skip to 4 minutes and 18 seconds But the average rate of contact between susceptible and infected individuals is a characteristic of the population. R0 will therefore vary from one place to another, from one situation to another, and also over time within one population if the contact rate changes.
Skip to 4 minutes and 37 seconds The R0 and the secondary attack rate are related to each other. So the R0 the average number of secondly cases per case in a totally susceptible population. The secondary attack rate– it’s a proportion of those exposed to the primary case that developed diseases as a result of that exposure. And they’re related in this way. If you imagine the secondary attack rate in the household, multiply the number of contacts, will give you the R0 for household contact. But then you have to think of the R0 for all the other situations.
Skip to 5 minutes and 6 seconds So add on the R0 to the secondary attack rate in the family, multiplied by the number of contacts in the family, plus the secondary attack rate in the community times the number of contacts in the community and so on. We’ll come back to R0 in further sessions.
Transmissibility: how do we measure infectiousness?
This lecture introduces two measures that describe infectiousness: the secondary attack rate and the case reproduction number.
The secondary attack rate (2o AR) is the proportion of those exposed to the primary case that develop disease as a result of the exposure. It only applies to person-to-person transmission. It will vary depending on the context (closeness and type of contact), as well as the disease. It can be estimated in situations in which the number of contacts is known, such as households or schools. For example, if 10 people in a household are exposed to the primary case and 2 get ill the secondary attack rate would be 2/10 or 20%.
The basic case reproduction number, R0 (R nought, pronounced “R nort”), is the average number of secondary cases per case in a totally susceptible population. As an infection spreads in a population some people become immune. The net case reproduction number, R, is the average number of secondary cases per case at a particular time, depending on the proportion of the population that is still susceptible.
R0 depends on 3 factors:
- duration of infectiousness
- probability of infection being transmitted per contact between a susceptible and infected individual
- average rate of contact between susceptible and infected individuals
It therefore varies not just from disease to disease, but between populations and in the same population over time, depending on the contact patterns.
To understand the difference between R0 and secondary attack rate, imagine you have one person with a new disease coming in contact with 10 people. That one person infects three people who then become ill. In that situation the secondary attack rate would be 3/10 (30%). The estimate of R0 would be 3: the one case has given rise to 3 further cases.
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