Skip to 0 minutes and 14 seconds JUDITH GLYNN: What can we do to reduce transmission? In this lecture, we look at the principles behind it.
Skip to 0 minutes and 22 seconds Remember the basic case reproduction number R0, the average number of secondary cases per case in a totally susceptible population. If the R0 is greater than 1, the number of cases increases, because each case, on average, gives rise to more than one new case. If the R0 equals 1– that means each case, on average, gives rise to just one new case– the number of cases is stable. And if the R0 is less than 1, each case on average gives rise to fewer than one new cases, and the number of cases decreases.
Skip to 0 minutes and 54 seconds If we look at this on the graph, looking at the number of new cases per week over time, if R0 is is greater than 1, the number of new cases increases. If R0 is 1, then it becomes stable. And if we get the R0 to less than 1, the number of new cases starts to go down. And it’s this, of course, that control programmes aim to achieve. For Ebola, the R0 measured in different outbreaks, in the early stages, is somewhere between 1.5 and 2. This is not particularly high. For example, for measles, the R0 is around 15, which is high and makes measles particularly difficult to control. For rubella, it’s around 6, and for flu, somewhere between 1.5 and 3.
Skip to 1 minute and 36 seconds Of course, it will vary for all of these diseases, depending on context. The R0 is not the only factor that affects the spread of an infectious disease. Another important factor is the serial interval. The serial interval is the time between the same stage of illness in successive cases in a chain of transmission, the average time, if one person transmits to another person, between the onset of illness of one person and the onset of illness in the other. And the rate of spread will depend on both the R0 and the serial interval. So in Ebola, let’s say the R0 was 2 in the early stages. The average serial interval has been measured at somewhere around two weeks.
Skip to 2 minutes and 19 seconds And in those conditions, the number of cases would double every two weeks. And this is approximately what was seen in Liberia in the early stages of the epidemic, between June and August– an approximate doubling of cases every two weeks. Remember that the R0 depends on three factors– the duration of infectiousness, the transmission risk– that’s the per-contact probability of infection being transmitted between a susceptible and infected individual, and the total contact rate, the average rate of contact between susceptible and infected individuals. Reducing R0 means tackling each of those three factors. So duration of infectiousness, for some diseases, can be reduced through treatment. The transmission risk may be reduced through barriers, through distance. This will depend on the type of infection.
Skip to 3 minutes and 10 seconds For example, for sexually transmitted infections, barriers would include condoms, other barriers, maybe gloves, to reduce actual physical contact, and distance to reduce possibility of transmission, as well. Reducing the total contact rate is reducing the likelihood that susceptibles and infectious people come together through isolation and through quarantine. So we have these different measures– treatment, barriers and distance, isolation and quarantine. What can we use for R0? Well, unfortunately, we can’t use treatment, because we don’t have any that reduces the duration of infectiousness. But we can use barriers and distance, isolation and quarantine. These last two factors, the transmission risk and the total contact rate, can be thought of together as the effective contact rate.
Skip to 3 minutes and 57 seconds This is the sort of contact in which infection can be transmitted from one person to another, and it is this that the control measures aim to reduce.
Reducing transmission: the principles
This lecture looks further at the basic reproduction number (R0), disease spread, and how to reduce transmission.
R0 is the average number of secondary cases per case in a totally susceptible population. Therefore:
- If R0 > 1 the number of new cases increases
- If R0 = 1 the number of new cases is stable
- If R0 < 1 the number of new cases decreases
The purpose of disease control is to get R0 less than 1, i.e. each case on average gives rise to less than one new case.
Another factor that determines the speed of spread of a disease in a population is the serial interval. The serial interval is the time between the same stage of illness in successive cases in a chain of transmission (e.g. between onset of fever in one person and onset of fever in the person they transmit the infection to).
For Ebola in the early stages of an epidemic R0 ≈ 2. The average serial interval ≈ 2 weeks. Therefore the number of new cases doubles every two weeks.
Since R0 depends on duration of infectiousness, per contact transmission risk, and rate of contact between infectious and susceptible individuals, transmission can be reduced by tackling each of these.
For some diseases, the duration of infectiousness can be reduced by treatment. For Ebola, there is no specific treatment, thus this is not an option.
The per contact transmission risk can be reduced by physical barriers (gloves, personal protective equipment) and by distance. Ebola is not airborne, so staying 2-3 metres away avoids transmission via droplets.
During the infectious period, the rate of contact between infectious and susceptible individuals can be reduced through isolation and quarantine measures.
Estimates of how the reproduction number has changed over time in the epidemic are shown in Figure 1 of this article in the New England Journal of Medicine. It peaked at 2, but by November 2014 was around 1 in Guinea, Liberia and Sierra Leone.
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