2.2 Critical immunisation threshold (qc)
The critical immunisation threshold (qc) tells us what proportion of the population need to be vaccinated, in order to control an infection, and it is calculated in exactly the same way as the herd immunity threshold.
qc = 1 – 1/R0 x100
Activity 1 Calculation of qc
In this activity we ask you to calculate qc for 7 different virus diseases expressed as a percentage of the whole population. Use the equation to determine the values of qc and enter them into Table 2.
Round your calculation to the nearest whole number.
| Virus | R0 | qc |
|---|---|---|
| Influenza | 2 | |
| Hepatitis C | 3 | |
| Zika | 4 | |
| SARS-CoV2 | 2.5 - 6 | |
| Mumps | 8 | |
| Chickenpox | 10 | |
| Measles | 16 |
When you have completed your calculations, click to reveal the answers.
What do you notice about the relationship between R0 and qc?
Answer
The higher the value of R0, the higher is the value of qc.
| Virus | R0 | qc |
|---|---|---|
| Influenza | 2 | 50% |
| Hepatitis C | 3 | 67% |
| Zika | 4 | 75% |
| SARS-CoV2 | 2.5 - 6 | 60 - 83% |
| Mumps | 8 | 87% |
| Chickenpox | 10 | 90% |
| Measles | 16 | 94% |
For diseases such as mumps, chickenpox and measles, the level of vaccination coverage needed to prevent outbreaks is very high. When vaccine coverage of these childhood diseases falls below the required level in the population, outbreaks of the diseases occur. Also, when vaccination levels fall below the qc level, the age at which unvaccinated children contract the disease is older. The reason for this is that on average it will take longer before they encounter an infective with the disease, because the disease is less common in partially-vaccinated populations. Moreover, for some infections, disease is more serious if contracted later in life.
Notice that if a population is protected because the level of vaccination exceeds the qc for a particular infection, then even people who are not immune have some protection, because the infectious agent no longer circulates in the community. However if everyone relies on other people being vaccinated, then the level of population immunity falls below qc and the infection returns. This has occurred in the UK recently for diseases such as measles, which require high levels of vaccine coverage to create herd immunity.
Next week we will look at how vaccines are formulated and how effective they are, but it is worth remembering from you calculations here that the qc values for SARS-CoV2 range from 60 – 83%.