1.4 Understanding RE
Because RE depends on the particular circumstances of an infection, the value of RE will be different dependent on the current local conditions.
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With reference to the COVID-19 pandemic: can you think of 5 different conditions that could affect the value of RE. There is a lot to think about here, so take your time before you reveal the answer.
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The proportion of people in the population who were resistant to infection with SARS-CoV2 varied over time. It was dependent on whether a person had been infected with SARS-CoV2 or had been vaccinated against it. Immunity gradually declines over time, if a person does not become reinfected with the same virus. Also, as the pandemic developed, new variants arose which could partly evade the immunity produced against previous strains. Effectively the new strains resulted in a more rapid decline in resistance than normally occurs and increased susceptibility to reinfection. All of these factors relate to the intrinsic susceptibility of the population. Also RE depends on how people interact with each other. Even before public health measures were introduced, many people started to reduce their social interactions so the level of effective contacts fell. Public Health measures, such as lock-downs, restricted access to public amenities and mask-wearing also all reduced the number of effective contacts between individuals.
We should also consider the underlying assumption for R0 and RE – that the population is homogeneously mixing. This is obviously a simplification. Some people live in large families, others in Institutions, and some people live alone. The intrinsic level of social contact varies greatly for each of these groups. Another important factor is what type of work people were doing. During the pandemic, some people could and did work from home. Others, because of the nature of their occupation, travelled to work and may have been meeting other members of the public as part of their work.
The number of effective contacts and the potential for transmission of an infectious agent depends on all of the factors noted here. Despite all of these variables affecting individuals, RE is still a very useful concept, when applied to the population, as a whole, since it informs public health policies.