Week 6: Epidemiology

Introduction

Do you remember the regular briefings given by the Chief Medical and Scientific Officers, at the height of the COVID-19 pandemic? There was a lot of concern about whether the number of infections were increasing or decreasing because that determined the public health measures needed to control the epidemic. Later, when vaccines became available, control of the epidemic shifted away from this as the vaccination programme did the heavy-lifting.

Public health measures such as closure of public amenities, reduced travel on public transport and mask-wearing reduced effective contacts between individuals and delayed the progress of the COVID-19 pandemic in different countries.

This week, you will be looking at aspects of epidemiology and modelling that underpinned the response to the pandemic.

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Transcript: Video 1 Introduction to Week 6

DAVID MALE:

This week, we’re going to take a look at some aspects of epidemiology and how it informs public health. A key concept in epidemiology is the reproduction number of an infection or its R-value. It describes the number of people who will become infected from a single infected individual.

At any one time, this depends on how many people are susceptible to infection and how they are mixing in the community. The R-value is important because it can tell us whether an epidemic is growing or shrinking. With an R-value greater than 1, an epidemic is growing.

And the larger the R-value, the faster the epidemic is growing. Understanding the rate of spread then helps determine what public health measures are needed to control an epidemic. In many countries, a major concern was that the health systems would become overwhelmed by a rapid influx of seriously ill patients.

By reducing the R-value, public health measures slowed the initial wave of infection, preventing hospitals from being overwhelmed. In addition, it gave doctors time to identify the most effective treatments for serious disease.

The closure of public amenities and restrictions on travel reduce the number of contacts between people. And other public health measures, such as social distancing, mask wearing, and good ventilation further reduced opportunities for the virus to transmit between individuals.

These regulations all contributed to a reduction in the R-value and slowed the progress of the COVID-19 pandemic in different countries. A few countries, which had close control of their borders, introduced quarantine for visitors and comprehensive programs of COVID testing.

And they were able to maintain very low levels of infection. But for most countries, waves of epidemic were checked only by implementation of widespread lockdowns. An important additional benefit of the public health measures was that it gave pharmaceutical companies time to develop vaccines and for countries to roll out their vaccination programs.

Vaccination can reduce the rate at which an infection spreads, as there are fewer susceptible people. However, once vaccines were widely available, their main benefit was to protect against serious disease and allow removal of the restrictions on normal life.

End transcript: Video 1 Introduction to Week 6

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Video 1 Introduction to Week 6

By the end of this week, you should be able to:

• define the meaning of R0, RE and RT
• understand the implications of different values of these variables
• outline the assumptions underlying the determinations of these variables.
• calculate the critical immunisation threshold for different diseases.