# Glossary

This website uses publicly available data sources and applies transformations and subject matter expertise in order to transform them into useful information.

## Basic Reproduction Number

The basic reproduction numbers ($$R_0$$) is defined as the average number of secondary cases caused by a single infection in a fully susceptible population . The important point here is that the population must be fully susceptible. We often estimate the $$R_0$$ using serology studies and can observe it indirectly when a new disease enters a population that have never seen said disease.

## Endemic

A disease or infection is said to be endemic if a level of infection is constantly maintained or at a steady state. For instance, the common cold could said to be endemic because a constant level of disease in the community.

## Force of Infection

The force of infection is the rate at which susceptible people (no previous contact with the infection and can be infected) are infected.

## Generation Time

The average time or distribution between two acquired cases. This includes the latent phase when the person may or may not be symptomatic and the period in which the person is infectious .

# Herd Immunity

Herd Immunity is the threshold by which the incidence of new cases will drop and is related to the number of available susceptible hosts and those who have acquired immunity through vaccination or natural infection. The herd immunity threshold is the target for vaccination campaigns which seek to achieve herd immunity through vaccination rather than natural infection. Herd immunity is roughly calculated using the Basic Reproduction Number, $$R_0$$.

$1-\frac{1}{R_0}$

So for example, the measles, which has an $$R_0$$ of about 16 would require that nearly 94% of the population would need immunity through vaccination or natural infection. This is a rough approximation for the herd immunity threshold because there are many factors that contribute to this calculation. For instance is has been shown that spatial heterogeneous can have an influence on the herd immunity threshold . Additionally, not all persons are candidates for vaccination (especially the very young and very old).

## Non-Pharmaceutical Interventions

Non-Pharmaceutical Interventions (NPIs) are interventions to slow or stop the spread of disease that do not include the use of drugs (most commonly vaccination). NPIs can include social and economic lockdown, restricting travel, physical distancing, wearing of personal protective equipment, and other social policies that reduce contact between persons . The primary purpose of these interventions is to slow the spread of disease by reducing the contact rate between persons .1 The aim of these policies is to reduce the spread of the disease such that the infection does not overwhelm the health care system and other interventions such as testing, tracing, isolating and supporting can be more effective .

## Reproduction Number

The reproduction number, $$R$$ or $$R(t)$$, is the average number of secondary cases caused a single infection. The important distinguishing factor between $$R_0$$ and $$R(t)$$ is that the population is not full susceptible and that there can be a time and behavioral impact. For instance, by practicing physical distancing or good hygiene we can lower the reproduction number, $$R$$, but the basic reproduction will remain unchanged.

## Serial Interval

The average time between two reported cases. This measure is what is more likely observed and reported than generation time. The Serial Interval is subject to testing delay and in the case of high asymptomatic transmission can actually be negative (e.g. a secondary case can test positive before the identified index case who may have communicated the disease without having symptoms).

## Serology

Serology or antibody tests look for the presence of specific proteins called antibodies in the blood. The presence of SARS-CoV-2 IgM and IgG in a person’s blood indicate that they had a previous SARS-CoV-2 infection regardless of if they had symptoms. Serology tests cannot tell us when someone had an infection only if they had an infection at some point. Serology tests are useful for understand the Basic Reproduction Number, $$R_0$$ of a virus as well as the force of infection. This is because serology tests capture the portion of the population who may have been asymptomatic or had mild symptoms and were not tested for SARS-CoV-2.

Super-spreaders are index cases (infected individuals) who go on to transmit to a higher expected number of people than would be expected. There is not a precise numerical value but a simple example would be one in which the basic reproduction number is 2 meaning that in a fully susceptible population we would expect the first infection to be based to two others. A super-spreader may instead go on to infect 8 people rather than 2. There is evidence to suggest that SARS-CoV-2 is transmitted by super-spreaders and super-spreading events. It is important to note that people might be super-spreader through no fault of their own . Our behavior, environment, and luck play a strong role in super-spreading. Someone who is asymptomatic and goes to a church choir practice could be deemed a super-spreader .

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1. See for more details on the POLYMOD survey which explored contact patterns in different groups and ages. These data are vitally important in understanding how disease is communicated↩︎

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