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What is the Herd Immunity Threshold (HIT)?

The herd immunity threshold (HIT) is a hypothetical fraction of a population. When a fraction of the population greater than HIT has immunity, a virus (or other infectious disease) will theoretically stop spreading through the population. HIT is a theoretical threshold and can not be known with exact certainty. This threshold will change depending on many factors. One factor is seasonal effects of summer and winter in temperate climates. This will change the HIT for respiratory viruses like flu and coronavirus. Another factor is how much a population facilitates viral transmission (e.g. not social distancing, hand shaking, hand washing, etc...).

Why does this site report herd immunity threshold (HIT) in terms of deaths?

There is notable uncertainty around

  1. the herd immunity threshold (HIT) for COVID-19, and
  2. the total number of infections (asymptomatic or symptomatic, confirmed or unconfirmed)

Due to these uncertainties, HIT is reported in terms of deaths on this website. This page outlines the calculations, evidence and questions relating to the HIT levels reported in terms of total deaths.

See ratio of deaths to infections for more details.

How is the Herd Immunity Threshold (HIT) calculated?

A simple calculation of HIT is (1 - 1/R) where R is an estimate of the basic reproduction number. More sophisticated models of the population can raise [8] or lower [7], [6] the threshold.

Aren't there really more COVID-19 deaths that are reported?

Although not known with perfect certainty, the total number of deaths can be estimated. These web pages and calculation use the daily death counts reported quickly by health authorities worldwide. The actual number of deaths due to COVID-19 is almost certainly higher with slower more careful analysis.

Opinion: Actual deaths due to COVID-19 will likely be somewhere around 50% higher after slower and more careful analysis. Maybe even twice as high.

Attributing deaths to causes is not simple. Death can have multiple causes. For an excellent discussion on counting COVID-19 deaths read What is counted as a death from COVID-19?

Will we get immunity to COVID-19?

Immunity for individuals often decreases over time. Immunity is short (~1 year) for mild coronaviruses OC43 and 229E (~1 year), but longer lasting (more than 3 years) for severe coronaviruses such as SARS-CoV-1 [1] (the ancestor of SARS-CoV-2 ). Due to births and decreasing immunity, the herd immunity threshold does not imply that a virus will stop spreading permanently and will die out.

Opinion: The vast majority of individuals with symptomatic COVID-19 will have immunity until a vaccine is found.

Sources