# COVID-19 herd immunity: where are we?
###### tags: `articles`

*[Fontanet A, Cauchemez S. COVID-19 herd immunity: where are we? Nat Rev Immunol 2020; 20: 583–84](https://scholar.google.se/scholar?hl=en&as_sdt=0%2C5&q=COVID-19+herd+immunity%3A+where+are+we%3F&btnG=)*

## Herd Immunity Threshold

- effective reproduction number $R=(1-p_C)(1-p_I)R_0$
- $R<1$ for herd immunity
- $p_C$ is relative reduction in transmission rates due to non-pharmaceutical interventions
- $p_I$ is the proportion of immune individuals
- $R_0$ is the reproduction number in the absence of control measures in a fully susceptible population
- herd immunity threshold $p_I=1-1/R_0$
- $2.5$ to $4$ for COVID-19
- $3$ for France
- herd immunity threshold requires $67$%
- drops to $62.5$% in age-specific contact pattern
- *[Britton, T., Ball, F. & Trapman, P. A mathematical model reveals
the influence of population heterogeneity on herd immunity to
SARS-CoV-2. Science 369, 846–849 (2020)](https://scholar.google.se/scholar?hl=en&as_sdt=0%2C5&q=A+mathematical+model+reveals++the+influence+of+population+heterogeneity+on+herd+immunity+to++SARS-CoV-2&btnG=)*
- drops to $50$% if we further assume the number of contacts varies substantially between individuals within the same age group
- may drop more since children are less suspectable
- *[Goldstein, E., Lipsitch, M. & Cevik, M. On the effect of age
on the transmission of SARS-CoV-2 in households, schools and
the community](https://scholar.google.se/scholar?hl=en&as_sdt=0%2C5&q=On+the+effect+of+age++on+the+transmission+of+SARS-CoV-2+in+households%2C+schools+and++the+community&btnG=)*
- even lower for some serological reason

## How to Achieve 50% Immunity

### Death

- re-infection
- not clear
- flu pandemics
- takes two to three waves
- COVID-19
- infection fatality ratio $0.3$%-$1.3$%
- $100,000–450,000$ to $500,000–2,100,000$ deaths for France and USA

### Vaccine

- six anti-SARS-CoV-2 vaccines have reached phase III trials
- prioritize highly exposed populations and those at risk of severe morbidity
- non-pharmaceutical intervention should be strictly enforced