What is the herd immunity threshold for COVID-19?

For the original SARS-CoV-2 strain (R₀ ≈ 2.5), HIT ≈ 60%. For the Delta variant (R₀ ≈ 5–7), HIT ≈ 80–86%. For Omicron (R₀ ≈ 15–20+), HIT may exceed 90–95%, making vaccine-only herd immunity extremely challenging.

Does herd immunity require everyone to be vaccinated?

No. HIT is the threshold fraction immune, whether through vaccination or prior infection. In heterogeneous populations with age-structured mixing, the effective HIT can be lower if immunity is concentrated in highly connected groups.

Herd Immunity Threshold Calculator

Calculate the herd immunity threshold HIT = 1 – 1/R₀. Once the fraction immune exceeds HIT, sustained transmission cannot occur. For measles (R₀ ≈ 15), HIT is about 93%; for COVID-19 variants, 70–90%.

Data set — comma or newline separated

Statistical Results — n = 8

Count (n)8

Sum (Σx)40

Min2

Max9

Range7

Q14

Q35.5

IQR1.5

Mean (μ)5

Median4.5

Mode4

Pop. Variance (σ²)4

Pop. Std Dev (σ)2

Sample Variance (s²)4.571428571

Sample Std Dev (s)2.138089935

Skewness0.65625

Excess Kurtosis-0.21875

How to Use

Enter the values for the scenario you are assessing. Calculate the herd immunity threshold HIT = 1 – 1/R₀. Once the fraction immune exceeds HIT, sustained transmission cannot occur. For measles (R₀ ≈ 15), HIT is about 93%; for COVID-19 variants, 70–90%. Use the herd immunity threshold result to inform your calculation.

Herd Immunity Threshold Calculator

How to Use

Frequently Asked Questions

What is the herd immunity threshold for COVID-19?

Does herd immunity require everyone to be vaccinated?