Max Vaccine Immunity Calculator

Model how vaccine-induced antibody titers decline over time using first-order exponential decay, and check today's estimated titer against a protective threshold.

Quick Facts

Decay model
First-order exponential
Titer(t) = Peak x e^(-t x ln2 / half-life) - the same kinetics used to model drug and antibody elimination.
Half-life varies
Vaccine- and assay-specific
Published antibody half-lives differ widely by vaccine and antigen, so use a half-life reported for your specific vaccine and assay rather than a generic number.

Your Results

Calculated
Current estimated titer
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Same units as peak titer
Immunity remaining
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Percent of peak titer left
Threshold crossing
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Days until/since threshold
Protection status
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Vs. your threshold input

Ready

Enter your peak titer, half-life, and days elapsed, then calculate to see the current estimated immunity.

About the vaccine immunity (antibody decay) calculator

After vaccination, antibody levels typically rise to a peak and then decline over time. This calculator models that decline with first-order exponential decay, the same kind of kinetics used to describe how a drug clears from the bloodstream, so you can see an estimated titer at any point after the peak and compare it with a protective threshold you specify.

How the calculation works

The tool applies the standard first-order decay equation: Titer(t) = Peak titer x e^(-k x t), where the decay constant k equals ln(2) divided by the antibody half-life. From that curve it also reports the percent of the peak titer remaining and, given the protective threshold you enter, the estimated number of days until (or since) the titer crosses that threshold.

Choosing a half-life and threshold

Antibody half-life after vaccination differs by vaccine platform, antigen, and which antibody or assay is measured, and reported values in the literature span from a few weeks to well over a year. A protective threshold, often called a correlate of protection, is only established for some vaccines and assays, such as a specific titer cutoff in a defined laboratory test. Enter the half-life and threshold that match published data for your vaccine and assay; without vaccine-specific numbers, treat the output as illustrative only.

What this model does not capture

Real antibody decline is often biphasic - a faster drop in the weeks after vaccination followed by a slower, more gradual decline - rather than one constant rate throughout. Antibody titer is also only one arm of immune memory: memory B cells and T cells can still support protection against severe disease after measured antibody levels fall, and this calculator does not model that cellular component. Treat the result as a simplified estimate of antibody kinetics, not a complete picture of immune protection.

When to consult a professional

This tool performs the standard first-order decay arithmetic for education and planning. It does not replace an actual antibody test, does not provide dosing or booster-timing advice, and should not be used alone to decide whether revaccination is needed - discuss your specific situation and any booster schedule with a healthcare provider.

Frequently Asked Questions

What formula does this vaccine immunity calculator use?
It models antibody titer decline with first-order exponential decay, the same kinetics used to describe drug elimination: Titer(t) = Peak titer x e^(-k x t), where k equals ln(2) divided by the half-life. You supply the peak titer, an antibody half-life, and the days elapsed, and the calculator returns the estimated current titer and the percent of peak immunity remaining.
What is a protective titer threshold?
Many vaccines have a correlate of protection: an antibody titer level above which a person is considered likely protected, based on population studies for that vaccine and assay. This calculator compares your estimated current titer against a threshold value you enter, since correlates of protection differ by vaccine and are not established for every disease.
Does antibody titer decay always follow a single exponential curve?
Not exactly. Published data often show a faster initial decline followed by a slower long-term decline, sometimes called a two-phase pattern, and antibody titer is only one part of immune memory alongside memory B cells and T cells. This calculator uses the single-phase exponential approximation for simplicity, so its output is an estimate, not a substitute for an actual antibody test.

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