Vaccine Efficacy Calculator
Calculate vaccine efficacy from clinical trial data: relative risk, 95% confidence interval, NNV, absolute risk reduction, and statistical significance.
Vaccine Immunity Duration Calculator
Estimate how long vaccine immunity lasts: model antibody decay, T-cell memory, and booster timing based on vaccine type, age, and immunological factors.
📊 Educational Tool: Immunity estimates are based on population-level research data. Individual immune responses vary significantly. This is not a diagnostic tool.
years
Estimated Antibody Level⧉
50%
Of peak level (100% adjusted)
Infection Protection⧉
~80%
Estimated protection against any infection
Severe Disease Protection⧉
~87%
T-cell memory sustains protection longer
Booster Status⧉
Not needed yet
~90 days until recommended
T-Cell Memory⧉
94% of peak
Long-lived cellular immunity
Age Factor⧉
100%
Immune response relative to young adults
Immunity Decay Timeline
| Days Post-Vaccination | Est. Antibody Level | Infection Protection | Severe Protection |
|---|---|---|---|
| 14 days (0 mo) | 90% | 95% | 95% |
| 30 days (1 mo) | 79% | 80% | 89% |
| 60 days (2 mo) | 63% | 80% | 88% |
| 90 days (3 mo) | 50% | 80% | 87% |
| 120 days (4 mo) | 40% | 60% | 76% |
| 180 days (6 mo) | 25% | 60% | 75% |
| 270 days (9 mo) | 13% | 40% | 62% |
| 365 days (12 mo) | 6% | 20% | 50% |
| 540 days (18 mo) | 2% | 20% | 46% |
| 730 days (24 mo) | 0% | 20% | 41% |
Antibody Decay Visualization
Peak
100%
30d
79%
90d
50%
180d
25%
365d
6%
730d
0%
Planning notes, formulas, and examples
About the Vaccine Immunity Duration Calculator
Vaccine-induced immunity does not last forever — antibody levels naturally decline over time, and the rate of decline depends on the vaccine type, number of doses, age, and individual immune factors. The Vaccine Immunity Duration Calculator models this antibody decay over time and estimates when protection drops below key thresholds.
Using published immunological data on antibody half-lives for different vaccine platforms (mRNA, viral vector, inactivated, protein subunit, and live attenuated), this calculator projects your estimated antibody level at any point after vaccination. It considers critical factors like age-related immune decline, immunosuppression, and hybrid immunity from prior infection.
Importantly, the calculator distinguishes between antibody-mediated protection (which wanes faster) and T-cell memory (which persists much longer and sustains protection against severe disease). This dual-layer model explains why vaccines continue to prevent hospitalizations and deaths long after their ability to prevent mild infection has declined.
When This Page Helps
Vaccine protection changes over time in different ways for antibodies, immune memory, and severe-disease protection. This calculator keeps those layers visible so booster timing, risk planning, and comparison between vaccine types are based on the same assumptions instead of a vague sense that protection has "worn off".
How to Use the Inputs
- Select the type of vaccine you received.
- Enter how many doses you have received.
- Enter the number of days since your last dose.
- Input your age.
- Indicate whether you are immunocompromised.
- Indicate whether you had a prior infection (hybrid immunity).
- Review estimated antibody levels, protection percentages, and booster recommendations.
Formula used
Antibody Level = Peak Level × (0.5)^(days / half-life)
Peak Level adjusted for: dose count, hybrid immunity, immunosuppression, age
Protection = Mapped from antibody level thresholds
Severe Protection = 0.5 × Infection Protection + 0.5 × T-Cell Memory Factor
T-Cell Memory = (0.5)^(days / cell memory half-life)Example Calculation
Result: ~25% antibody level, ~60% infection protection, ~72% severe disease protection
mRNA antibody half-life ~90 days. After 180 days (2 half-lives), antibodies drop to ~25% of peak. T-cell memory remains strong at ~90% of peak, sustaining severe disease protection.
Tips & Best Practices
- Getting a booster before immunity wanes completely (rather than after) produces a stronger anamnestic response.
- Hybrid immunity (infection + vaccination) provides broader and more durable protection than either alone.
- Immunocompromised individuals should discuss additional or earlier booster doses with their physician.
- T-cell memory provides a safety net even when antibody levels are low — severe disease protection lasts longer.
- For travel to high-risk areas, consider timing a booster 2-4 weeks before departure for peak protection.
- Annual boosters may be appropriate for elderly and immunocompromised individuals, similar to influenza.
The Two Layers of Vaccine Immunity
Vaccine immunity operates through two complementary mechanisms: humoral immunity (antibodies) and cellular immunity (T cells). Antibodies are proteins that circulate in the blood and can neutralize pathogens before they infect cells. They are the first line of defense and primarily prevent infection. T cells, particularly cytotoxic CD8+ T cells, destroy infected cells and are critical for preventing severe disease and death.
After vaccination, antibody levels peak at 2-4 weeks, then begin a biphasic decline: a rapid initial phase as short-lived plasma cells die, followed by a slower decline maintained by long-lived memory B cells and plasma cells in the bone marrow. T-cell memory, in contrast, is established within weeks and can persist for years to decades.
Why Vaccines Prevent Severe Disease Longer Than Infection
The observation that vaccines continue to prevent hospitalizations long after they stop preventing mild infections is explained by the different thresholds required. Preventing infection requires high levels of circulating neutralizing antibodies at mucosal surfaces. Preventing severe disease requires enough immune memory (both antibody and cellular) to mount a rapid response after an infection has already begun. Since the severe disease threshold is lower, protection against severe outcomes persists much longer.
Individual Variation in Immune Responses
Immune responses to vaccination vary enormously between individuals. Factors include age, genetics, nutritional status, stress, sleep, concurrent medications (particularly immunosuppressants), and prior immune experience. Two people receiving the same vaccine on the same day may have 10-fold different antibody levels at one month. This calculator uses population-level averages and should be interpreted as a general guide, not an individual prediction.
Sources & Methodology
Last updated:
Methodology
This worksheet uses dose timing, waning, and booster assumptions to sketch immunity planning context. It is a comparison aid, not a serology result or a substitute for clinical guidance.
Sources
- Immunization schedules and booster guidance (CDC) — Timing and dose-interval context for immunity planning.
- Vaccine recommendations and schedules (WHO) — Reference on dose timing, boosters, and immunization planning.
Frequently Asked Questions
After vaccination, plasma cells produce antibodies at high levels, but many of these cells are short-lived. Over weeks and months, antibody production shifts to long-lived memory B cells that maintain lower but more stable levels.
No. T-cell memory persists much longer than circulating antibodies and provides protection against severe disease. Even with low antibody levels, memory B and T cells can mount a rapid response upon exposure.
Hybrid immunity combines vaccine-induced and infection-induced immunity. Studies show this produces stronger and more durable protection than either alone, with higher peak antibody levels and broader immune memory.
Immune function (immunosenescence) declines with age. Older adults typically produce lower peak antibody levels and their antibodies decline faster, which is why booster doses are more important for elderly populations.
The optimal booster timing depends on your risk profile. Generally, when estimated antibody levels drop below 25% of peak and community transmission is occurring, a booster is recommended. Immunocompromised individuals may need boosters sooner.
Live attenuated vaccines (like MMR) typically provide the longest immunity, often lasting decades. Among newer platforms, protein subunit and mRNA vaccines with boosters show durable responses, though long-term data is still emerging.
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