Immune memory can last nearly 90 years, while some protection fades within a decade

Key takeaways

• In a 2008 study, all 32 people tested who had lived through the 1918 flu pandemic still had antibodies that reacted to that virus nearly 90 years later¹.
• Antibody levels fade at very different speeds: one Oregon study estimated a half-life of 11 years for tetanus but more than 200 years for measles and mumps³.
• Surviving tetanus doesn’t make you immune, because the toxin is so potent, so protection has to come from vaccine doses and boosters⁹.
• Measles can erase immune memories a child already had: in 77 unvaccinated Dutch children, it wiped out 11% to 73% of their antibodies against other germs⁶.
• For COVID-19, a 2023 review found that protection against severe illness held up far longer than protection against catching the virus¹⁴.

Eric Altschuler wanted to know how long the body can remember a germ. The rehabilitation doctor tracked down 32 New Jersey residents who were toddlers or young children during the 1918 flu pandemic, each of whom had lived close to someone sick with it².

He had no lab to test their blood, so the National Institutes of Health put him in touch with immunologists, among them Christopher Basler and James Crowe². Their blood, from people born in 1915 or earlier, was tested nearly 90 years after the pandemic¹.

Every one of the 32 had antibodies that reacted with the 1918 virus¹. From survivors’ blood, the team isolated B cells, the white blood cells that make antibodies, and produced five antibodies that strongly neutralized the virus¹. In mice, those antibodies protected against a lethal 1918 infection¹.

Basler told PBS the results showed that immune responses can last close to an entire human lifetime².

Yet the same immune system lets tetanus protection slide toward the bare minimum within about 10 years⁹. And one virus, measles, can delete memories the body already had⁶.

This guide explains how immune memory works, why it lasts for decades against some germs and only years against others, and what that means for boosters. If you’d rather build immunity the natural way, it also covers what infection teaches the body, and what it can cost.

Memory lives in a few kinds of long-lived cells

The first time your body meets a new germ, its learning defenses need time to respond. Our companion article on immunity across the lifespan explains how that learning changes with age.

What stays behind afterward is immune memory. CDC’s Pink Book, its textbook on vaccine-preventable diseases, describes certain memory B cells that keep circulating in the blood and living in the bone marrow for many years⁸. If the same germ returns, they multiply and pour out antibodies quickly to restore protection⁸.

That memory comes in layers. Some cells keep a steady supply of antibodies in the blood. Other memory B cells wait in reserve.

In a 2007 study from Oregon Health & Science University, the number of memory B cells didn’t track antibody levels for five of the eight vaccine and virus targets tested³. The authors suggested the two groups of cells are regulated on their own, with different jobs³.

T cells, the immune system’s hunters and coordinators, keep memories too. They can fade on a different clock than antibodies, as the smallpox research below shows⁴.

CDC also separates two kinds of immunity. Passive immunity, such as the antibodies a baby borrows from its mother, is temporary⁸. Active immunity, which your own body builds after infection or vaccination, usually lasts many years and often a lifetime⁸.

Some memories last a lifetime, and others fade in a decade

The Oregon team tracked antibody levels in 45 people for up to 26 years³. From those readings, they estimated each antibody’s half-life, the time it takes for a level to drop by half³.

The spread was huge. Tetanus antibodies had a half-life of 11 years and diphtheria 19 years³. Chickenpox virus antibodies had a half-life of about 50 years, and measles and mumps more than 200 years³.

By our math, an 11-year half-life means a tetanus antibody level falls to about a quarter of its starting point in 22 years. With a 200-year half-life, a measles level would still be above three-quarters of its start after 80 years.

In other words, antibodies against the viruses outlasted those against the two vaccine targets, as HealthDay’s summary of the study pointed out³. Tetanus and diphtheria shots target bacterial toxins, not viruses.

Smallpox vaccination showed the same staying power. In a 2003 study of 306 vaccinated volunteers, antibody levels held steady from 1 to 75 years after vaccination⁴⁵. More than 90% of people vaccinated 25 to 75 years earlier still had substantial antibody or T-cell immunity, or both, against vaccinia, the virus in the vaccine⁴.

T-cell memory did fade, slowly, with a half-life of 8 to 15 years⁴. Doses beyond two added no long-term antibody gain⁵.

One caution: these studies measured immune markers, not protection. With natural smallpox gone, researchers may never be able to say for sure which markers signal protection against it⁵.

Protection fades for three different reasons

“Waning immunity” sounds like one problem. It’s really three.

The antibody level drifts down

Tetanus is the clearest case. By 10 years after the last dose, most people’s antitoxin levels only approach the minimal level considered protective, CDC says⁹. That’s why routine boosters come every 10 years⁹.

Whooping cough, or pertussis, fades faster. Today’s shots, DTaP for children and Tdap for teens and adults, use an “acellular” design, developed over safety concerns and linked to fewer side effects than the older whole-cell vaccine¹¹.

Protection from the acellular shots doesn’t hold as long. In a 2018 report from CDC’s vaccine advisers, the Advisory Committee on Immunization Practices (ACIP), vaccine effectiveness, a measure of how much a shot cuts risk, was 98.1% in the first year after the fifth childhood DTaP dose but 71.2% five or more years later¹⁰. For Tdap, one study found effectiveness of 73% in the first year and 34% after two to four years¹⁰.

CDC’s pertussis guidance, updated in December 2025, says immunity after Tdap wanes after a few years¹¹. CDC’s textbook adds that antibody levels drop substantially within a year of a Tdap shot, so a dose in one pregnancy doesn’t cover the next¹¹. Immunity after catching whooping cough isn’t permanent either¹¹.

The germ changes

Immune memory is precise. The 1918 survivors’ antibodies neutralized the 1918 virus and also bound a 1930 swine flu strain, but not more recent human flu viruses¹.

So a strong memory of an older virus may miss a newer one. Seasonal shots follow that logic: as of October 7, 2026, CDC’s pregnancy vaccine page recommends a 2026–2027 COVID-19 vaccine for all adults¹⁵.

Blocking infection fades faster than blocking severe illness

A 2023 review by researchers at the University of Calgary and the World Health Organization looked at protection against the Omicron variant of COVID-19¹⁴. Protection against catching it dropped substantially by 12 months, whether people had been infected, vaccinated or both¹⁴.

But people with “hybrid” immunity, from infection plus vaccination, kept more than 95% protection against hospitalization and severe disease for 12 months¹⁴. The work was funded by WHO’s COVID-19 Solidarity Response Fund and the Coalition for Epidemic Preparedness Innovations¹⁴.

Our reading: this fits how CDC describes memory cells. Antibodies at the front door fade, but reserve cells can still gear up quickly once an infection starts⁸.

Measles can erase memories you already had

In 2013, measles spread through an orthodox Protestant community in the Netherlands⁶. Researchers had blood samples from 77 unvaccinated children, ages 4 to 17, taken before and after their infections⁶.

They used VirScan, a lab tool that tracks tens of thousands of antibodies against proteins from thousands of viruses and other microbes⁶. Two months after measles, the children had lost 11% to 73% of their existing antibody repertoire⁶. The children with the worst measles lost the most⁷.

A comparison group of 32 children who had the MMR vaccine kept their antibodies⁷. In macaque monkeys, more than half of the antibodies were still missing five months after recovery, with no sign of returning⁶.

A second team, writing in Science Immunology, looked at B cells themselves. Measles left the B-cell pool only partly rebuilt and shrank families of memory cells⁶. Colin Russell of the University of Amsterdam compared the effect to resetting the immune system to infancy⁷.

Velislava Petrova of the Wellcome Sanger Institute estimated that rebuilding the lost protection could take about five years. It would likely require meeting those germs again, by infection or vaccination⁷.

The irony is sharp. Measles antibodies are among the most durable known³. The cost lands on everything else the child’s body had learned.

For current measles numbers in the United States and the policy debate, see our article on vaccine schedules.

Infection or vaccination: which teaches better?

Many readers trust the body’s own way of learning, and infection does build immunity. But the answer to “which is better?” depends on the germ.

Live vaccines behave like infection. Live, weakened vaccines must copy themselves in the body to work, and CDC says the response is virtually identical to a natural infection⁸. Yet the measles vaccine didn’t cause the antibody loss seen after measles infection⁶⁷.

Some infections teach nothing. Tetanus toxin is so potent that the disease doesn’t produce immunity⁹. In 2018, ACIP wrote that immunity to tetanus toxin is “rarely if ever acquired naturally”¹⁰.

Some vaccines beat infection. In one study cited by ACIP in 2007, only 54% to 69% of women with new HPV infections had antibodies afterward¹³. Vaccination produced antibody levels higher than infection did¹³. More than 98% of vaccinated people make antibodies to each covered type, with no waning seen through at least 10 to 12 years¹².

Some combine best. For COVID-19, infection plus vaccination protected best¹⁴. The review’s first author, Niklas Bobrovitz, still warned that no one should try to get infected, since even mild cases can lead to long COVID¹⁴.

Infection’s price can be steep. Tetanus has been fatal in about 11% of recent U.S. cases⁹. Measles can strip away years of other protection⁶.

How long protection lasts: a germ-by-germ guide

This table is our own summary of the studies and CDC pages cited. Use it to frame questions, not as a schedule. For who recommends which doses as of 2026, see our vaccine schedule article.

Germ	Memory after infection	Memory after vaccination	How it fades, and boosters
Measles	Very durable; antibody half-life over 200 years in one study3	Live vaccine response is virtually identical to infection8	Infection, not vaccination, erased 11% to 73% of other antibodies67
Tetanus	None, even after surviving the disease9	Antitoxin starts above the protective level of 0.1 IU/mL, then drifts down9	Nears the minimum by about 10 years; boosters every 10 years, sooner for some wounds9
Whooping cough	Not permanent11	DTaP 98.1% effective in the first year after the fifth dose10	Down to 71.2% after 5 years; Tdap fell from 73% to 34% in one study10; Tdap in each pregnancy11
HPV	As few as 60% of infected people make detectable antibodies13	Over 98% make antibodies, at higher levels than infection1213	No waning seen through at least 10 to 12 years12
Smallpox	Disease no longer exists	Antibodies stable for up to 75 years; T cells halve every 8 to 15 years4	Beyond two doses, no lasting antibody gain5
Flu	1918 survivors still had antibodies to that virus nearly 90 years later1	Not measured in that study	Those antibodies missed later human flu strains1
COVID-19	Protection against reinfection dropped substantially by 12 months14	Protection against infection dropped the same way14	Infection plus vaccination kept more than 95% protection against severe disease for 12 months14; CDC lists a 2026–2027 shot for adults15

How strong is the evidence?

Claim	Evidence level	Basis
Memory against some germs can last most of a lifetime	Strong evidence	1918 survivors, smallpox vaccine recipients and long antibody half-lives134
Tetanus and whooping cough protection wanes, so boosters matter	Strong evidence	CDC and ACIP data910
Measles infection erases part of a child’s existing antibody memory	Strong evidence	Before-and-after blood tests in 77 children, a second lab method and monkey studies67
Rebuilding the memory measles erases takes about five years	Promising but early	One research team’s estimate, not yet confirmed7
Infection always gives better immunity than vaccination	Evidence says no, or unsafe	Tetanus infection gives none; HPV vaccine makes more antibody than infection913
Infection plus vaccination guards best against severe COVID-19	Strong evidence	2023 review by University of Calgary and WHO researchers14

Boosters: why they exist and how to weigh them

Inactivated vaccines, made from killed germs or pieces of them, usually don’t protect after one dose. CDC says the first dose usually primes the immune system, and protection develops after the second or third⁸. Their response is mostly antibodies, which fall over time, so some need boosters⁸.

A booster isn’t a sign that your body failed. It’s a reminder for a memory that naturally fades, much as the tetanus numbers show. Prevention, self-reliance and respect for the body’s own defenses all fit here: the memory a vaccine leaves is made by your own cells.

Vaccine advice is also in flux. Our article “How to read competing vaccine schedules now that the advice has split” explains the 2025 and 2026 changes to the federal advisory committee, the court case that followed and who recommends what now.

Questions to bring to a clinician

1. Which germs am I protected against now, and how do we know: records, past illness or a blood test?
2. For this booster, how much does protection drop without it, in numbers?
3. Is an antibody test meaningful for this germ? For HPV, CDC says no antibody level is known to mark protection¹².
4. Does my age, pregnancy or health change the timing?
5. What are the common side effects, and the rare ones to watch for?

When to get help

• After a dirty, puncture, crush or burn wound, ask about a tetanus shot if your last dose was five or more years ago. For a clean, minor wound, the line is 10 years⁹.
• If you’ve had fewer than three tetanus doses, or don’t know, ask about a shot for any wound. Dirtier wounds may also need tetanus immune globulin, a dose of ready-made antibodies⁹.
• Lockjaw, then a stiff neck, trouble swallowing and rigid belly muscles are the first signs of tetanus⁹. CDC’s textbook notes it has been fatal in about 11% of recent U.S. cases, and that keeping the airway open during spasms is critical⁹. For when to call 911, see “The symptoms that mean call 911 now.”

Questions readers ask

The bottom line

Your immune memory can be astonishingly durable. Survivors of the 1918 flu still carried antibodies to it nearly 90 years later, and smallpox vaccine antibodies held steady for 75 years. But memory doesn’t last equally for every germ. Tetanus and whooping cough protection drifts down, flu and COVID-19 viruses change, and measles can erase memories you already had.

Boosters exist to cover those gaps, and the case for each one is specific to the germ. Bring the questions above to a clinician you trust, ask for numbers, and make the choice that fits your health.

Keep exploring: immune system and aging.

Related reading

• Babies borrow their mothers’ antibodies, and older adults’ infections may bring no fever
• How to read competing vaccine schedules now that the advice has split
• Your immune system works more like a team than a volume knob, and some habits help it
• What a clinician must tell you before you agree
• The symptoms that mean call 911 now

Sources

1. Yu X, Tsibane T, McGraw PA, et al., Basler CF, Crowe JE. “Neutralizing Antibodies Derived From the B Cells of 1918 Influenza Pandemic Survivors.” Nature, 2008;455(7212):532–536 (University of Texas Medical Branch research record). researchexperts.utmb.edu
2. PBS NewsHour. “90 Years Later, 1918 Flu Lives on in Antibodies, Research” (on Yu X, et al., Nature, 2008). PBS, August 22, 2008. pbs.org
3. HealthDay. “Humoral Immunity Has Long Duration to Viral Antigens” (on Amanna IJ, Carlson NE, Slifka MK, “Duration of Humoral Immunity to Common Viral and Vaccine Antigens,” New England Journal of Medicine, 2007;357:1903–1915). November 7, 2007. healthday.com
4. Hammarlund E, Lewis MW, Hansen SG, et al. “Duration of Antiviral Immunity After Smallpox Vaccination.” Nature Medicine, 2003;9:1131–1137. nature.com
5. Center for Infectious Disease Research and Policy (CIDRAP), University of Minnesota. “Protective Effect of Smallpox Vaccine May Last Decades.” August 2003. cidrap.umn.edu
6. American Association for the Advancement of Science. “In Unvaccinated Children, Measles Cripples Immune Protection” (on Mina MJ, et al., Science, 2019, and Petrova VN, et al., Science Immunology, 2019). AAAS, October 31, 2019. aaas.org
7. PBS NOVA. “How Measles Virus Triggers Immune System ‘Amnesia.'” PBS, 2019. pbs.org
8. Centers for Disease Control and Prevention. “Chapter 1: Principles of Vaccination.” Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book), 14th ed., page reviewed March 22, 2024. cdc.gov
9. Centers for Disease Control and Prevention. “Chapter 21: Tetanus.” Pink Book, updated April 25, 2024. cdc.gov
10. Liang JL, Tiwari T, Moro P, et al. “Prevention of Pertussis, Tetanus, and Diphtheria with Vaccines in the United States: Recommendations of the Advisory Committee on Immunization Practices (ACIP).” MMWR Recommendations and Reports, 2018;67(2), April 27, 2018. cdc.gov
11. Centers for Disease Control and Prevention. “Chapter 16: Pertussis.” Pink Book, updated April 12, 2024, and “Pertussis Vaccination Recommendations,” updated December 2, 2025. cdc.gov and cdc.gov
12. Centers for Disease Control and Prevention. “Chapter 11: Human Papillomavirus.” Pink Book, updated April 11, 2024. cdc.gov
13. Markowitz LE, Dunne EF, Saraiya M, et al. “Quadrivalent Human Papillomavirus Vaccine: Recommendations of the Advisory Committee on Immunization Practices (ACIP).” MMWR Recommendations and Reports, 2007;56(RR-2), March 23, 2007. cdc.gov
14. University of Calgary. “UCalgary and WHO Researchers Find Hybrid Immunity Best Protection Against COVID-19” (on Bobrovitz N, et al., Lancet Infectious Diseases, 2023). January 20, 2023. ucalgary.ca
15. Centers for Disease Control and Prevention. “Vaccine Recommendations Before, During, and After Pregnancy.” CDC, updated October 7, 2026. cdc.gov