Protective Measles Immunity Cannot Be Reduced to a Cutoff in an Immunological Readout

Protective Measles Immunity Cannot Be Reduced to a Cutoff in an Immunological Readout

Why predictions of a coming crisis of measles susceptibility in the vaccinated elderly overlook the most important variable: herd immunity

Geert Vanden Bossche

Aug 29, 2026

Take-home message:

In highly vaccinated populations, protection against measles of more vulnerable individuals, such as the elderly, is not determined solely by whether immunological readouts exceed predefined minimum thresholds.

A lack of understanding of herd immunity leads some scientists to assume that immunological correlates of protection (CoPs) against measles must take the form of predefined minimum thresholds.

At first glance, Robert Malone’s recent article, “Measles Immunity: No Twice-Vaccinated Generation Has Reached Sixty-Five”, appears impressively scientific.

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It contains historical timelines, immunological terminology, references to antibody (Ab) waning, discussion of CoPs and citations to peer-reviewed literature. Its central observation is also factually correct: no generation vaccinated twice against measles in childhood has yet reached conventional old age.

On closer examination, however, the argument is fundamentally weak.

The problem is not the observation that vaccine-induced measles Ab concentrations may decline with time. They do.

The problem is the inference that this necessarily creates a presently unknowable ─ and potentially dangerous ─ future susceptibility among elderly vaccinees.

That reasoning treats protection against measles largely as an individual serological property while insufficiently accounting for the epidemiological environment in which that individual lives.

For a highly contagious infection such as measles, the probability of exposure is itself one of the dominant determinants of disease risk.
Maintaining sufficiently high population immunity dramatically reduces circulation of the virus and therefore protects individuals whose own immunity may be incomplete, waning or impossible to induce.

That is precisely what herd immunity is for.

And it fundamentally changes the interpretation of declining Ab concentrations.

The first problem: chronological cohorts are not immunological compartments

Malone writes that essentially every American above a certain age was naturally infected, whereas younger generations were vaccinated and therefore probably never encountered wild measles virus.

That creates an appealingly simple division: the naturally immune elderly on one side and a progressively aging vaccine-dependent population on the other.

Reality is less tidy.

Before measles vaccination, virtually all children did indeed acquire measles. CDC historical data indicate that, in the decade preceding introduction of vaccination, nearly all children had contracted measles by age 15. For precisely that reason, birth before 1957 remains accepted as presumptive evidence of immunity in the United States. But presumptive is not synonymous with universal: even CDC guidance recognizes exceptions. [1,2]

More importantly, the opposite assumption ─ that people born and vaccinated in the vaccine era were immunologically insulated from wild-type measles ─ is equally problematic. Measles was not declared eliminated from the United States until 2000. Consequently, vaccinated individuals who lived during the decades preceding elimination could still have been exposed to circulating wild-type measles virus, potentially boosting pre-existing vaccine-induced immunity without necessarily developing clinically recognized measles. This possibility is explicitly acknowledged in a recent New England Journal of Medicine review, which notes natural boosting among persons vaccinated during the pre-elimination era. Although that review specifically discusses the 1967–1989 cohort because the routine two-dose schedule was not introduced until 1989, the biological opportunity for exposure and immune boosting did not suddenly disappear in 1989; it progressively diminished as transmission was suppressed and was only considered interrupted nationally when measles was declared eliminated in 2000. [3]

Thus, vaccination year or birth year cannot simply be translated into an immunological history, and Malone’s implicit separation between naturally exposed older generations and younger vaccinees who supposedly never encountered wild virus is much too categorical.

Antibody waning is real but Malone overinterprets what it means

The strongest evidence cited for waning vaccine-induced measles immunity comes from Bolotin and colleagues’ systematic review and meta-analysis. [4]

That paper indeed found declining geometric mean Ab concentrations after two doses of measles-containing vaccine. This observation deserves attention. But the remainder of their findings is at least as important:

During the available 20 years of observation, average Ab concentrations did not fall below the commonly used 120 mIU/mL threshold and the proportion of seropositive individuals did not significantly decline after either one or two vaccine doses. [4]

The authors extrapolated that mean concentrations might cross 120 mIU/mL approximately 51 years after the second dose. But the uncertainty surrounding that extrapolation is enormous: its confidence limits extended approximately from 31 to 137 years, and the authors themselves explicitly cautioned that data beyond 20 years were sparse. [4]

However, an extrapolation is not an observation and, more fundamentally, declining circulating Ab concentrations cannot simply be equated with proportional loss of clinical protection.

The immune system does not consist of a serum Ab concentration frozen at the moment a blood sample is taken. Memory B cells and T help-assisted Ab recall responses can contribute to protection following re-exposure. Reviews of measles immunity have specifically noted that individuals with low or even undetectable neutralizing Ab levels may nevertheless remain protected from clinical measles, consistent with contributions from immune mechanisms beyond immediately measurable circulating Ab. [5]

Indeed, CDC’s review of measles vaccine immunity states that most vaccinated individuals who apparently lose measurable Ab display an anamnestic response when revaccinated, suggesting persistence of immunological memory. This does not prove that waning Ab titers are irrelevant. They clearly are not. [2]

It does mean that serological waning and loss of protection are not interchangeable concepts.

The ‘protective threshold’ is a correlate, not an immunological cliff

Malone correctly points out that the frequently cited measles neutralizing-Ab threshold of approximately 120 mIU/mL rests on a surprisingly small evidence base. [7]

The original Chen et al. study is particularly striking. Among nine subjects with pre-exposure plaque-reduction neutralization titers at or below 120, eight subsequently fulfilled the clinical measles case definition, whereas none of 71 subjects above that level did. A later systematic review by Bolotin and colleagues concluded that the available evidence supporting 120 mIU/mL as a universal CoP is indeed scant. Only five studies fulfilled their criteria, laboratory methodology varied, and some clinically infected individuals had pre-exposure concentrations above the nominal threshold. [6,7]

But this observation does not support the conclusion Malone appears to draw from it.

The fact that an exact threshold is imperfectly defined does not mean we know almost nothing about protection. For measles, high titers of virus-neutralizing Abs remain the strongest established CoP against measles. [5,7]

However, the evidence tells us that a CoP is not necessarily an absolute binary cutoff separating ‘protected’ from ‘susceptible’ individuals under every epidemiological circumstance.

That distinction becomes particularly important for measles.

Whether an individual is effectively susceptible cannot be inferred from predefined cutoffs of immunological readouts alone. It also depends on whether exposure occurs, its duration and intensity and on the immunological memory available to respond following exposure.

CDC specifically notes that breakthrough measles is more likely after prolonged or intense exposure ─ even among fully vaccinated persons. Conversely, vaccinated breakthrough cases tend to develop milder disease and appear less likely to transmit infection than immunologically naïve cases. [8]

This is why trying to infer future population disease burden from a predefined threshold of one or more immunological readouts alone is conceptually inadequate.

The missing variable: herd immunity

This is the central weakness of Malone’s analysis.

For measles, individual susceptibility cannot sensibly be discussed independently of population immunity.

Measles is extraordinarily contagious. Consequently, maintaining elimination requires extremely high immunity coverage, commonly approximated at around 95% for a two-dose vaccination program. When that level is maintained relatively uniformly, sustained transmission becomes difficult. [9]

This creates indirect protection for individuals whose immunity is inherently weak, incomplete or declining. This is to say that an elderly vaccinated person whose circulating neutralizing-Ab concentration has declined does not face the same risk when living in a population with sustained >95% effective immunity as that same person would inside a dense cluster of susceptible individuals during an outbreak.

The serum concentration could be identical. The disease risk is not.

This seemingly simple epidemiological fact profoundly changes the interpretation of Ab waning.

It also explains why herd immunity does not require every individual to possess some uniformly high Ab concentration. A vulnerable individual can remain free of measles despite weak or even undetectable circulating Ab simply because sufficiently high immunity in the surrounding population prevents infectious virus from reaching that person. [5]

That is not immunological protection in the strict individual sense. It is epidemiological protection created by interruption of transmission.

For vulnerable people ─ the immunocompromised, infants who cannot yet be vaccinated and potentially older adults with diminished immune competence ─ that distinction can be lifesaving.

What the recent modelling study actually tells us

The 2024 analysis by Robert, Suffel and Kucharski is particularly informative because it explicitly addresses waning in the context of transmission dynamics rather than treating Ab concentrations in isolation. [10]

Their modelling of measles in England found evidence consistent with slow waning of vaccine-derived protection. Breakthrough infections became relatively more frequent among twice-vaccinated individuals aged 15 years or older. But the estimated waning rate was approximately 0.039% per year in their preferred model. [10]

More importantly, the authors concluded that the vaccine remained highly protective for decades and that most transmission remained connected to unvaccinated individuals. [10]

Their estimate that vaccinated cases generated approximately 83% as many onward transmissions as unvaccinated cases is sometimes interpreted alarmingly. But that number describes transmission conditional on having become a case. It should not be confused with the probability that a vaccinated person will become a case in the first place, which remains dramatically lower. [10]

This distinction is crucial!

A vaccine that strongly reduces the probability of infection while only moderately reducing infectiousness among the comparatively rare breakthrough cases can still have an enormous effect on population transmission. That is precisely how vaccination contributes to herd immunity.

The same modelling study therefore supports two propositions simultaneously:

1. secondary vaccine failure exists and slowly increases with time; and [10,11]

2. maintaining high vaccination coverage remains highly effective in suppressing measles transmission. [10]

There is no contradiction between them.

Where the real danger lies

The epidemiological danger is therefore not principally that the first twice-vaccinated generation will one day celebrate its 65th birthday.

The danger is allowing effective population-level immunity to deteriorate.

Measles outbreaks in elimination settings remain concentrated disproportionately in unvaccinated and undervaccinated populations and particularly in geographically or socially clustered susceptible communities. CDC surveillance explicitly identifies these pockets of under-vaccination as a major determinant of post-elimination outbreaks. [9]

Waning immunity becomes epidemiologically important mainly when measles is introduced into populations in which vaccine coverage has declined, substantial cohorts of older adolescents or adults have partially waned vaccine-induced immunity or significant numbers of individuals have impaired immune competence because of underlying disease or immunosuppression. [4,9,10]

This matters because herd immunity is not simply a national percentage. A country may report apparently reassuring average vaccination coverage while containing schools, religious communities, neighborhoods or social networks in which effective immunity is far below the level required to interrupt measles transmission. Once virus enters such a cluster, the infectious pressure rises dramatically.

It is under those conditions that partially waned immunity in previously vaccinated adults becomes much more relevant.

In other words:

Waning immunity increases the importance of herd immunity; it does not make herd immunity obsolete.

This is also why routine childhood measles vaccination remains enormously important for protecting people who cannot rely exclusively upon their own immunity.

At the individual level, the two-dose MMR schedule is estimated to be approximately 97% effective against measles, but this figure should not be interpreted as evidence that the same degree of protection is maintained indefinitely after vaccination. [2,9]

What should actually be studied?

There is certainly room for better research.

Long-term longitudinal studies of vaccine-derived immunity would be scientifically valuable. Improved characterization of neutralizing Ab, memory B-cell responses, T-cell immunity and age-related immune decline could refine our understanding of secondary vaccine failure. [5,11]

Likewise, prospective observational studies need not ─ and ethically obviously should not ─ deliberately expose individuals to measles! Carefully designed serological cohorts linked to naturally occurring exposure and outbreak surveillance could provide useful information.

But such research should not begin from the assumption that a universal immunological number will somehow predict whether tomorrow’s elderly population is safe.

The relevant outcome emerges from an interaction between at least three variables:

individual immune status# ─ exposure intensity# ─ population immunity#

Ignoring the latter two risks generating an exquisitely measured biomarker that poorly predicts the phenomenon we actually care about: clinical disease.

The wrong public-health conclusion

Malone goes on to suggest that public health has effectively allowed an assumption ─ that childhood vaccination protects throughout life ─ to harden into an article of faith before the first fully twice-vaccinated generation reaches advanced age. The title of Malone’s article, “No Twice-Vaccinated Generation Has Reached Sixty-Five,” suggests that the absence of direct observations in elderly twice-vaccinated cohorts leaves their long-term protection as an important unresolved vulnerability.
That is rhetorically powerful but scientifically misleading.
The effectiveness of current measles-control strategies does not depend on every twice-vaccinated individual maintaining lifelong sterilizing immunity. No serious immunologist needs such an absolute claim to justify the current strategy. Even if vaccine-induced immunity partially wanes in some individuals, protection against disease may persist through immunological memory while high population immunity substantially reduces the probability that susceptible or partially susceptible individuals will be exposed in the first place. This already explains why vaccine breakthrough disease remain uncommon despite waning vaccine-induced immunity.

The most rational response to uncertainty about waning immunity is therefore not to frighten middle-aged vaccinated adults with the prospect of an approaching immunological cliff at age 65.
It is to preserve high, homogeneous population immunity against measles, as this is the ecological condition that keeps vulnerable individuals safe.
That means maintaining routine childhood immunization, identifying genuine immunity gaps, preventing accumulation and clustering of susceptible individuals while maintaining excellent outbreak surveillance.

If measles transmission is efficiently interrupted, an elderly person’s declining Ab concentration becomes much less consequential because the virus has difficulty reaching that person in the first place.

That is the central principle missing from Malone’s analysis. And for measles ─ one of the most transmissible human viruses known ─ this is not a peripheral detail. It is the heart of measles control!

References

1. Centers for Disease Control and Prevention. History of Measles.

2. Centers for Disease Control and Prevention. Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book), Chapter 13: Measles.

3. Paules CI, Marston HD, Fauci AS. Measles 2025. N Engl J Med.

4. Bolotin S, et al. In Elimination Settings, Measles Antibodies Wane After Vaccination but Not After Infection: A Systematic Review and Meta-Analysis. J Infect Dis. 2022;226:1127–1139.

5. Haralambieva IH, et al. Current perspectives in assessing humoral immunity after measles vaccination. Expert Rev Vaccines. 2019.

6. Chen RT, et al. Measles Antibody: Reevaluation of Protective Titers. J Infect Dis. 1990;162:1036–1042.

7. Bolotin S, et al. What Is the Evidence to Support a Correlate of Protection for Measles? A Systematic Review. J Infect Dis. 2020;221:1576–1583.

8. Centers for Disease Control and Prevention. Clinical Questions about Measles.

9. Centers for Disease Control and Prevention. Manual for the Surveillance of Vaccine-Preventable Diseases, Chapter 7: Measles.

10. Robert A, Suffel AM, Kucharski AJ. Long-term waning of vaccine-induced immunity to measles in England: a mathematical modelling study. Lancet Public Health. 2024;9:e766–e775. doi:10.1016/S2468-2667(24)00181-6.

11. Zhang Z, et al. Outbreak of measles among persons with secondary vaccine failure, China, 2018. Hum Vaccin Immunother. 2020;16:358–362.

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