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Originally Posted by caned & able  |
Yah, I don't know that anyone denies that the possibility of waning immunity is a meaningful and understudied public health issue. (A bit more on the very same
here.)
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Originally Posted by caned & able American Journal of Epidemiology (1984 – Vol. 120, No. 1: 39-48)
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| “A computer model was created to study the effect of the measles elimination program in the United States on the number of susceptibles in the population. The simulation reveals that in the prevaccine era, approximately 10.6% of the population was susceptible to measles, most of whom were children less than 10 years of age. With the institution of the measles immunlzation program, the proportion of susceptibles in the population fell to 3.1% from 1978 through 1981, but then began to rise by approximately 0.1% per year to reach about 10.9% in the year 2050. The susceptibies at this time were distributed evenly throughout all age groups. The model did not consider the potential effect of waning immunity.” |
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OK. Presumably a standard SIR model, but we don't know any of the details or their caveats. As it explicitly
doesn't consider waning immunity, what one is likely seeing is just a combination of (1) an accumulation of population in which the vaccine didn't "take" in the first place and who nonetheless were never infected and (2) less circulating disease overall. The result isn't qualitatively surprising.
Anyway, just to finish off the abstract,
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| The results of this study suggest that measles elimination in the United States has been achieved by an effective immunization program aimed at young susceptibles combined with a highly, naturally immunized adult population. However, despite short-term success in eliminating the disease, long-range projections demonstrate that the proportion of susceptibles in the year 2050 may be greater than in the prevaccine era. Present vaccine technology and public health policy must be altered to deal with this eventuality. |
Of course, public health policy
has been altered since the publication of this computational study 26 years ago, with the introduction of a second measles dose. Assuming it does what it's intended to do, this would be an additional input to the model, changing the quantitative results. How much, we don't know. [A more modern example of modeling the accumulation of susceptibles can be found
here (PDF).]
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Originally Posted by caned & able |
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| Although the titer of vaccine-induced antibodies is lower than that following natural disease, both serologic and epidemiologic evidence indicate that vaccine-induced immunity appears to be long-term and probably lifelong in most persons. Most vaccinated persons who appear to lose antibody show an anamnestic immune response upon revaccination, indicating that they are probably still immune. Although revaccination can increase antibody titer in some persons, available data indicate that the increased titer may not be sustained. Some studies indicate that secondary vaccine failure (waning immunity) may occur after successful vaccination, but this appears to occur rarely and to play only a minor role in measles transmission and outbreaks. |
This, on the other hand, tends to undercut the severity of the issue, especially if titer thresholds are an imprecise long-term proxy for immunity.
The upshot would seem to be something that was known in the first place, that measles is hard to eradicate. The contribution to this difficulty from waning immunity is still every bit as unclear as the actual extent of the waning itself. Indeed, the consequences of a mostly adult, 10.9% measles-susceptible population independent of any such effect, as suggested by Levy's model, aren't exactly obvious. (It can't continue to increase linearly forever. Is there actually a steady state solution? Does it turn over and oscillate? If so, how, and on what timescale? Etc.)
