I've come to the conclusion that, now that we've started vaccinating for varicella, we have to continue, even if the vaccine itself isn't that good as vaccines go.
The reason: the first effect of a vaccine is not to eliminate the disease from the population, but to raise the average age of infection into the ranks of those not yet vaccinated. So if the average age of infection before the vaccine was five or six, five years after the introduction of the vaccine, the average age will have risen by seven or eight years (depending on the prevalence of the disease in the population, it will go up higher than just the age difference, because there will be fewer vectors of infection running around.) If vaccination continues so that most babies are vaccinated, then the average age of infection factored into the decreased incidence of the disease effectively eliminates the disease for all but a few people, possibly all of them, and herd immunity is achieved.
The problem comes if vaccine uptake is incomplete. A study was done on the vaccine for rubella in Greece, where it was not mandatory and uptake was slow during the eighties. The study found that, when only 50% of the population was vaccinated, the effect was to keep just enough disease vectors in the population that the average age of infection settled on the early twenties. Rubella is most dangerous to pregnant women and their babies, so there was rash of brain-damaged babies due to measles infection in Greece throughout the decade.
This ceases to be a problem if the people who refuse to vaccinate their babies against varicella are willing to either ensure that they get chicken pox naturally, or get them the vaccine later if they don't catch it by a certain age. But the result of incomplete immunity would end up being worse than no vaccine at all.
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Date: 2010-11-21 09:29 pm (UTC)The reason: the first effect of a vaccine is not to eliminate the disease from the population, but to raise the average age of infection into the ranks of those not yet vaccinated. So if the average age of infection before the vaccine was five or six, five years after the introduction of the vaccine, the average age will have risen by seven or eight years (depending on the prevalence of the disease in the population, it will go up higher than just the age difference, because there will be fewer vectors of infection running around.) If vaccination continues so that most babies are vaccinated, then the average age of infection factored into the decreased incidence of the disease effectively eliminates the disease for all but a few people, possibly all of them, and herd immunity is achieved.
The problem comes if vaccine uptake is incomplete. A study was done on the vaccine for rubella in Greece, where it was not mandatory and uptake was slow during the eighties. The study found that, when only 50% of the population was vaccinated, the effect was to keep just enough disease vectors in the population that the average age of infection settled on the early twenties. Rubella is most dangerous to pregnant women and their babies, so there was rash of brain-damaged babies due to measles infection in Greece throughout the decade.
This ceases to be a problem if the people who refuse to vaccinate their babies against varicella are willing to either ensure that they get chicken pox naturally, or get them the vaccine later if they don't catch it by a certain age. But the result of incomplete immunity would end up being worse than no vaccine at all.