One of the newest vaccines on the CDC schedule is the varicella vaccine, which protects against chickenpox. Introduced to the schedule in 1995, the chickenpox vaccine is one of those that parents may be more inclined to question, especially if they had chickenpox themselves. I’ve heard the following concerns or questions:
- “I had chickenpox, and I’m fine, so is a vaccine really necessary?”
- “Aren’t the complications very rare and not serious?”
- “It’s really new, so has it been out long enough to know that it’s safe?”
- “If my child gets the vaccine and it wears off, aren’t they at higher risk for getting the chickenpox as adults, when it’s more dangerous?”
All of these are valid questions or concerns if the parents haven’t researched these questions thoroughly. The first is a classic example of survivorship bias, and, along with the second, shows lack of knowledge about how serious – even fatal – the illness can be. The third sounds reasonable until a person better understands how rigorous vaccine safety trials and post-licensure monitoring is (including subsequent studies showing the vaccine’s low risk of adverse events). And the fourth is exactly what a recent study in the journal Pediatrics addressed.
Previous studies have already addressed whether the chickenpox vaccine works. Deaths from chickenpox decreased by about two thirds within the first six years after the vaccine was licensed, and six years later, deaths were nearly eliminated. But the question remained whether more infections might occur among teens now that younger children would be immune to the disease.
So researchers at Kaiser Permanente in Oakland did a series of studies before and after the introduction of the vaccine to look at who was getting sick. The first study was conducted from 1994-1995, just before the vaccine was introduced. The subsequent waves occurred in 2000, 2003, 2006 and 2009.
During each one, between 8,400 and 8,900 members of Kaiser Permanente of Northern California, all aged 5 to 19 years old, were randomly selected to be surveyed by telephone regarding whether they had had the chickenpox in the past year or ever, regardless of whether they had been vaccinated or not. To ensure the sample focused on the older kids, it included at least 1,000 members aged 5 to 9, at least 1,000 aged 10 to 14, and at least 6,000 aged 15 to 19.
In addition, the hospital records for the entire insurance network were examined to determine hospitalization rates from chickenpox for all ages during each year of the study.
The result? From the first year the vaccine was introduced (1995) until 14 years later, the number of chickenpox cases dropped from about 26 per 1,000 individuals per year to 1 per individual per year. In other words, infections declined by about 90% to 95% across all age groups – including the older kids.
Likewise, the hospitalization rate dropped across all ages by about 90%, from 2.13 hospitalizations per 100,000 network members to 0.25 hospitalizations per 100,000. The Kaiser Permanente of Northern California network included about 2.3 million individuals in 1994, which rose to 3.1 million through 2009, so that’s a pretty substantial sample size of the population for the study.
The drop in hospitalization implies that no increase in infections among adults was seen either, though most adults would have probably had chickenpox as kids and remained immune to it.
The survey also asked about vaccination status to see what the rates of immunization coverage were throughout the network. The uptake of the vaccine increased sharply in the first decade of the millennium, from 51% in 2000 to 99% in 2009 among kids aged 5 to 9, and from 11% to 95% among 10- to 14-year olds.
Even among teens (15 to 19), vaccination rates increased from 3% to 54% over that decade, and by 2009, 91% of all children and teens who had not had chickenpox had been vaccinated against it. (In fact, 99% of all kids aged 5 to 14 had been vaccinated.) At the same time, as would be expected, the percentage of children and teens who were not protected against chickenpox (either by vaccination or disease-provided immunity) decreased from 1995 to 2009, from 18% to 6%.
“These findings suggest that vaccination coverage was high in the younger age groups and that catch-up vaccination programs have been effective at reaching susceptible children and adolescents,” the authors noted – perhaps revealing one reason why the burden of disease did not shift to older kids. If few enough individuals have the disease because so many are vaccinated, then there aren’t many sick folks around to allow the disease to travel through the population.
Meanwhile, the infections themselves steadily dropped as well. While 85% of kids surveyed had had the chickenpox when the study in 1995 was conducted, that number declined to 80% in 2000, 71% in 2003, 59% in 2006 and 38% in 2009. The decrease was even more dramatic among the youngest children: for those aged 5 to 9, an overall 76% reported in 1995 that they had ever had chickenpox, but by 2009, only 5% had ever had the disease.
But the important finding – the one the study set out to determine – was that infections in teens dropped over this period as well. While 86% of teens aged 15 to 19 had ever had chickenpox in 1995, only 48% reported ever having had it in 2009. So over the years, vaccinations increased, infections decreased, and the burden of illness never shifted to the older kids.
What do all these numbers mean? Vaccination coverage went up, infections went down, and older kids or young adults were not picking up infections even as more of the younger kids were vaccinated. So at this point, there is no evidence that vaccinating against chickenpox is putting older children at higher risk for contracting the illness.
Complications, or Why Chickenpox is More Dangerous Today Than When You Had It
But what about those other questions that parents ask about the vaccine – mainly the idea that chickenpox isn’t that serious? It’s true that chickenpox was never a major killer in the way that measles and many other vaccine-preventable diseases were and can be. But that doesn’t mean that chickenpox can’t be fatal – it most certainly can be, even for children. As recently as the 1990s, approximately 100 to 150 people died from chickenpox each year, not to mention the 10,000 to 13,000 who were hospitalized.
Most of those who died had underlying conditions or were at higher risk, such as being pregnant or having HIV, but even healthy children and adults died from the disease. Other serious complications from chickenpox include pneumonia, encephalitis (inflammation of the brain), blood stream infections, toxic shock syndrome and bacterial infections. In fact, that last one is the reason chickenpox is a bit more frightening today than it was 20 years ago.
Twenty years ago, MRSA, or methicillin-resistant Staphylococcus aureus, was far less common and concerning than it is today. It was around – it had been in the US since at least the 1960s – but it began dramatically increasing in the 1990s and has continued climbing since. This bacteria lives all throughout our environment and can easily infect the skin lesions caused by chickenpox. Whereas antibiotics could have treated such an infection a couple decades ago, more of the bacteria has now developed a resistance to methicillin, making it far more dangerous if it infects chickenpox lesions.
So, while a child who catches the chickenpox is far more likely than not to survive the disease, they may not survive unscathed, and they’re still playing odds that are less in their favor than those from the vaccine. In fact, as Melinda Wenner Moyer describes at Slate, a child who gets the chickenpox has a 1 in 400 chance of ending up in the hospital from the infection but only a 1 in 2,000 chance of suffering a side effect from the vaccine – which is likely to be a minor side effect in any case.
Another reason to get the chickenpox vaccine, as Moyer also discusses, is that it reduces a child’s risk of getting shingles later on. Shingles are horribly painful, but they develop from the latent varicella virus continuing to live on in those who have had wild chickenpox. A person vaccinated against chickenpox who never catches chickenpox is very unlikely to develop shingles. (And so far, there is no evidence that the introduction of the chickenpox vaccine has led to an increase in shingles cases. However, it is possible in rare circumstances – usually when a child is immuno-compromised – to develop a mild case of shingles as a result of the chickenpox vaccine.)
So, in the less than two decades that the chickenpox vaccine has been available, we’ve seen a decline in chickenpox cases, a decline in hospitalizations, a decline to near-zero in deaths, an increase in immunizations, no shift of cases to older individuals and no increased risk of shingles. We also know MRSA is a greater threat now than it was only a couple decades ago, making chickenpox lesions more of a risk than they once were. All of this makes the decision to forgo the chickenpox vaccine – or, worse, to intentionally infect a child with chickenpox so they have “natural immunity” – unwise.Update: For more discussion and information from a science mom, whose two children recently had chickenpox, check out Southwark Belle’s blog post here: http://southwarkbelle.blogspot.co.uk/2014/06/some-poxy-questions-why-doesnt-uk.html