Monday, November 28, 2011

Varicella Vaccination Among Older Kids Protects Infants

Turning on the news this morning, I was greeted with a story that caught my ear. Published online, ahead of print, a new study in the journal Pediatrics, "Varicella in Infants After Implementation of the US Varicella Vaccination Program", looked at the incidence of varicella (chickenpox) among infants under 12 months of age.

The varicella vaccine was approved in 1995 and is recommended for all children aged 12 months to 12 years. It is a "live" virus vaccine, meaning that the vaccine uses intact, though weakened, virus particles that grant immunity without causing full-blown disease. Vaccines of this sort are generally more effective than "killed" virus vaccines, which only use part of the virus. Because of the approved age range for the vaccine, it does not provide direct protection to infants under 12 months of age, but it can provide protection through herd immunity.

So what did the researchers find?

In the observational study, the authors looked at chickenpox incidence among infants under 12 months of age from 1995 to 2008. The population was broken up into 0-5 months and 6-11 months. They found that incidence among the total infant population fell 89.7% over the period examined. They also noted that complications and severity of disease was lower in the younger age group vs. the older group. The authors speculate that this may have been due to maternal antibodies (which generally provide some protection to infants for about 6 months).

The results of this study add to an examination by the CDC of the impact of the varicella vaccine from 2003. The CDC looked at all cases, finding that while incidence remained steady prior to approval of the vaccine, the rate of new cases declined beginning in 1999. A more detailed look at the disease and vaccine can be found in the the CDC's Pink Book chapter on varicella (PDF). This resource also notes that the vaccine is 70%-90% effective, and even in cases where it fails to prevent infection, it is about 90%-100% effective in ensuring a milder course of illness.

Since chickenpox is only known to infect humans, this study shows promise that we may, one day, eliminate the virus from the planet, much like we have with rinderpest among cattle, smallpox among humans and are on the verge of doing with polio. We have more evidence that vaccination protects far more people in the community than just the person getting the vaccine. As the study authors note:
The varicella vaccination program has resulted in substantial indirect benefits for infants, who are not eligible for vaccination. Presence of maternal varicella-zoster virus antibodies might explain attenuated disease in very young infants likely born to mothers with history of varicella. Although varicella disease incidence has declined, exposure to varicella-zoster virus continues to occur. Improving varicella vaccination coverage in all age groups will further reduce the risk of varicella exposure and protect those not eligible for varicella vaccination.
To those who oppose vaccination against chickenpox, claiming it is a "mild" disease or a "right of passage", consider this. We have a tool that could, given enough time and use, render itself useless. This tool could eliminate, completely, a disease that before its use cause 11,000 hospitalizations and 100 deaths per year in the U.S. Even mild courses of illness mean a week of lost school time. While children may enjoy having an excuse not to go to school, making up the missed homework is no fun. Furthermore, there is a cost for parents who need to miss work to care for their itchy young'n. That may be as simple as using a vacation day that could have otherwise been used for more important or fun family time; or, it may mean days of lost wages, which some cannot afford to lose. The more people use the tool, the sooner we can stop using it altogether. The more you fight against its use, the longer it will be used.

Think on that.

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