Chicken Pox: What's going on?

During the 2007-2008 school year, something very interesting happened with our elementary school aged students. We had a chickenpox epidemic, affecting approximately 33% of the student population in our 500-student elementary school.

It started in October 2007 with one kindergarten student was sent to me showing 7 raised blisters on his head and face. I thought it resembled chickenpox; admittedly I was rusty on this childhood illness as it had become more rare since the introduction of the chickenpox vaccine. The student had all of the symptoms of chickenpox:

He had a sore throat, had a slight fever, his parents reported that he had been sick over the previous weekend with an upset stomach and a cough. Now, the raised blisters on his face were itching to the point that he could not stop scratching.

Reviewing the vaccination history, this child had received the chickenpox vaccine when he was about 12 months old, as recommended by the CDC. In fact, in 2005, the chickenpox vaccine was required for every child in Michigan before entering school.

It was first thing in the morning, so to be more comfortable with assuming a chickenpox possibility, I counted the blisters and logged them on a diagram. My next step was to recheck in one hour while contacting the local Public Health Department. One hour later, the blister count had increased to 16, with new blisters in the scalp and abdomen. The child was sent home with the presumption of chickenpox. He was taken to his pediatrician, and unfortunately, the chickenpox diagnosis was denied. The reason--he had had the vaccine as an infant.

Within 2 weeks, similar symptoms were showing up in our elementary school students across the board; attempts to contain the illness to grade-level, or geographic location in the school building was impossible. I was reporting to the Public Health Department 5-15 students weekly with chickenpox. By Christmas, pediatricians in the city were confirming the diagnosis. By Easter, the chickenpox had affected approximately 180 students, all of whom had had the vaccine as an infant.

But here is another interesting question that remains unanswered: This epidemic was limited to elementary school students only. No older siblings in the Middle School or High School were affected. While this question continued to be raised by myself and the local Public Health Department, by the end of the school year letters were sent by the Health Department to all school age children in our district encouraging a chickenpox booster shot as recommended by the CDC.

In 2007, the new CDC guidelines for chickenpox vaccination:

1. All healthy children 12 months through 12 years of age should have two doses of chickenpox vaccine, administered at least 3 months apart. Children who have evidence of immunity to varicella do not need the vaccine.
2. People 13 years of age and older who do not have evidence of immunity should get two doses of the vaccine 4 to 8 weeks apart.
3. Chickenpox vaccination is especially important for certain groups of susceptible adults.

**An interesting side note for further discussion; my own child erupted in chickenpox on the last day of school in 2005 at the age of 7. He had had the vaccine at age 18 months along with his older brother and sister, who were vaccinated at age 4 1/2 and 6 years old. Neither of the older children showed any signs of chickenpox during or after his illness.

While the CDC denies any change in effectiveness of the vaccine based on the age of the person receiving the dose (see http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5604a1.htm "Risk factors for Vaccine Failure"), why is it that in these two incidents, children who received the vaccine at an older age did not develop the chickenpox when exposed to their siblings with active chickenpox who had received the vaccine as infants?