My seven-year-old son developed a rash today that I believe to be chickenpox. My concern is that a friend of mine had her 10-day-old baby at my home last evening before we were aware of the illness. My son had no contact with the infant, as he was in bed during the visit, but I have read that chickenpox is contagious up to two days prior to the actual rash. Is there cause for concern at this point? What should I tell my friend to look for?

D.

Chickenpox is the common name for varicella infection. The varicella virus infects the vast majority of people in the United States -- and the rest of the world, for that matter. Most people can recognize chickenpox. It appears as a rash of vesicles (small blisters) that starts on the face and trunk and spreads to the rest of the body. The vesicles, which are surrounded by red areas, are often described as looking like "dewdrops on rose petals." A classic feature is that the vesicles are in different stages of resolution at the same time. Each new "crop" of lesions forms for three to five days, and then they all begin to crust over. You are correct in that a person with chickenpox can be contagious for 48 hours before the first vesicle is seen. A patient continues to be contagious until about the time all of the lesions have crusted over.

The fact that your son did not come in close contact with the infant means he most likely did not transmit the virus. Of concern, though, is the fact that newborns are at higher risk of complications of varicella, including pneumonia. The highest risk is in premature infants and newborns whose mothers developed chickenpox just a few days before delivery. What would be important to know is whether your friend has had chickenpox before. If she has, then the baby is probably somewhat protected because of the antibody your friend would have passed to her baby. Even if your friend has not had chickenpox, the baby is past the time of highest risk, which is the first few days after birth.

There is a very effective means to prevent infection after exposure. A form of antibody to varicella called varicella-zoster immune globulin (VZIG) can be given up to 48 hours after exposure and still prevent disease. It has some effect even 96 hours after exposure. Experts recommend giving VZIG to newborns whose mothers develop varicella five days or fewer before delivery or up to two days after. It is also recommended for pregnant women and immunocompromised people who have a significant exposure to the virus. On the other hand, the varicella vaccine, which is currently recommended for all children in the United States, will only work if given long before exposure.