My four-year-old son recently experienced chicken pox. He was unvaccinated, but of the 10 children in his preschool to be infected, half were. What is the efficacy of the chicken pox vaccine? And are these children who were exposed to the virus twice at a higher, lower or unchanged chance of getting shingles when they are adults? Finally, what is the uppermost incubation period for the virus (to see if his siblings were also infected)? A.D., Jerusalem Prof. David Branski, chairman of pediatrics at the two Hadassah University Medical Centers in Jerusalem, replies: The efficacy of the varicella vaccine is good. About 95 percent of children from one to 12 years are immunized from one dose, while those above 13 years should have two doses to increase the seroconversion rates from 80 percent to 99%. It seems that the incidence of herpes zoster - which causes chicken pox and sits dormant on the nerves, with the possibility of reawakening and causing the very painful condition of shingles decades later - is higher after having a natural chicken pox infection than after vaccination. The incubation period for chicken pox is usually 14 to 16 days, but it can be from 10 to 21 days. As for half of the children who received the vaccine getting chicken pox - I am not sure about the details, but if it is true, then I simply cannot explain. I am 64 and have several chronic conditions. My health fund tries to give me generic medications rather than commercially known products. Do generic drugs have a different molecule from the original drugs? Is there any risk in taking them? V.I., Kiryat Ono Dr. David Lipschitz, an American physician and author of the book Breaking the Rules of Aging, advises: It always makes sense to go for the generic drug whenever you have a choice. In more than 50 years of clinical experience, there is no scientific trial that has ever shown brand-name medications are superior to their generic counterparts. The US Food and Drug Administration requires that the active ingredients be identical before it approves a generic medication. There can be a difference, however: The composition of the pill or the capsule may be different. It may vary in shape, have different fillers or flavoring and possess a different color. But to fully understand the effect of any differences, the FDA also requires the makers of the generic drug to perform pilot tests to demonstrate that the blood levels of the active ingredient in a subject's blood and the method and rate at which it is cleared from the body be identical to the brand-name equivalent. Nevertheless, many people - both doctors and patients - refrain from using generic drugs. Detractors of generic medications often complain that the generic pill may be absorbed differently from patient to patient or that there may be adverse reactions, such as allergies, to an ingredient in the generic drug. Recently, there was an article in The Los Angeles Times by a writer with depression who felt that she responded differently to generic Zoloft, causing her symptoms to worsen. She then found out that many patients, when switched from the brand-name Wellbutrin XL 300 to the generic version of the antidepressant, complained of worsening depression, panic attacks, anxiety and thoughts of suicide. Wellbutrin XL is a slow-release medication, which means that the drug is gradually released from the tablet over a 24-hour period. Studies by a private laboratory showed that 34% of the active ingredient in the generic drug was released in two hours, compared to 8% percent in the brand name. It was postulated that the difference in release was contributing to the symptoms. The FDA is investigating this complaint. While it is possible that a slow-release medication could be different from one pill to another, significant differences between generic and brand-name drugs are extremely rare. And for every one person who complains that the generic is ineffective, millions take it without any adverse reactions. When it comes to choosing brand-name therapy over generics, many patients see the powerful effect of the mind over the body. With the "placebo effect," if you believe that a medication will help you, there is a good chance that it will, and vice versa. If you switch from a brand-name to a generic medication, keep an open mind and rest assured that it will work. If the generic drug does not seem to work, discuss the problem with your pharmacist or doctor. Find out whether others are experiencing similar problems. If you are absolutely convinced that the drug is ineffective, consult with your physician, who may get permission from the health insurer to prescribe an alternate medication. Rx for Readers welcomes queries from readers about medical problems. Experts will answer those we find most interesting. Write Rx for Readers, The Jerusalem Post, POB 81, Jerusalem 91000, fax your question to Judy Siegel-Itzkovich at (02) 538-9527, or e-mail it to firstname.lastname@example.org, giving your initials, age and residence.