A recent study assures that vaccines for measles-mumps-rubella and varicella, or chicken pox, are equally effective in extremely preterm infants also even though their immune systems are not as developed as full-term babies.

"The assumption has always been that it would be OK, that very early babies would have enough immunity, but no one had formally researched the subject," said Carl D'Angio, M.D., associate professor of Pediatrics at the University of Rochester Medical Center, and author of a paper on the subject in Pediatrics this month. "I'm happy be able to reassure my colleagues and parents that it is OK."

The study, which included 16 term and 16 extremely preterm (born before about 6 1/2 months of pregnancy) infants born between May 2002 and May 2005, is the first of its kind to examine the antibody levels of measles, mumps, rubella and varicella in this population before and after vaccination. It was published in this month's Pediatrics.

The same number of preterm infants and full-term infants in the study reached a level of immunity considered protective against the diseases. This positive outcome, however, was not guaranteed because preemies' immune systems do not always react the same way as full-term infants.

For instance, a change in the types of vaccine used in the UK apparently resulted in an increase in Haemophilus influenzae type b infections, like bloodstream infection or meningitis, in children. Children born prematurely appeared to be particularly at risk for this, alluding to potential problems this population may have with responding to vaccines.

That isn't the case for the MMR and chicken pox vaccines. "Now we can all breathe a sigh of relief. We were right," D'Angio said. Although not generally life-threatening, measles, mumps, rubella and chicken pox can all have serious complications. While many adults contracted chicken pox as a childhood rite of passage, the varicella virus killed 100 and hospitalized 11,000 people every year before the vaccine became available.

Generally, the virus causes a rash, itching, fever and fatigue, but it can also cause severe skin infections, scars, pneumonia and brain damage. And a person who has had chicken pox can get a painful rash called shingles years later, according to the Centers for Disease Control and Prevention.

The measles virus causes a rash, cough, runny nose, eye irritation and fever, but it can also lead to ear infections, pneumonia, seizures, brain damage, and rarely, death. The mumps virus causes fever, headache and swollen glands, but it may also cause deafness, meningitis, painful swelling of the testicles or ovaries, and rarely, death. The rubella virus causes a rash, mild fever and arthritis, but in pregnant women, it can lead to a miscarriage or birth defects, according to the CDC.