An experimental vaccine is effective half the time in stopping cytomegalovirus infection in women in their child-bearing years, researchers report in the March 19 New England Journal of Medicine.
No vaccine currently exists for cytomegalovirus, which can cause birth defects when it infects a pregnant woman. Because of this risk, vaccine researchers have targeted the virus for decades — without any clear benefit until now.
“This is the first vaccine that really shows prevention from infection with cytomegalovirus,” says Walla Dempsey, a microbiologist and immunologist at the National Institute of Allergy and Infectious Diseases in Bethesda, Md., who wasn’t a researcher on this study.
Most people get infected by cytomegalovirus as children and have few complications or even symptoms. Nearly two-thirds of women in child-bearing years have already been infected with cytomegalovirus.
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But roughly 27,000 first-time cytomegalovirus infections occur in pregnant women in the United States every year. These women don’t harbor home-grown antibodies — generated from a previous infection — that would lower the risk of re-infection, says Robert Pass, an infectious disease pediatrician at the University of Alabama at Birmingham, who coauthored the new study. As a result, such women have a one-in-three chance of passing the virus along to their fetuses via the placenta. At birth, infected babies have an 11 percent chance of having symptoms that include hearing damage, visual impairment, mental retardation and diminished motor skills.
From 1999 to 2006, Pass and his colleagues recruited 464 women in their childbearing years who had tested negative for cytomegalovirus antibodies.
Half the volunteers were randomly assigned to receive the vaccine in three shots spaced over six months. The other half got placebo shots over a similar time span. The researchers attempted to follow each participant for 42 months.
Eighteen of those who received the real vaccine developed a cytomegalovirus infection during the study, compared with 31 of those who got placebo shots.
One-fourth of each group dropped out of the study before receiving all three shots. Some had become pregnant, while others just moved away or quit the study. These women were monitored for less than the targeted 42 months.
To ascertain the effect of the vaccine, the researchers employed a calculation called “person-years” in which they used the risk of infection per person per year of participation. For example, a woman in the trial for three years would contribute three person-years to the analysis.
By this measure, vaccinated women had 3.3 infections per 100 person-years, whereas those getting the placebo had 6.6 infections per 100 person-years. Thus, getting the vaccine halved the infection rate.
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The vaccine uses a cytomegalovirus surface molecule called glycoprotein B to attract the attention of the immune system, which then generates antibodies against the protein and the virus itself. The vaccine is owned by drug developer Sanofi Pasteur, which provided partial funding for this study. The company will need to succeed in a larger randomized trial to gain regulatory approval.
“My understanding is that they are likely to go forward with a glycoprotein vaccine,” Pass says.