Infectious Diseases: No RSV, Please

INFECTIOUS DISEASES No RSV, PleaseEach year, 10,000 U.S. infants die of lower-respiratory infections — more than half of them caused by a microbe with the forbidding name of “respiratory syncytial virus.” Researchers have now devised a new vaccine against RSV.

Unhappily, it turns out to be a partial failure, according to evidence presented at a recent Manhattan virology conference. In fact, RSV has once again confronted virologists with the paradox that a vaccine sometimes worsens the dis ease it theoretically prevents.

Most adults carry telltale antibody showing that they have been infected with RSV at some time. The virus probably caused what seemed to be only a bad cold, and since the mild infection left no permanent damage, it was for gotten. But to infants under six months old, RSV is a far more serious threat.

It causes inflammation of the lower respiratory tract, bronchiolitis (as distinct from upper-respiratory bronchitis) and pneumonia.

If Not, Why Not? Up to about six months of age, babies carry in their bloodstream their mothers’ antibody against RSV. By the basic rules of virology, this should protect them. Obviously, it doesn’t. Why not?

At the Manhattan conference, Dr.

Robert M. Chanock of the National Institute of Allergy and Infectious Dis eases proposed an unorthodox explanation. The inherited antibody, he suggested, actually makes babies more vulnerable to the potentially fatal dis ease. First, Dr. Chanock distinguished between several kinds of antibody (“im-munoglobulins”), which immunologists label alphabetically. Only type G passes the placenta and gets into the fetal blood; the others are developed later.

Type A is most important in RSV disease, because it is a fixed antibody attached to cells lining the nasal passages and the bronchial tree. Once entrenched there, it usually chokes off an RSV infection.

The under-six-months baby, suggested Dr. Chanock, still has little or no immunoglobulin A to fight off RSV. So the virus gets to his bronchioles and lungs. There, it wreaks havoc by causing 50 or more cells to merge into giant combines. Oxygen exchange is so impaired that the baby has asthma-like spasms. To make matters worse, said Dr. Chanock, the G antibody circulating in the blood just below the lungs’ surface actually combines with virus particles to form more damaging complexes.

Although some virologists differed with Dr. Chanock about details, they agreed that the newly developed vaccine against RSV must not be given to infants under six months, as it appears to increase the risk and severity of pneumonia. A similar phenomenon is now being seen in connection with another viral disease: circulating antibody from killed-virus vaccine against measles also seems to make a child more susceptible to severe disease if he later receives live-virus vaccine.