Because of aggressive childhood immunization programs, the rate of measles, mumps, and rubella has declined dramatically in the United States, with a simultaneous reduction being seen in congenital rubella. Nevertheless, maintaining population immunity is essential because these infections may be introduced from other countries. Currently, 65 percent of the rubella cases recognized in the United States occur in adults, resulting in an increased risk of complications and congenital infections. Similarly, mumps and measles are now uncommon conditions but have the potential for serious complications such as meningitis and encephalitis. An estimated 10 to 18 percent of women of reproductive age in the United States may be susceptible to these diseases because of failure of childhood immunization or fading immunity.
The American College of Obstetricians and Gynecologists (ACOG) recommends screening all pregnant women for rubella immunity and offering postpartum immunization with combined measles, mumps, and rubella (MMR) vaccine to susceptible mothers. However, only 21 percent of U.S. hospitals surveyed follow the ACOG recommendations. In addition, there are concerns that the ACOG strategy does not benefit women who have immunity to rubella but are susceptible to mumps, measles, or both. Haas and colleagues studied women obtaining pre-natal care at a military hospital to document susceptibility to one or more of these diseases and estimate the likely outcome of different screening and immunization strategies.
All women presenting for prenatal care were screened for immunity to measles, mumps, and rubella based on immunoglobulin G titers. Demographic and reproductive data were collected, and patients were questioned about immunization history, specifically if they had received childhood immunizations and boosters as a teenager or after a previous pregnancy. Cost comparisons for the various strategies were based on actual laboratory and clinical costs plus average retail vaccination costs.
During the six-month study, 973 pregnant women were screened. These women ranged in age from 17 to 43 years (mean, 24.2 years) and 36.8 percent were primigravidas. Among all pregnant women screened, 9.4 percent were susceptible to rubella, 16.5 percent to measles, and 16.3 percent to mumps. Overall, 32.6 percent of the women were susceptible to at least one of these viruses. Approximately one fourth of the women who were immune to rubella were susceptible to mumps or measles. Most of the women (75.3 percent) did not know if they had received any booster vaccinations. Even among women who reported receiving an adult booster vaccination, 25.8 percent had susceptibility to one or more of the three illnesses.
From these data, the authors modeled four strategies for screening and immunization targeting measles, mumps, and rubella in 1,000 adults. They calculated that strategy A (screening only for rubella susceptibility and providing rubella immunization to susceptible patients) would fail to immunize 280 women susceptible to one or more of the viruses. Strategy B (screening for rubella susceptibility and providing MMR vaccine to susceptible patients) would fail to immunize the 232 women who were immune to rubella but susceptible to mumps, measles, or both. Strategy C would screen for susceptibility to all three viruses and provide the MMR vaccine if the patient was not immune to one of them. Strategy D would screen for susceptibility to all three viruses and provide a rubella-only vaccine for women susceptible to rubella alone; the MMR vaccine would be provided for women susceptible to measles or mumps. Strategies C and D would protect all mothers from all three viruses, with strategy D being slightly less expensive.
The authors conclude that at least one third of pregnant women are currently susceptible to one or more of the three viruses. History, including recall of booster immunizations, is not a reliable indicator of immune status. The authors argue that current ACOG guidelines are poorly followed and provide a suboptimal strategy. They recommend that strategies based on screening for antibody titers to all three viruses be introduced to protect young women and their unborn children.
editor’s note: For family physicians, the implications of this study go well beyond pregnant patients. Are about one third of healthy young adults in the United States susceptible to at least one of the formerly “common childhood viral exanthems”? What about aging baby boomers? Does their naturally acquired childhood immunity persist, or are some of them at particular risk because boosters have never been recommended for persons born after 1957? Add the increasingly mobile population and popularity of international travel and we could have a recipe for more infections in U.S. adults. Although herd immunity is likely to prevent or curtail epidemics, older patients are likely to be more severely ill and have more serious complications. A more basic concern is diagnosis. Rubella was never easy to diagnose, even when it was common in the community. Measles can also be tricky, especially in an adult with malaise, red eyes, a cough, and an atypical rash. Even mumps could be challenging in an adult, because parotiditis may be unilateral or not a prominent feature. If the immunity levels in adults are low and decreasing, perhaps we will see a change in recommendations for adult immunization in the future.—a.d.w.