Outbreaks can occur despite vaccination

No immunization is 100% effective, say infectious disease experts.

By Susan J. Landers — Posted March 27, 2006

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Washington -- Last summer's outbreak of mumps at a New York camp probably affected more kids than it should have, because no one recognized the swelling that characterizes the disease. Plus, many campers had been vaccinated, so suspicion was low.

Mumps is rare in the United States, and physicians as well as parents have become less likely to suspect it. That was unfortunate for the 31 campers and counselors infected by a 20-year-old counselor who had arrived from the United Kingdom where an outbreak of mumps was raging, noted a report in the Feb. 24 Morbidity and Mortality Weekly Report.

The case illustrates the need to maintain broad vaccination programs that continue to provide immunity for diseases rarely seen, as well as the need to recognize that vaccines aren't 100% effective.

The New York case also demonstrates the importance of recognizing the signs and symptoms of mumps and that physicians should suspect mumps independent of vaccination history, diagnose the disease by using laboratory testing and report it immediately to local health authorities, according to the MMWR report. In this instance, delays in diagnosis and isolating those infected, as well as the failure to report it quickly, likely caused a much larger outbreak than should have occurred, federal authorities said.

The outbreak also raised the question of the effectiveness of at least the mumps portion of the measles, mumps and rubella vaccination.

But don't worry, say several experts. The vaccine is 95% effective after the first dose and nearly 98% effective after the second. In the New York case, it is thought there was enough mixing of infected and healthy counselors and campers that even those who were vaccinated got sick, said Claire Pospisil, a spokesperson for the New York State Dept. of Health.

Though the MMR vaccine is considered to provide lifelong immunity, no vaccine is 100% effective. Some people might have never developed immunity, said Joseph Bocchini, MD, a member of the American Academy of Pediatrician's Committee on Infectious Diseases. "There is about a 5% incidence of vaccine failure."

There is also something of a hierarchy of effectiveness.

Hepatitis A and B vaccines seem likely to maintain effectiveness for decades, while "varicella does look like it has some waning immunity," said Robert Frenck, MD, professor of pediatrics at the University of California, Los Angeles.

There has been discussion of the need for a varicella booster, said Martin Myers, MD, director of the National Network for Immunization Information, and the topic is likely to come up at the June meeting of the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices.

Keeping a close watch

"We know a great deal about the duration of immunity as it is something that is monitored closely, particularly for new vaccines," Dr. Myers said. Such vigilance detected the occurrence of measles among people who were vaccinated with one dose of the MMR vaccine, leading to the recommendation for a second dose. "Monitoring vaccine effectiveness is one of the really important things that the public health service does, and people don't realize it."

It's also clear immunity to the pertussis vaccine wanes after 10 years, said Joseph Bocchini, MD, a pediatrician at Louisiana Health Sciences Center in Shreveport, La., and a member of the AAP's Committee on Infectious Diseases.

A recently approved acellular pertussis vaccine for adolescents and adults is designed to shore up that waning immunity and indirectly provide needed protection to vulnerable newborns who don't begin the DTaP series until they are 2 months old.

The newer vaccines, including Haemophilus influenzae type B, or Hib, appear to be effective, although the longevity of their effectiveness is being studied.

Given the possibility that new information can result from such observation, physicians are advised to maintain contact with patients to advise them of possible changes in medical advice, including their vaccination needs, Dr. Bocchini said. Vaccine registries on state and national levels are probably the best way to compile this vital information, he added.

Such registries could be particularly important when children leave pediatric care to enter the practice of an internist or family physician, Dr. Bocchini said. "They need to have their immunization records, and it's easy to lose them when they are on paper."

The registries, long recommended by the CDC and strongly supported by the AMA, might be catching on. In Louisiana, for example, 50% of the physicians are recording all immunizations for children. "It's helpful but we aren't where we need to be yet," Dr. Bocchini said.

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Rating vaccine effectiveness

Haemophilus influenzae type b: This vaccine is one of only two that are more effective at providing immunity than natural infection. The other is tetanus.

Hepatitis B: The series with recombinant hepatitis B vaccines are 95% effective at inducing seroimmunity. Immunity is probably lifelong, but undetermined, because the vaccine has only been in use 20 years.

Hepatitis A: Between 97% and 99% of healthy 2-year-olds (or older) who receive two doses of the vaccine are protected from HAV infection. Immunity usually develops within four weeks of the initial dose. After two doses, immunity is estimated to last for at least 20 years.

Measles, mumps and rubella: 95%of those who receive the MMR vaccine at 1 year of age or older are immune after the first dose. Immunity is lifelong. For the measles, the second dose increases protection to 99.7%.

Meningococcal disease: In older children and adults, the MPSV4 vaccine is 85% to 100% effective at preventing infection from the strains of the meningococcus used in the vaccine, and protection lasts for at least three years. Children younger than 2 years old respond poorly to the vaccine. Compared with MPSV4, protection from MCV4 is expected to last longer.

Diphtheria: The DTaP vaccine's overall effectiveness rate is 95% -- 100% effective for tetanus, 95% effective for diptheria and 59% to 89% effective in preventing pertussis.

Polio: The IPV vaccine is 90% effective after two doses and 99% effective after three doses. Duration of immunity is not certain but probably quite long.

Varicella: Varicella vaccine is 85% to 90% effective for prevention of varicella and 100% effective for prevention of moderate or severe disease.

Source: The National Network for Immunization Information

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External links

AMA on improving immunization rates (link)

Centers for Disease Control and Prevention on vaccine registries (link)

National Network for Immunization Information (link)

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