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Personal Health Jane Brody on health and aging.

After more than 50 years as a medical and science writer, I’m pained to see that childhood immunizations, perhaps the most important health and lifesaving advance of the last century, are being seriously eroded by misinformation and scaremongering.

The result has been a resurgence in outbreaks of preventable diseases that, in earlier generations, robbed children like me of many weeks of school, left long-lasting physical and emotional scars and, in some cases, ended their lives.

Parents who probably never saw a single case of any of these diseases have chosen to “opt out” of protecting their offspring with vaccines strongly recommended by the American Academy of Pediatrics and the Centers for Disease Control and Prevention, vaccines that have been shown to be safe and effective. Citing personal or religious objections to immunizations, these parents have insisted that their children be allowed to attend school without them.

California recently joined West Virginia and Mississippi in passing legislation that eliminates exemptions from vaccinations for personal or religious reasons. Children with medical conditions that preclude safe immunizations remain exempt. All other parents who choose not to have their children immunized will have to home-school them.

Passage of the law in California was helped by a frightening outbreak of measles that started at Disneyland in January, resulting in at least 125 cases, the vast majority occurring among unvaccinated people.

Those refusing immunizations often expect their families to be protected by what is known as “herd immunity” — high rates of protection among everyone else. But to be effective, herd immunity for a highly contagious disease like measles requires vaccination rates of 95 to 99 percent of people, a rate higher than what a scientific analysis indicated prevailed among those exposed through the California cases. That rate could have been “as low as 50 percent and likely no higher than 86 percent,” according to a study by Maimuna S. Majumder, a researcher at Boston Children’s Hospital, and colleagues, published in JAMA Pediatrics.

Measles, a viral infection that is transmitted through the air, can result in a week of high fevers, sometimes serious complications like encephalitis and seizures, and death. Those who are not immunized are up to 35 times as likely to get measles.

Measles vaccine, made from a modified live virus, is usually given to babies at 12 to 15 months in one shot combined with vaccines for mumps and rubella, the so-called M.M.R. vaccine. A second dose is administered from ages 4 to 6, before kindergarten.

The same schedule is used for a vaccine against chickenpox. Lest you think this disease is a necessary rite of childhood, I speak from experience that it can be misery incarnate, as well as cause serious complications like infections with flesh-eating bacteria, pneumonia, sepsis and encephalitis. As an 8-year-old, I suffered through three stiflingly hot weeks with an impossibly itchy bodywide rash. To keep from scratching, I soaked for hours a day in oatmeal baths between being fanned by my grandmother. Still, I ended up with scars on my chest, back and face that lasted for decades.

Mumps, which inflames the glands under the ears or jaw, attacked both sides of my face at once, making it a challenge to sleep. Complications from mumps can include meningitis, encephalitis and deafness. Twenty to 50 percent of boys infected after puberty develop testicular inflammation that can cause sterility.

Rubella, formerly known as German measles, is usually a mild illness that is most dangerous to pregnant women. It can cause miscarriage or birth defects that include deafness, vision disorders, heart defects and intellectual disability. The M.M.R. vaccine should not be given to children who are taking steroids or who have cancer or a disease that impairs the immune system; they are especially dependent on protection through herd immunity.

In 1998, a fraudulent report linked the M.M.R. vaccine to autism, the early signs of which are often first noticed in 1-year-olds. The report, later retracted, caused an international backlash against this vaccine and spawned rising disease rates. The controversy refuses to die despite multiple well-designed studies that found no such link, including a study of more than 95,000 children published in April.

Some parents balk at the seemingly large number of vaccines administered to young children, who can get as many as 23 shots by age 2. The vaccines are given at such young ages because this is when children are most vulnerable to the diseases and any immunity acquired from their mothers has worn off, the C.D.C. explains.

According to Dr. Paul Offit, an infectious disease specialist at Children’s Hospital of Philadelphia, young children readily handle the immune challenges of multiple vaccines. For example, studies have shown the five-in-one vaccine Pediarix against hepatitis B, polio, tetanus, diphtheria and pertussis is as safe and effective as giving each of these vaccines individually.

Pertussis, or whooping cough, is another serious, potentially deadly childhood disease now on the rise in the wake of diminishing immunization rates. About half of infants who get pertussis must be hospitalized.Complications of the disease include pneumonia and seizures, and it can be fatal. Pertussis vaccine is made from killed bacteria, usually given as DTap with vaccines for diphtheria and tetanus, in five doses starting at 2 months.

Still another serious disease of young children, Haemophilus influenzae type b, or Hib, was long a leading cause of bacterial meningitis among children younger than 5. Before the Hib vaccine was developed in the 1980s, about 20,000 children were infected each year, and up to 6 percent of them died. The Hib vaccine, given in four doses before 15 months, lowered the number of serious cases by more than 99 percent. Researchers at the University of California San Francisco reported in May that by preventing chronic inflammation from recurrent Hib infections, the vaccine could also reduce the risk of acute lymphoblastic leukemia in young children.

This is the second of two columns on how vaccines protect children.

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