Measles antibodies passed by pregnant women to their babies in the womb, protecting infants early in life, actually disappear quicker than had been previously recognized, leaving babies vulnerable to the sometimes fatal infection for much of their first year of life, according to a new study.

The authors say their findings underscore the importance of widespread immunization against measles, which produces a phenomenon known as “herd immunity” — whereby measles viruses brought into the country by sick travelers can’t get to the vulnerable because so many people are protected. In the case of measles, that’s particularly important for infants; in babies, the risk of complications from the illness and of death is higher than in older children.

“One of the best ways to protect young infants is to make sure those around them are vaccinated. This not only protects infants but also other groups who are vulnerable to measles, such as immune-compromised individuals who cannot be vaccinated,” said Dr. Michelle Science and Shelly Bolotin, the first and senior authors of the paper, published Thursday in the journal Pediatrics.

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Women who have been infected by or immunized against measles pass the protection their immune systems develop — known as maternal antibodies — to their fetuses during pregnancy. It was previously known that babies born to women vaccinated against measles lose maternal antibodies sooner than infants born to women who were infected with measles.

But the new study charts how quickly this critical early-life protection actually erodes. Some babies were unprotected in their first month of life; all babies in the study were susceptible to measles by the time they were 6 months old.

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Because of maternal antibodies, the Centers for Disease Control and Prevention recommends the first of two measles vaccination not be given until a child is between 12 months and 15 months. If babies are vaccinated too early, maternal antibodies actually impede a baby’s ability to develop a good immune response to measles vaccine.

“Prior to this research, in general, it was assumed that infants were immune to measles for at least the majority of their first 6 months of life,” Science and Bolotin told STAT in an email. “Since infants are immunized routinely at 12 months in Ontario, our findings have revealed a wide susceptibility gap between the time that infants lose the protection that they’ve received from their mother and when they are protected by vaccination at 12 months.”

The authors, most of whom work either at the Hospital for Sick Children in Toronto or Public Health Ontario, suggested there is no easy way to fill this susceptibility gap. There’s evidence that babies vaccinated before 12 months don’t develop as strong an immune response from the vaccine, even after maternal antibodies are gone. The thinking is that the immune system has to mature more before the vaccine triggers a good “take.”

Likewise, giving pregnant women booster shots isn’t an option, they said, because the vaccine contains live but weakened measles viruses. Live-virus vaccines are typically not given to pregnant women.

But there was little doubt, judging from the data, that infants are losing protection against the measles faster the expected.

The researchers used stored blood samples from children who had been treated at the Hospital for Sick Children, where Science is an infectious diseases physician, to check measles antibody levels. Blood samples from children with suspected or confirmed immune system diseases were not included.

They analyzed 196 stored samples, deliberately choosing a number by each month under the age of 1; some had been drawn from babies in their first month of life. One-fifth of the babies under a month old did not have antibody levels high enough to protect them; by the time babies were in their third month — 2 months old — two-thirds weren’t protected. By six months, all the babies were vulnerable to measles.

A commentary published with the article argued that the findings should not change current U.S. policy about when to first vaccinate young children against measles.

“Despite ongoing outbreaks, the epidemiology does not support lowering the age for measles vaccination in the United States,” wrote Huong McLean, of Marshfield Clinic Research Institute and Dr. Walter Orenstein of Emory University School of Medicine, suggesting that in most recent years there have been so few measles cases in the country that susceptible babies are protected by the lack of transmission around them — the herd effect.

This year has been a notable exception, with the U.S. reporting more measles cases than have been recorded in a quarter-century. As of early November there had been 1,261 cases, most occurring in outbreaks that lasted so long they threatened the country’s measles-free status. In most recent years there have been fewer than 200 cases reported to the CDC.

“These data are interesting and will be evaluated, and we’re always evaluating whether our recommendations are effective, but the implications of changing a long-standing recommendation are substantial,” said Dr. Thomas Clark, deputy director of the CDC’s division of viral diseases. “The decision-making process would require a lot more data, including any meaningful changes in disease incidence in this age group.”

Kimberly Thompson, president of the nonprofit Kid Risk, which conducts modeling studies on infectious diseases, agreed that changing the complex schedule of when children get immunizations is no easy matter. But she disagreed with the notion that maintaining herd immunity is the only available option for protecting babies against measles.

She suggested it would be worthwhile studying whether giving women of childbearing age a measles booster shot could better arm the babies some of them would go on to have. She even suggested it would be worth studying whether giving a measles booster in pregnancy could be done safely and would have the desired effect.