乌鸦传媒

A first measles vaccine given at 12 or 13 months of age is less likely to provide lasting protection than a shot at 15 months or later, researchers reported.

In a case-control study conducted in the wake of a large measles outbreak, people who received the shot earlier were about six times as likely to become ill as those whose vaccination was later, according to of the Quebec National Institute for Public Health in Quebec City.

The "unexpected vulnerability" could cause measles control efforts to fail and needs more research to understand what's causing it, De Serres and colleagues said .

But outside experts told 乌鸦传媒 the study is probably not enough by itself to warrant changes either in recommendations or in clinical practice.

What would help would be a prospective study comparing antibody levels in babies who get the vaccine on schedule or later, commented of Children's Mercy Hospital in Kansas City, Mo.

"The problem ... is that it will take 10 years to get that data," he told 乌鸦传媒.

When measles vaccines were first put into clinical use, they were given at 9 months but that was quickly changed when it became clear that efficacy was greater when the shot was given later, De Serres and colleagues noted.

In Canada, the first vaccine shot has been recommended at 12 months since 1970, with a booster some time later. Since 1998, the recommendation of the U.S. Advisory Committee on Immunization Practices has been to give the first shot between 12 and 15 months, followed by a booster.

But U.S. children usually have a scheduled visit to the doctor at 12 months so they mostly get the shot earlier rather than later, commented , of UH Rainbow Babies & Children's Hospital in Cleveland.

"This needs a lot more study before we decide ... that there's going to be a total change in policy," she told 乌鸦传媒. "I'm not going to change my practice until the experts within the American Academy of Pediatrics say we should move to a different time."

A large outbreak in 2011 in Quebec, centered on a single high school, mostly affected people who had not been vaccinated. But it also seemed to show that, among people who had been given two doses of the vaccine, those who got the first shot at 15 months or later were better protected.

De Serres and colleagues studied all confirmed cases in the province among students, ages 5 to 17, who had been given two doses of the measles, mumps, rubella (MMR) vaccine, starting at 12 months or older, and who had measles between Jan. 1 and Dec. 31, 2011.

Controls -- also recipients of two vaccine shots starting at 12 months or older -- were selected at random from the provincial vaccination registry and matched for age and school attended.

The participants included 102 cases (including 41 from the original outbreak school) and 510 controls. Some details of the investigation were given in 2012 at the Interscience Conference on Antimicrobial Agents and Chemotherapy in San Francisco.

Outside of the original outbreak school, 90% of the cases and 72% of controls had their first shot at 12 or 13 months of age. In contrast, 4.8% of cases and 19.6% of controls got the first dose at 15 months or later.

In a multivariate analysis, the risk of measles outside the outbreak school was 6.2 times higher (95% CI from 1.33-29.3) when the first dose was given at 12 or 13 months compared with at 15 months or later. The increase was significant (P=0.02).

In a pooled analysis including the outbreak school, the risk was 5.2 times higher (95% CI from 1.91-14.26) and was significant (P=0.0013).

What's "really interesting," McDavid commented, is that most of the cases in the outbreak either had not been vaccinated or could not be shown to have been immunized.

So the take-home message from the outbreak is really about the importance of making sure people get vaccinated against the disease, she said.

Harrison cautioned that the study has several limitations, in addition to its retrospective nature, that make it unlikely to change practice in the U.S.

He noted that people in the study got their second dose much closer to the first than is common practice in the U.S. "It's not incorrect to give the dose at that time," he said, "it's just a different way to do it."

The researchers noted that 70% of their cases and controls were born to mothers who had had measles disease, while the comparable age groups in the U.S. were much more likely to have derived immunity from vaccination.

That's important, Harrison noted, because mothers with actual measles disease will pass more antibodies to the fetus and the infant will carry them longer, likely reducing the impact of an early vaccination.

"So this does not apply to U.S. moms," he said, most of who had been immunized.