The vast majority of mainstream medical organizations and professionals now seem convinced that no MMR-autism connection exists.

The review, which concludes that MMR shots are safe and effective, appears in the most recent issue of the Cochrane Library, a publication of the Cochrane Collaboration, an international organization that evaluates medical research.

Despite many reputable scientific studies to the contrary, some doctors and parents continue to believe that a mercury-based preservative once used in some vaccines is related to an increase in autism diagnoses.

Earlier Study Sparked Concerns

Since the 1971 introduction of MMR, measles, mumps and rubella -- which can be life-threatening -- have become rare in most developed countries. Measles disappeared entirely in the United States in 1993.

Although effectiveness studies have focused on the single vaccines rather than on the combined injection, "we have no reason to doubt the effectiveness of MMR," say the authors of the review, led by Dr. Vittorio Demicheli, an Italian epidemiologist.

Concerns about the safety of MMR arose in 1998, when The Lancet published a study linking the injections with the onset of autism and Crohn’s disease -- an inflammation of the gastrointestinal system -- in 12 children. In the United Kingdom, the study triggered a collapse in vaccination rates and subsequent measles outbreaks.

Since that time, intensive studies of MMR safety have been conducted around the world, and they have not confirmed the link between MMR and autism or bowel disease. A majority of the original Lancet report's authors published a formal retraction in 2004.

The new comprehensive review of studies adds to the weight of evidence supporting the safety and efficacy of MMR. "MMR remains an important preventive global intervention," conclude the authors.

Short-Term Effects

Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

Demicheli and coauthors analyzed 139 studies of children up to age 15, eventually selecting 31 of the highest-quality reports for review. Ten of these focused on long-term adverse effects and ranged in size from several hundred participants to more than 500,000. They took place in the United States, United Kingdom, Denmark and Finland.

The review confirmed that the combined vaccine may cause expected short-term effects -- such as irritability, fever, rash and joint stiffness. Very rarely, more serious side effects, including bleeding disorders and seizures, may occur. Nevertheless, "no credible evidence of an involvement of MMR with either autism or Crohn’s disease was found," say the authors.

The quality of the studies was not perfect, concedes Demicheli. Cochrane reviews generally focus on studies that compare an experimental group to a control group that receives different treatment.

When broad vaccination programs have been underway for many years, there are no people to serve as a comparison group. However, "if you keep finding consistent results across different settings, across different study designs, it's reasonable that what you have found is true," he contends.

In the United States, the vaccine is normally given twice between the ages of 1 and 6 years.

One Infection at a Time?

Although the vast majority of mainstream medical organizations and professionals seem convinced by the accumulated epidemiological evidence, some still maintain that an MMR-autism connection exists. One of the more vocal is Dr. F. Edward Yazbak, an American pediatrician and grandfather of an autistic boy.

Yazbak believes that a small percentage of genetically predisposed infants are unable to detoxify mercury-containing preservatives once used in first-year vaccinations and were thus injured by a blow to their immune system by MMR. "Our body is made to deal with one infection at a time," he notes.

Although mercury has now been removed from pediatric vaccines, Yazbak remains convinced that current inoculation recommendations carry unwarranted risks. He promotes "reasonable vaccination," recommending that single vaccines for measles, mumps and rubella be available at no additional cost.

But giving the vaccinations at different times could leave people exposed to the diseases, according to the US National Immunization Program. For instance, if the rubella vaccine were delayed, some children could contract the disease and transmit it to pregnant women. "Ironically, infection of pregnant woman with 'wild' rubella virus is one of the few known causes of autism," according to the organization's website.

Need for Improvements in Vaccine Safety Research

The MMR controversy points out the need for improvements in vaccine safety research, say the Cochrane reviewers. "It is impossible to make all the evaluations before the vaccine is in current use," says Demicheli.

To explore new hypotheses retrospectively, he says, researchers need detailed data collected in high-quality information systems. "We need well-organized vaccination registers with standardized definitions of adverse events."