NEW YORK (Reuters Health) - A new study provides further evidence that the measles-mumps-rubella vaccine is not associated with an increased risk of autism.

An autistic child looks from behind his hand during a therapy session at the Stars and Rain School for autistic children in Beijing March 23, 2009. REUTERS/Jason Lee

Concerns that the MMR shot could cause autism were first raised a decade ago by British physician Andrew Wakefield, who, based on a study of 12 children, proposed that there was a link between the vaccine and bowel disease and autism.

That research has since been widely discredited, and numerous international studies have failed to find a connection between MMR vaccination and autism.

This latest study included 96 Polish children ages 2 to 15 who had been diagnosed with autism. Researchers compared each child with two healthy children the same age and sex who had been treated by the same doctor.

Some of the children had received the MMR vaccine, while others had not been vaccinated at all or had received a vaccine against measles only.

Poland has been slower to introduce the MMR than other European countries, but over the past decade, the vaccine has slowly been replacing the measles-only shot.

Overall, the study found, children who had received the MMR vaccine actually had a lower risk of autism than their unvaccinated peers. Nor was there any evidence of an increased autism risk with the measles-only vaccine.

“Parents should be convinced about the safety of MMR vaccine,” lead researcher Dr. Dorota Mrozek-Budzyn, of Jagiellonian University in Krakow, wrote in an email to Reuters Health.

She noted that the infectious diseases the MMR shot prevents can sometimes have serious complications.

Measles, for instance, can lead to pneumonia or brain inflammation, and one or two children die out of every 1,000 who contract the virus, according to the U.S. Centers for Disease Control and Prevention. Mumps can cause painful testicular swelling, brain inflammation and, in rare cases, hearing loss.

Most of the children in the current study had received either the MMR or measles vaccine, according to a report in the Pediatric Infectious Disease Journal.

Of the 96 children with autism, 8 had received no vaccine against measles, while about 41 percent had received the MMR shot and half had received the measles-only vaccine.

Among the healthy children, 55 percent had gotten the MMR shot, while 45 percent had received the measles vaccine; only one child remained unvaccinated.

When the researchers looked only at children who had been vaccinated before their autism diagnosis, they found that children who had received the MMR vaccine had an 83 percent lower risk of autism than unvaccinated children. Similarly, the measles-only vaccine was associated with a 56 percent lower risk.

When the researchers looked at children who had been vaccinated before showing any symptoms of autism, MMR vaccination was again linked to a lower risk of the disorder. The measles-only vaccine showed no effect on autism risk.

The study does not answer the question of why vaccinated children had a lower autism risk. But one possibility, according to Mrozek-Budzyn, is that some children started showing potential signs of autism, or possibly other health problems, before receiving the MMR or measles vaccine. Doctors or parents may then have avoided vaccination.

SOURCE: Pediatric Infectious Disease Journal, May 2010.