Vaccines for measles, mumps, rubella, and varicella (chickenpox) are highly effective and do not cause autism, say researchers who reviewed 138 studies that included 23 million children.

"In terms of safety, we know from previous studies all around the world that the risks posed by these diseases far outweigh those of the vaccines administered to prevent them," said lead author Dr. Carlo Di Pietrantonj. "In this review, we wanted to look at evidence for specific harms that have been linked with these vaccines in public debate — often without rigorous scientific evidence as a basis."

The infectious diseases can cause serious illness, disability and death. Measles is a leading cause of childhood death worldwide. Rubella poses a serious risk in pregnancy because it can cause miscarriage or harm the fetus.

The MMR (measles, mumps, rubella) vaccine protects against all three infections. There's also a combined MMR and varicella vaccine (MMRV), or the chickenpox vaccine can be given separately at the same time (MMR+V).

The new study was published April 20 in the Cochrane Review.

"Overall we think that existing evidence on the safety and effectiveness of MMR/MMRV/MMR+V vaccines supports their use for mass immunization," Di Pietrantonj said in a Cochrane news release. He's with Italy's Regional Epidemiology Unit SeREMI. 

For the new study, the researchers reviewed 51 studies of 10 million children that assessed the effectiveness of MMR and chickenpox vaccines, and 87 studies of 13 million children that assessed risks posed by the vaccines.

One dose of vaccine was 95% effective in preventing measles, rising to 96% after two doses. The rate of cases in children who received one dose would be 0.5%, compared with 7% in unvaccinated children.

One dose of vaccine was 72% effective in preventing mumps, rising to 86% after two doses. The rate of cases in children with two doses would be 1%, compared with 7.4% in unvaccinated children.

The results for rubella and chickenpox were also encouraging. One dose of vaccine was 89% effective in preventing rubella. One study found that after 10 years, the MMRV vaccine was 95% effective at preventing chickenpox infection. If exposed to chickenpox, five out of 100 vaccinated children would catch it.

In addition, the researchers analyzed two studies with nearly 1.2 million children that examined the association between vaccination and autism. Diagnosed cases of autism were similar in vaccinated and unvaccinated children.

Two other studies with more than 1 million children found no evidence of a link between the MMR vaccines and the following diseases and conditions: encephalitis, inflammatory bowel disease, Crohn's disease, cognitive delay, type 1 diabetes, asthma, dermatitis/eczema, hay fever, leukemia, multiple sclerosis, gait disturbance, and bacterial or viral infections.

"Campaigns aimed at global eradication should assess epidemiological and socioeconomic situations of the countries as well as the capacity to achieve high vaccination coverage. More evidence is needed to assess whether the protective effect of MMR/MMRV could wane with time since immunization," Di Pietrantonj said.