Kids who go to daycare or attend playgroups are 30 per cent less likely to develop the most common form of childhood leukemia than those who do not, finds a new study.

The research is being presented Monday at the second Children with Leukemia Conference in London.

Researchers at the University of California, Berkeley reviewed data on 6,108 children with acute lymphoblastic leukemia and 13,704 who did not have the disease. The studies included interviews with parents about their children's attendance at daycare or participation in playgroups. The length of time in daycare, and the extent of the children's exposure to other kids were documented.

Twelve of the 14 studies included in the Berkeley analysis found that kids in daycares and with higher exposure to other children were less likely to develop leukemia than those who were more sheltered. Two studies found no connection between increased social interaction and fewer cases of leukemia.

"Combining the results from these studies together provided us with more confidence that the protective effect is real," said Dr. Patricia Buffler, professor of epidemiology at Berkeley's school of public health, in a release. "Analyzing the evidence in this way gives a more reliable answer to the question and a more precise estimate of the magnitude of the effect."

According to data cited in the study, scientists think that childhood leukemia is the result of a genetic mutation in the womb followed by a second trigger in childhood such as an infection — or its timing.

The authors believe that children exposed early in life to infections may develop immunity to the disease. They theorize that if a child's immune system is not forced to defend itself, it may overreact to infections encountered later in childhood, leading to the development of leukemia.

Acute lymphoblastic leukemia affects 1,091 children between the ages of 0 and 14 in Canada per year, according to the Canadian Cancer Society. It accounts for 80 per cent of all childhood leukemia cases.

Children between the ages of two and five are most at risk of developing the disease.

Leukemia is a disease of the blood-forming tissue known as bone marrow. It affects white blood cell production, the body's defense system.