So says Patricia Buffler, PhD, of the University of California at Berkeley. She and her colleagues reviewed 14 studies on leukemia and kids' social contacts with other children, including at day care and in play groups.
Together, the studies included about 6,100 kids with leukemia and 13,700 children without leukemia. The kids' parents answered questions about the children's social exposure to other kids.
"We calculated an overall estimate of effect, which suggests that the reduction in risk may be as high as 30% [and] with the better studies as high as 40%," Buffler tells WebMD.
These findings show that "early social contacts, as estimated from day care and other settings, appear to be consistently and significantly with a decreased risk of childhood leukemia," says Buffler, who presented the findings today in London at the Causes and Prevention of Childhood Leukemia conference.
"More research needs to be done to establish this, but it's been proposed that the earlier the child is exposed to a variety of infectious agents, the better the immune system is primed," Buffler explains. Exposure to other kids at day care and in play groups provides opportunity for common childhood infections, which may help the immune system respond more effectively, according to this theory.
"I think the earlier the child is exposed, the better," Buffler says. "The peak age for acute lymphoblastic leukemia [the most common type of leukemia in young children] is 2-5 years of age, so the exposures of interest would take place prior to that."
The pattern Buffler's team noted wasn't just about day care. "We looked at all types of social contact, not just day care, and all types of social contact were found to be protective," says Buffler. She adds that the pattern was weaker for kids with many siblings because by being around their brothers and sisters, those kids had plenty of contact with other children even if they didn't go to day care.
There are three things to keep in mind about Buffler's review.
First, the reviewed studies were observational, so they don't prove that social contacts prevent childhood leukemia. "These types of studies can only point to or provide clues about what might be involved -- in this instance, infection and a disregulated immune system," says Buffler.
Second, the theory about infection and leukemia risk hasn't been proven. The reviewers can't promise that social contact prevents childhood leukemia, and they're not blaming childhood leukemia on insufficient social contact.
Third, leukemia is rare among kids. It occurs in about one of every 29,000 U.S. children per year, according to the National Cancer Institute. Though a 30% or 40% drop in the relative risk of leukemia may sound large, the overall risk of developing leukemia is still low.
Still, "the epidemiological data are fairly consistent and hopefully will stimulate more research on what mechanisms might be involved," says Buffler.