Last week, the U.S. Court of Federal Claims began hearing arguments about whether a childhood vaccine that protects against measles, mumps and rubella caused autism in a 12-year-old (Cedillo versus Secretary of Health and Human Services). Here is a look at the studies behind the controversy.
In 1998, a gastroenterologist named Dr. Andrew Wakefield, then at the Royal Free Medical School in London, examined 12 children with bowel problems. Nine had autism, a disorder that affects 1 in 150 youngsters. The parents of eight recalled that the symptoms started soon after the children received a vaccine against measles, mumps and rubella (the MMR vaccine). Wakefield postulated in an article in the Lancet that the vaccine might cause autism.
“Then he held a press conference and went a bit further,” says Rachel Casiday of Durham University in the United Kingdom, who studies peoples’ attitudes toward MMR and autism. Wakefield recommended the United Kingdom’s Department of Health provide the three vaccines separately. The department refused, due to scientific, logistical and economic reasons. Casiday said this response probably made people think the government wasn’t taking the issue seriously, and some parents of autistic children began to wonder.
Autism researcher David Mandell, of the University of Pennsylvania School of Medicine in Philadelphia, says the U.S. government made similar mistakes, with discussions about the potential link being held behind closed doors at the Centers for Disease Control and Prevention in Atlanta. “That gave off the appearance of impropriety,” he says.
In 2004, 10 of 12 coauthors of Wakefield’s paper retracted the claims in it.
Because of the public’s concern, various governments investigated the possible autism-vaccine link with more rigorously-designed studies. Researchers focused on two vaccine components: thimerosal, a mercury-based preservative, and the killed virus that provides the immunization against measles.
Scientists started with studies that looked at how many children were being vaccinated and how many new cases of autism were diagnosed, to see if there was a link between the two, says epidemiologist Craig Newschaffer of Drexel University School of Public Health in Philadelphia. A 2001 study in California, for example, found that between 1980 and 1994, the number of MMR vaccines administered every year rose about 14%, but the autism rates rose 373%. Thus, vaccine administration could not account for the rise in cases of autism.
A 2005 Japanese study of children in Yokohama between 1988 and 1996 found that MMR vaccination rates dropped over that time, but autism rates rose, even after vaccinations stopped in 1993.
But the best evidence compares rates of autism in children who received the vaccine with rates in children who did not, Newschaffer says. A 2002 Denmark study of more than 537,000 children -- 440,000 of whom had received the vaccine -- between 1991 and 1998 found the same autism risk in vaccinated and unvaccinated children. No link was found between vaccination date and onset of autistic symptoms.
Wakefield had proposed the measles virus hid in tissues and induced the immune system to cause autism. But a 2006 report found that autistic children had no more measles virus in their blood cells than other children. Also, no connection has been found between children contracting measles, and autism.
Still more studies have found that autism rates continued to rise after thimerosal was removed from vaccines.
“Those studies just kept piling up that showed no association between MMR or thimerosal exposure and autism,” Newschaffer says. To date, he says, about a dozen studies have investigated autism’s link to one or the other, and none has found one. “Among the scientific community, it’s pretty generally accepted that there is no link.”
Newschaffer says researchers can’t rule out a small percentage of highly susceptible individuals that the vaccine might affect, and so the search is still on for a possible link between either measles or mercury and the disease. The idea is not unreasonable, Newschaffer adds, but finding susceptible individuals is difficult.
Another unresolved issue is whether autism is actually increasing or physicians are getting better at diagnosing it. “It is extremely, extremely hard to tell the difference,” Newschaffer says.
Mandell says he sympathizes with parents who see their toddlers all of a sudden lose the skills they’ve acquired. “It’s human nature to search for an answer: ‘Why does this happen?’ ” he says.
But regardless of where the disease is coming from, most people still get their children vaccinated. “There wasn’t a wholesale revolt against the vaccine,” Casiday says. “And in the last few years, it’s been on the rise.” And that, she says, is good news for the public welfare.