The Centers for Disease Control and Prevention (CDC) of the the United States are planning a wide-ranging study that will examine the possible link between vaccines and autism, two sources with knowledge of the issue told Reuters.

It is not clear whether U.S. Health and Human Services Secretary Robert F. Kennedy Jr. who has long promoted skepticism over some vaccines, is involved in the planned CDC study. The CDC and the Health Department were not immediately available for comment.

The CDC’s decision was made amid one of the largest measles outbreaks the U.S. has experienced in the past decade, with more than 150 cases and two deaths in Texas and New Mexico. The outbreak appears to have been fuelled by declining vaccination rates in areas of the US where parents have been convinced that vaccines are doing more harm than good to their children.

Kennedy, whose responsibilities include oversight of the CDC, has long expressed doubts about the safety of the triple vaccine for measles, mumps and rubella (MMR). At last week’s cabinet meeting, Kennedy initially downplayed the news that a school-age child had died of measles in Texas – the first such death in a decade – calling such outbreaks commonplace.

Over the weekend, Kennedy published an opinion piece on Fox News promoting the role of vaccination, but also told parents that vaccination is a personal choice and urged them to consult their doctor.

Autism diagnoses have increased significantly in the US since 2000, heightening public concern. Many researchers attribute this increase to the fact that medical testing is now more widespread, but it has also broadened the range of behaviors considered indicative of the condition. However, some are also arguing that vaccines are to blame, citing a 1990s study, now refuted, by British researcher Andrew Wakefield. That study linked an increase in autism cases to the use of the triple childhood vaccine.

The causes of autism are unclear. No study has found a link between autism and vaccines or other drugs. Scientists speculate that the neurological features of autism occur during the period when the fetus’s brain is developing in the womb. Studies link autism to maternal health problems during pregnancy or complications during childbirth.

In his speech to Congress this week, US President Donald Trump addressed the rising cases of autism in children. “Well, we’re going to find out what’s going on and no one is better than Bobby (the health secretary) and his colleagues to do that,” he said.