A US judge on Monday dealt a blow to the Trump administration by blocking the government from implementing major vaccine policy changes made by health chief Robert F. Kennedy Jr.

The federal court in Massachusetts abruptly halted key components of the overhaul that vaccine skeptic Kennedy has rolled out over the past year, also staying the decisions put forth by the health secretary's hand-picked panel that makes immunization recommendations.

Under Kennedy the US Department of Health and Human Services has announced sweeping changes to the pediatric vaccine schedule, cutting the number of shots universally recommended including for diseases like flu and hepatitis A.

The health secretary additionally stacked a key immunization advisory panel (ACIP) with figures whose anti-vaccine sentiment mirrors his own, having fired the previous members of the influential group.

Several leading medical groups including the American Academy of Pediatrics and the American College of Physicians sued over the policy changes, and on Monday Judge Brian Murphy contended that the organizations were likely to succeed in court on their argument that the shifts were "arbitrary and capricious."

"There is a method to how these decisions historically have been made -- a method scientific in nature and codified into law through procedural requirements," the judge wrote.

"Unfortunately, the government has disregarded those methods and thereby undermined the integrity of its actions."

The decision blocks implementation of the recommendations made by the Kennedy-era ACIP committee.

It also said the January memo that upended the childhood vaccine schedule and reduced universal immunization recommendations had not followed proper procedure, as it bypassed the ACIP panel entirely, a move Murphy determined was unlawful.

Health and Human Services spokesperson Andrew Nixon told AFP in a statement that the department "looks forward to this judge's decision being overturned."

An HHS official told AFP a two-day meeting of the ACIP committee scheduled for this week had been postponed.

The American Academy of Pediatrics called the order "historic and welcome," saying in a statement the changes had "sowed chaos and confusion for parents and pediatricians across the country."

"This decision effectively means that a science-based process for developing immunization recommendations is not to be trifled with and represents a critical step to restoring scientific decision-making to federal vaccine policy that has kept children healthy for years."

Likely appeal
The ruling will almost certainly be appealed, opening the door to a lengthy legal roller coaster that will pit major medical organizations against the federal government, as the question of who has the final say on shaping immunization policies weaves its way through the courts.

Since Kennedy's revamped ACIP began meeting last year, they have made numerous decisions that have alarmed the medical establishment.

Prominent experts in the medical field have said the panel's advice has endangered public health and the nation's children.

For example, the panel no longer recommends that all newborns in the United States receive a hepatitis B vaccine, a highly infectious disease that can lead to incurable liver damage.

In his decision Monday, the judge wrote that "of the fifteen members currently on ACIP, even under the most generous reading, only six appear to have any meaningful experience in vaccines -- the very focus of ACIP."