By Mrinalika Roy and Michael Erman

Dec 5 (Reuters) - Vaccine makers expressed concern on Friday's decision by a U.S. advisory panel to scrap its long-standing recommendation that all infants receive a hepatitis B vaccine at birth, a shift that public health experts fear will undermine decades of public health advances.

Merck MRK.N, whose Recombivax HB has been a staple of the U.S. childhood immunization program, said it was "deeply concerned" by the decision of the CDC's Advisory Committee on Immunization Practices (ACIP), warning it "puts infants at unnecessary risk of chronic infection, liver cancer and even death."

The company said the universal birth dose, which was instituted in 1991, has driven a 99% drop in acute hepatitis B cases in children and young adults and argued there is no evidence that delaying it provides any benefit. Infectious disease experts, as well as organizations representing pediatricians, pharmacists and public health professionals decried the move.

Hepatitis B, which can spread from mother to child during birth, can cause severe liver disease and early death, and has no cure. According to the National Foundation for Infectious Diseases, the universal hepatitis B birth dose has prevented more than 500,000 childhood infections, cut infant cases by 95% and averted an estimated 90,100 deaths.

Many of the committee members, which were appointed by U.S. Health and Human Services Secretary Robert F. Kennedy Jr., a longtime vaccine skeptic, criticized the vaccine safety data and said that the U.S. vaccine schedule was out of step with other countries, particularly Denmark, that have low hepatitis B rates.

GSK GSK.L said it stands behind the science supporting its vaccine and is awaiting the CDC's formal adoption of the recommendation to assess its impact.

Its vaccine, Engerix-B, has been approved since 1989, with 1.4 billion doses administered worldwide.

Merck and GSK shares fell about 1% each following the vote. U.S.-listed shares of Sanofi SASY.PA, another maker of hepatitis B shots, rose about 0.7%.

The panel now recommends only infants born to mothers who test positive for hepatitis B should receive the birth dose. Parents of infants whose mothers test negative are advised to decide, in consultation with a healthcare provider, when or whether to begin the vaccine series.

Merck urged the committee to return liaison organizations and frontline clinicians to its work groups, calling discussions led by medical and scientific experts "essential to informing sound, evidence-based recommendations that safeguard public health."

(Reporting by Mrinalika Roy in Bengaluru; Editing by Alan Barona)

((mrinalika.roy@thomsonreuters.com))