With the United States facing its largest single measles outbreak in 25 years, Health and Human Services Secretary Robert F. Kennedy Jr. will direct federal health agencies to explore potential new treatments for the disease, including vitamins, according to an H.H.S. spokesman. The decision is the latest in a series of actions by the nation’s top health official that experts fear will undermine public confidence in vaccines as an essential public health tool.

The announcement comes as Mr. Kennedy faces intense backlash for his handling of the outbreak. It has swept through large areas of the Southwest where vaccination rates are low, infecting hundreds and killing two young girls. On Friday, the Centers for Disease Control and Prevention reported more than 930 cases nationwide, most of which are associated with the Southwest outbreak.

Critics have said Mr. Kennedy has focused too much on untested treatments — such as cod liver oil supplements — and offered only muted support for the measles vaccine, which studies show is 97 percent effective in preventing infection.

The decision to put more resources into potential treatments, rather than urging vaccination, could have grave consequences at the center of the outbreak.

“We don’t want to send the signal that you don’t have to get vaccinated because there’s just a way to get rid of it,” said Jennifer Nuzzo, an epidemiologist at the Brown University School of Public Health.

Scientists have already thoroughly studied various vitamins and medications as potential treatments formeasles, said Michael Osterholm an epidemiologist at the University of Minnesota.