U.S. health regulators cleared a second COVID booster shot for older adults this week, as public health experts are concerned another wave of infection could result from an even more contagious mutated strain of the already highly transmissible Omicron variant.

The U.S. Food and Drug Administration (FDA) authorized fourth doses of either Pfizer (PFE  )-BioNTech (BNTX  ) or Moderna (MRNA  ) vaccines on Tuesday for adults age 50 years and older, as well as a fifth dose for younger individuals with compromised immune systems. People 12 and older with weakened immune systems are now eligible for a fifth Pfizer dose, while those 18 and older with the same conditions are eligible for Moderna.

The U.S. Centers for Disease Control and Prevention (CDC) quickly endorsed the FDA's decision, also recommending that all adult who received two doses of Johnson & Johnson's (JNJ  ) vaccine get third shots of either Pfizer or Moderna vaccines. The CDC ruled that adults who received the J&J shot and a booster of Pfizer or Moderna are not eligible for a third dose, unless they are age 50 and older, or have compromised immune systems.

All of the new booster shots are to be administered at least four months after their last dose, both agencies ruled.

Dr. Rochelle Walensky, director of the CDC, said in a press statement that receiving an additional booster dose is "especially important" for those age 65 and older and those age 50 and older with underlying medical conditions as " they are the most likely to benefit from receiving an additional booster dose at this time." Walensky added that the agency, alongside the FDA and other public health authorities, as evaluating the possible need for additional shots for all Americans.

In a rare move, the FDA and CDC both made this decision without holding a meeting of their respective advisory committees. The authorizations come just two weeks after Pfizer and Moderna asked the FDA to permit a second booster based on data from Israel, CNBC reports. In that study, scientists found the death rate due to Omicron infection was 78% lower in those age 60 and above who received a fourth vaccine dose compared to those who have only received three.

The FDA's vaccine advisory committee is scheduled to meet in early April to discuss broader booster shot use.

Dr. Peter Marks, the FDA's top vaccine official, said earlier this week that fully vaccinated Americans may need another booster shot this fall as the immunity levels provided from earlier doses is expected to wane as transmission of the virus increases in the colder months.

"It would not be surprising if there is a potential need for people to get an additional booster in the fall along with a more general booster campaign if that takes place," Marks told reporters on Tuesday, quoted by CNBC. "It may be that a decision is made that rather than the vaccine we currently have, which are called vaccine against the prototype virus, that we will move to a vaccine that is either against one of the variants or something else."