Some Americans should get more than one shot of the updated COVID vaccines because their age or certain health conditions make them more vulnerable to severe infections, U.S. health officials advised this week.

Six months after their first shot, people 65 and older and those who are immunocompromised should receive a second dose of the vaccines that rolled out this fall, according to new guidance approved by a vaccine advisory panel and backed by the U.S. Centers for Disease Control and Prevention on Wednesday.

Folks with weakened immune systems also have the option of getting three or more doses if their doctors deem it prudent.

"This vote allows people to make the best decisions possible to keep themselves and their loved ones safe from COVID-19," CDC Director Mandy Cohen said in a news release announcing the new recommendations. "CDC will continue to educate the public on how and when to get their updated vaccinations so they can risk less severe illness and do more of what they love."

The latest move makes it easier for those most at risk for dying or becoming very ill from COVID to get additional protection, the agency noted.

"The recommendation acknowledges the increased risk of severe disease from COVID-19 in older adults and those who are immunocompromised, along with the currently available data on vaccine effectiveness and year-round circulation of COVID-19," the CDC said in its news release. "The recommendation also provides clarity to healthcare providers on how many doses should be given per year to people who are moderately or severely immunocompromised and is meant to increase coverage of this second dose for that group."

Experts noted the new advice gives doctors the latitude to decide how many COVID shots a patient might need.

“If the person is willing to get it, great -- we now have the green light to give it to them, in no uncertain terms,” Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center in Tennessee, told the New York Times.

Despite the availability of updated shots, COVID vaccination rates have been dismal: As of Oct. 12, less than 12 percent of adults reported receiving an updated vaccine, the latest CDC data shows.

While the vaccines offer some defense against infection and the chances of hospitalization and death, immunity wanes over time.

“There’s just a greater need than ever to have a more durable strategy, vaccination-wise,” Dr. Marc Sala, co-director of the Northwestern Medicine Comprehensive COVID-19 Center in Chicago, told the Times.

COVID has proved to be a wily virus, spreading all year long and spiking unpredictably. That makes it tough to protect people, especially those with compromised immune systems. The CDC's new recommendations reflect that reality, Schaffner said.

“This is not a way we usually give vaccines,” he said. But, “we have to move into this new way of providing vaccine protection.”