Americans eligible for a second COVID booster shot—including those 50 and older and the immunocompromised—may want to consider waiting before getting a fourth jab, the U.S. Centers for Disease Control and Prevention said Friday, as it launched an online tool to help individuals determine if they qualify.

Per the agency’s booster guidance website, updated Friday, those who are eligible for a second booster should consider how likely they are to get “very sick” from the virus based on preexisting health issues and potential community exposure.

“If you are eligible, can you wait?” asks the guidance, urging those who have had COVID-19 in the past three months or who feel “that getting a second booster now would make you not want to get another booster in the future” to consider holding off.

“A second booster may be more important in the fall of 2022, or if a new vaccine for a future COVID-19 variant becomes available,” the guidance states.

Individuals ages 50 and older, those 12 and older who are immunocompromised, and those who received two doses of Johnson & Johnson’s Janssen vaccine are eligible for a second booster shot, according to the CDC.

Updated guidance encourages those eligible to think again based on “the benefits and risks of a second booster,” but does not expound. The CDC did not respond to questions from Fortune on Friday as to what the potential risk and benefits are of a second booster, in particular, or whether there are concerns about the continued efficacy of current vaccines, especially in light of immune-evading Omicron subvariants.

It also failed to respond to questions about potential supply concerns, given a lack of renewed COVID funding by Congress. The government is running out of money to fund its efforts to combat the virus, and a $22.5 billion March ask by the Biden administration to fund, among other things, boosters and variant-specific vaccines, was pared down to $10 billion. But it’s still stuck in limbo, caught up in immigration politics, The Hill reported this week.

Meanwhile, Politico reported Friday that the White House is preparing to ration COVID vaccines in light of the continued funding impasse.

“Among the sacrifices being weighed are limiting access to its next generation of vaccines to only the highest-risk Americans—a rationing that would have been unthinkable just a year ago, when the White House touted the development and widespread availability of vaccines as the clearest way out of the pandemic,” Politico’s Adam Cancryn wrote, adding that White House officials are “increasingly concluding that these types of difficult choices will soon have to be made.”

A March 15 White House memo warned that Congress’ failure to fund additional COVID efforts “will have severe consequences as we will not be equipped to deal with a future surge.”

U.S. COVID cases have risen steeply in recent weeks, and hospitalizations have also been on the rise. Earlier this month the White House warned of a coming surge this fall and winter that could more than double the amount of COVID infections the U.S. has recorded thus far.

“Without funding, the United States will not have enough additional boosters or variant-specific vaccines, if needed, for all Americans,” the March White House memo warned.