The CDC says to think again before getting a second COVID booster. Is it rationing vaccines?

The government wants you to wait on getting that fourth vaccine shot, and it won’t say exactly why.

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.