By Katherine Dillinger and Brenda Goodman, CNN

Only about 17% of eligible Americans have gotten the updated Covid-19 boosters.

The US Centers for Disease Control and Prevention updated its recommendations on the Covid-19 vaccines on Wednesday to allow another dose of the bivalent booster for people who are 65 and older or who have weakened immune systems and who “want the option of added protection” against the coronavirus.

The move aligns with Tuesday’s US Food and Drug Administration actions to allow these groups to get additional booster doses ahead of the fall vaccination campaign. On Wednesday, members of CDC’s Advisory Committee for Immunization Practices met to discuss the changes and expressed their support for them, although the committee did not vote.

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Monovalent mRNA vaccines, which protect only against the original strain of the coronavirus, will no longer be recommended in the United States, the CDC says. The updated bivalent shots from Moderna and Pfizer/BioNTech protect against the original strain as well as the BA.4 and BA.5 subvariants of the Omicron variant.

The CDC now recommends the updated vaccines for everyone 6 and older, even if they didn’t finish the two-dose series of the original shots.

People who are 6 and older who have already gotten a dose of the updated vaccine do not need more shots now unless they’re 65 and older or have a weakened immune system, the agency says.

The updated boosters have been available in the US since September under emergency use authorizations, or EUAs, which restrict how they may be given. The FDA amended these EUAs on Tuesday to allow certain groups to get an additional dose ahead of other people.

The FDA also urged anyone who hasn’t gotten their first dose of the bivalent booster to do so. Only about 17% of eligible Americans – fewer than 1 in 5 – has gotten a recommended booster dose.

As time has passed, many adults with reduced immune function because of their age or an underlying health problem have been asking doctors whether they need another dose of the bivalent vaccines.

The CDC has reported early data showing that the effectiveness of the bivalent vaccines, even against emergency room visits and hospitalizations, has already started to wane.

But the agency had previously not been free to make a “permissive use” recommendation about the boosters that would allow doctors to offer additional doses to vulnerable people.

The FDA says it intends to make decisions about future vaccinations for most Americans after receiving recommendations on the fall strain composition from its advisory committee in June.