By Julie Steenhuysen

CHICAGO (Reuters). The U.S. Centers for Disease Control and Prevention extended the availability of COVID-19 vaccine booster shots Friday to all Americans, in an effort to protect against the rapid-growing Delta variant.

The agency recommended booster shots for those 65+ or high-risk COVID patients. It also stated that they were open to choosing a new vaccine.

Today, more Americans have to choose which booster they want. These are the opinions of experts:

THE LATEST GUIDELINES

Prior CDC booster guidance had been based upon specific health and age requirements. This left many people uncertain about their eligibility.

That is what the new guidance intends to clarify. All adults aged 18 or older, who have received their second doses of Pfizer/BioNTech are eligible for this guidance. Moderna A booster shot is available for those who received the vaccine less than six months ago.

Advisors to the CDC suggested boosters for adults 50-plus. Prior guidance suggested boosters for anyone 65 or older.

A lot of people aged between 18 and 64 were curious if boosters could be granted to them under previous guidance. These allowed for individuals with serious medical conditions such as obesity, diabetes, or other chronic diseases. People at higher risk because of their occupations and living conditions were also eligible.

For adults who initially received the single-dose Johnson & Johnson (NYSE:) vaccine, guidance remains unchanged: they should receive any one of the authorized COVID-19 boosters two months later.

Which BOOSTER IS THE RIGHT?

U.S. booster guidance allows individuals to mix-and-match, whether they are using the original vaccine or another one.

Although these choices can seem overwhelming, Dr. Monica Gandhi from the University of California San Francisco tried to make it simple.

“Fundamentally, you can get whatever you want for your booster, except for something very particular: Johnson & Johnson should be followed by an mRNA (from Pfizer (NYSE or Moderna) Period

The FDA cleared a second J&J shot based on data showing increased efficacy against COVID-19 to 94%, up from 72% as a single-dose vaccine.

But a U.S.-government study of mixed booster shots found people who followed a J&J shot with an mRNA booster had significantly higher levels of protective antibodies.

Mix and match gives doctors more flexibility to recommend to patients who are at high risk of side effects that they try another vaccine. This policy also makes it possible that not all pharmacies or doctors’ offices will stock the three types of boosters.

PFIZER OR MODERNA

According to Dr. Paul Offit (an infectious disease specialist at Penn and member of FDA’s vaccine advisory board), there is strong evidence that boosters are needed for those who have received mRNA vaccines.

The decision for younger people who were initially vaccinated by an mRNA vaccine is more complex. Data presented to a CDC advisory panel on Friday showed that protection from Pfizer/BioNTech appears to wane faster than Moderna shots. However, both vaccines still prevent hospitalization and deaths.

Pfizer’s shot has 30 micrograms and its booster also contains 30. Moderna used 100-microgram shots for its original shot, however the booster has been approved in a reduced dose. Moderna’s low-dose booster isn’t yet certain if it will be as durable as the original shots.

Kathryn Edwards is a Vanderbilt University Medical Center vaccine researcher. She said that Moderna’s lower dosage may decrease the severity of side effects like fever or body aches, which are associated with higher doses.

Moderna and Pfizer vaccines have been associated with myocarditis, a side effect of heart inflammation in young men. However, data presented Friday to CDC advisors suggests that a booster dose will not increase this risk.