began administering the first COVID-19 booster vaccines that have been updated to better match the latest circulating coronavirus strains.

The new vaccines, from mRNA vaccine makers Pfizer/BioNTech and Moderna, are bivalent, meaning that along with the original version of the coronavirus, or SARS-CoV-2, they also specifically target the BA.4 and BA.5 omicron subvariants.

In what’s being called a “fall booster ‘reset,’” people will no longer count the number of vaccine doses they’ve received.

For both Pfizer/BioNTech and Moderna, half of the mRNA in the vaccine includes the instructions used in the earlier vaccines for cells to make the spike protein of the original coronavirus strain.

The dual components are why the boosters are referred to as “bivalent.” For the same reason, you may hear the original vaccines being called “monovalent.”.

It doesn’t matter if you’ve never been boosted or if you’ve received multiple boosters already — everyone who meets the age requirements and has had their primary vaccinations can get the updated booster as long as they are two months or more out from their last COVID-19 dose.

The updated boosters are only authorized as booster shots, so they can’t be given to people as primary vaccinations.

As a result, all booster doses will be the bivalent ones — except for kids 5 to 11 years old, who are currently only eligible to receive Pfizer/BioNTech’s original booster dose for their age group.

Moderna tested this booster in about 600 adults who had received two primary doses and one original booster, and at least three months later were given a second original booster or a BA.1 bivalent booster.  In blood tests, there was a stronger antibody response a month out in those who had received the BA.1 booster against both BA.1, BA.4/5 and the original virus, as well as against a variety of other variants.

The Pfizer/BioNTech BA.1 bivalent booster was tested in a similar way, in about 600 people over the age of 55, with a median of about six months in between the booster doses.

Although similar studies are being done in people for the BA.4/5 bivalent booster now, those results aren’t in yet.

However, experiments from both companies show that mice previously vaccinated and then boosted with the bivalent BA.4/5 vaccines have higher BA.5 neutralizing antibody responses than those boosted with the original vaccine.

Moderna also challenged, or intentionally infected, vaccinated and boosted mice with BA.5 and found animals who received the bivalent BA.4/5 booster were better protected in their lungs than those boosted with the original vaccine.

An unpublished study posted as a preprint following authorization of the new boosters also found that mice given Moderna’s BA.4/5 booster had a stronger and broader antibody response compared with those given the original booster, which appeared to translate into better protection against BA.5 in the lung.

Regulators considered the abundance of data indicating the original shots are safe and effective, particularly against severe disease, and they had the clinical data from the highly similar BA.1 bivalent boosters along with the animal data.

Numerous other countries have opted to go with updated BA.1 bivalent vaccines, which do have clinical data (although the European Medicines Agency, which recommended BA.1-adapted boosters in early September, also recommended the BA.4/5 boosters on Sept. 12).

As for when the human data on the bivalent BA.4/5 booster will be available, Dr. Peter Marks, the head of vaccines at the FDA, said on Aug.

The real question is not whether the booster will increase protection — they will, scientists told us — but whether and to what degree the updated booster will be better than the original boosters.

The exact formulation found in the new boosters has not yet been tested in people, but the revised vaccines are very similar to the original vaccines that have now been given hundreds of millions of times in the U.S.

In addition, both companies tested the slightly different BA.1 omicron-specific bivalent booster in people, and found no new safety concerns.

Given the similarity in design and manufacturing process as the original vaccines, the FDA felt very confident authorizing the boosters.

Like the original vaccines, scientists do expect the updated shots will carry a small increased risk of myocarditis and pericarditis, or inflammation of the heart muscle and its surrounding tissue, particularly in young males.

Some animal studies suggest that giving an omicron-only vaccine as a first vaccine dose in animals could be detrimental, he added, but that’s not what is being given to people.

Although two months is the minimum amount of time to wait since the last COVID-19 dose before getting the updated booster, many experts, including Offit and Wherry, suggest that people wait longer than that since last being vaccinated or being infected with SARS-CoV-2.

As with other COVID-19 vaccines, the reformulated boosters will be available at pharmacies, community health centers, and some clinics and doctor’s offices.

Again, the only booster available to people 12 years and older will be the updated one, so even if it’s not advertised as being new or bivalent, that’s what you will receive.

The temporary, expected side effects of vaccination are usually on par with or only slightly worse in people getting both shots compared with those just getting a COVID-19 dose.

This year, for the first time, the CDC is preferentially recommending that people over 65 years of age get a high dose or adjuvanted flu vaccine instead of a standard flu vaccine, given evidence that those shots may work better for this group.

21, the head of vaccines at the FDA said that the agency was “only a matter of weeks away” from authorizing updated boosters for kids 5 to 11 years old and “a few months away” for children under 5

The company, she said, is currently conducting a study of primary series BA.1 bivalent vaccines and original and BA.1 bivalent boosters in children 6 months to 5 years old,  which is expected to be finished by the end of 2022

She added that Moderna was “exploring” ways of testing BA.4/5 bivalent vaccines in children for use as primary vaccinations and boosters