Moderna announced Thursday it had asked the United States drug regulator for emergency authorization for a second booster shot of the company's COVID-19 vaccine for all adults.

The request to the US Food and Drug Administration (FDA) would "allow for a fourth dose of our #COVID19 vaccine in adults 18 years of age and older who have received an initial booster" of any approved COVID jab, Moderna said on Twitter.

The request comes days after Pfizer-BioNTech, makers of the other COVID mRNA vaccine, also requested emergency approval for a second booster shot, but their request was limited to adults aged 65 and older.

Moderna said in a separate statement that its request had a broader scope so the Centers for Disease Control and Prevention (CDC) and health care providers could have "flexibility" in determining appropriate use of the second booster, such as for seniors or immunocompromised people.

The request was based partly on recent data from the United States and Israel about how well the Moderna shot protected against the Omicron variant of the coronavirus.

Pfizer also based its request off two Israeli studies, which it said in a press statement show "an additional mRNA booster increases immunogenicity and lowers rates of confirmed infections and severe illness."

The first Israeli study showed that "rates of confirmed infections were 2 times lower and rates of severe illness were 4 times lower among individuals who received an additional booster dose," compared to those who only had one.

The analysis was limited to people age 60 and older who received their second booster four months after their first.

The second study—an analysis of Israeli health care workers 18 and up—showed that antibody levels in those who received a second booster were significantly higher than those who did not.

Since the initial regimen of both the Pfizer-BioNTech and Moderna vaccines is two doses, a second booster would be most individuals' fourth jab.

Recent studies have offered evidence that while a third mRNA vaccine dose raises antibody levels above those of the initial regimen, a fourth dose only returns individuals' levels to that same highly-elevated level.

© 2022 AFP