Late Thursday, Moderna Inc requested emergency use authorization from US health regulators for a second COVID-19 booster shot, as an increase in cases in some parts of the world fuels fears of another pandemic wave.

The US biotechnology company stated that its request covered all adults over the age of 18 to allow the US Centers for Disease Control and Prevention (CDC) and health care providers to determine the appropriate use of an additional booster dose of its vaccine, including for those at increased risk of COVID-19 due to age or comorbidities.

Moderna's request is significantly more expansive than Pfizer Inc and its German partner BioNTech SE's application to US regulators earlier this week for a second booster shot for people 65 and older.

Without commenting specifically on the efficacy of a fourth shot, Moderna stated that its submission was based in part on data recently published in the United States and Israel in response to the emergence of the omicron variant.

The FDA did not immediately respond to a request for comment from Reuters.

Health officials in the United States, including top infectious disease expert Dr. Anthony Fauci, have raised the possibility of a fourth shot, primarily for older adults, and preparing for another outbreak.

According to the CDC, vaccine efficacy diminishes over time, and a third shot helps restore it. However, it has not released comprehensive data on age or health status to substantiate the claim.

While COVID-19 infections are declining in the United States and much of the rest of the world, they increase in China. In the United Kingdom and Europe, the downward trend in COVID cases has reversed as economies have opened and the second variant of omicron circulates.