Moderna asked the US Food and Drug Administration on Thursday to authorize a second booster of its COVID-19 vaccine for all adults, regardless of which booster they first received. 

On Tuesday, Pfizer and BioNTech asked the FDA to authorize a second booster of their vaccine, but only for adults 65 and older -- a much smaller segment of the population. 

Moderna said its request for all adults is designed to give health care providers and the US Centers for Disease Control and Prevention flexibility in their recommendations for a second booster. The CDC decides how vaccines roll out to the public after the FDA approves them.

The company's decision to apply for a second booster is based in part on data from the US and in Israel while the omicron variant was circulating, Moderna said. While COVID-19 vaccines continue to provide protection against hospitalization and death, omicron has evaded some vaccine protection against infection. Older adults and people with certain health conditions are also more at risk of severe breakthrough infections. 

If the FDA authorizes Moderna's request, the CDC would decide how the shots are recommended to the public and which people should seek out a second booster. 

This week's submissions from Moderna and Pfizer highlight the questions surrounding fourth shots or extra boosters. Some scientists have questioned the use of additional boosters of vaccines that were created for earlier variants and have encouraged other solutions like further development of pan-coronavirus vaccines that could work against all variants. 

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