COVID-19 vaccine makers’ requests to authorize another round of booster shots have prompted a debate splitting the pharmaceutical companies and vaccine experts who say a fourth jab is not yet needed.

Pfizer and partner company BioNTech, as well as Moderna, the makers of the two-dose mRNA vaccines, have each requested expanded use authorization from federal regulators to administer another round of booster shots to adults, bringing the total number of shots to four.