Moderna has received reports of accidental underdosing of the Spikevax bivalent booster vaccines, where a 0.25 mL dose (equivalent to 25 µg) was administered instead of 0.5 mL (50 µg). In most cases, underdosing was due to dose confusion, since the booster dose volume for the original monovalent Spikevax vaccine used earlier in 2022 was 0.25 mL (equivalent to 50 µg). • Spikevax bivalent booster vaccines have recently been approved by MHRA for use in individuals 18 years of age and older. The correct dose is 0.5 mL (50 µg).