I’m confused about how to correctly interpret expiration dates on vaccines.

The expiration date is the date by which the vaccine should be used. Vaccines may be used up to and including this date unless otherwise stated in the manufacturer’s product information. The expiration date is based on the assumption that the vaccine has been properly handled and that it has not become contaminated.

Some vaccines expire within a certain time after opening or after reconstitution. Multidose vials that contain bacteriostatic agents that prevent the growth of bacteria and may be used until the expiration date printed on the vial unless they become contaminated. Single-dose vials, COVID-19 vaccine multidose vials, and manufacturer-filled syringes do not contain bacteriostatic agents. Once the cap has been removed or the sterile seal has been broken on these vaccines, they should be administered. Lyophilized (freeze-dried) vaccine must be used within a specified time frame after it has been reconstituted. You may find an educational piece from Immunize.org titled Vaccines with Diluents: How to Use Them helpful. It’s available at www.immunize.org/catg.d/p3040.pdf.