Experts say mixing and matching different two-dose COVID-19 vaccines is likely safe and effective, but data still needs to be collected to be sure. COVID-19 shots are all designed to stimulate your immune system to produce virus-fighting antibodies, though the way they do so varies. To check that mixing them would be OK, scientists are testing combinations of various two-dose COVID-19 vaccines. Limited data so far suggests an AstraZeneca shot followed by a Pfizer shot is safe and effective, and comes with an elevated likelihood of side effects. That’s likely because mixing and matching vaccines can sometimes produce a stronger immune response.