This megastudy is a massive randomized controlled trial. By randomizing participants to 10 different intervention conditions simultaneously, the investigators will be able to compare the effectiveness of different interventions to one another and to a control group (in which individuals will only receive the usual communications from their partner organization) to identify which interventions significantly increase vaccination rates. Pharmacy customers will be randomly assigned to receive one of the interventions designed by team scientists to encourage vaccination or to a control group. The baseline intervention will be based on the top-performing SMS intervention identified in the investigators previous megastudies on encouraging vaccination (Milkman et al., 2021b, 2022). Pharmacy customers will receive SMS messages conveying that a COVID booster vaccine is reserved or waiting for them at the pharmacy. Additional behavioral science messaging strategies will be tested by building off of this baseline intervention.

Inclusion Criteria:

• Patient of a large retail pharmacy that has opted into receving SMS messages from the pharmacy and has received their primary COVID vaccination series.

Exclusion Criteria:

• The patient unsubscribed from texts before the send date and time of the patient's SMS/MMS message
• The patient received a bivalent COVID booster before the send date and time of the patient's SMS/MMS message.

As treated participants will receive text messages, there is no scope for blinding. Care providers will not be made aware of subjects' participation in the study, or assigned treatment arms. The study team will only receive data on subjects' assigned arms and outcomes at the end of the study.