Intervention
Participants received one of four intervention messages according to their randomised group. The intervention was a short piece of written material (a message of approximately 70–140 words) designed to encourage uptake of booster doses of COVID-19 vaccines. The material was modified from a public email communication campaign disseminated by an Australian health authority in November 2021. These modifications were made by the research team, based on the current state of evidence on the types of messages that may have an effect on intention to vaccinate. The control message informed recipients of eligibility requirements for a booster dose of COVID-19 vaccine. The four intervention messages added to this message, each using a different persuasive strategy. 