Subscale and included items	Scale / response options	Cronbach’s alpha
PAPM STAGE •Which of the following best describes your thoughts about having a COVID-19 booster vaccine, once Public Health authorities recommend you have one?	“I’ve not yet thought about having a booster vaccine” (Stage 2); “I’m not yet sure about having a booster vaccine, but will probably (not) have it” (Stage 3); “I’ve decided I don’t want to have a booster vaccine” (Stage 4); “I’ve decided I do want to have a booster vaccine” (Stage 5); and “I have had the booster vaccine” (Stage 6)	n/a
PREVIOUS COVID-19 VACCINE EXPERIENCE
Previous COVID-19 vaccine: Overall experience •Having the vaccine was ... painful / tolerable •I believe the vaccine was ... worthless / valuable •After having the first COVID-19 vaccine, I experienced … strong side effects / no side effects •After having the second COVID-19 vaccine, I experienced … strong side effects / no side effects •Overall, I found having the vaccine was … a terrible experience / a good experience	5-point scale	0.70
Previous COVID-19 vaccine: Healthcare environment •The staff were respectful •The staff were friendly •The staff were helpful •My care was well organized •My privacy was respected •The waiting area was comfortable	5-point scale (not at all – very much)	0.87
Previous COVID-19 vaccine: Patient-provider connection •I trusted them •They paid attention to my individual needs •They gave me enough information •They gave me support and encouragement	5-point scale (not at all – very much)	0.91
Previous COVID-19 vaccine: Access •Booking my second COVID-19 vaccine was easy •Getting to the place where I had my second COVID-19 vaccine was easy •I was happy with the venue where I received my second dose	5-point scale (strongly disagree – strongly agree)	0.79
Previous COVID-19 vaccine: Perceived benefits •My health is better protected now that I have been vaccinated •I have done all I can to protect my family and friends now that I have been vaccinated •I am less nervous, anxious, or on edge now that I have been vaccinated	5-point scale (strongly disagree – strongly agree)	0.68
PSYCHOLOGICAL CONSTRUCTS
HEALTH BELIEF MODEL
Perceived severity •Despite already being fully vaccinated (have received at least two doses), I believe I would be very sick if I got COVID-19	5-point scale (strongly disagree – strongly agree)	n/a
Perceived susceptibility •There is a high chance I will come into contact with a person with COVID-19 over the next few weeks •I believe I will still have good immunity to COVID-19, six months after having the vaccine •I believe my immune system is strong enough to protect me against COVID-19	5-point scale (strongly disagree – strongly agree)	†0.33
Perceived benefits •A COVID-19 booster vaccine will protect me against catching COVID-19 •A COVID-19 booster vaccine would reduce the severity of a possible future COVID-19 infection •If I have a COVID-19 booster vaccine, I would be less likely to spread COVID-19 to others •A mass COVID-19 booster vaccine programme will protect the vulnerable from catching COVID-19 •A mass COVID-19 booster vaccine programme, will help protect the NHS •A mass COVID-19 booster vaccine programme could prevent the re-introduction of protection measures such as social distancing	5-point scale (strongly disagree – strongly agree)	0.87
Perceived safety •I believe that an approved COVID-19 booster vaccine will be very safe •I’m NOT concerned about the possible side effects of a COVID-19 booster vaccine	5-point scale (strongly disagree – strongly agree)	0.70
THEORY OF PLANNED BEHAVIOUR
Vaccine attitudes •Having a COVID-19 booster vaccine would be beneficial •Having a COVID-19 booster vaccine would be tolerable	5-point scale (strongly disagree – strongly agree)	0.78
Subjective norms •My family would expect me to have a COVID-19 booster vaccine •My GP would expect me to have a COVID-19 booster vaccine •‡My employer would expect me to have a COVID-19 booster vaccine •My family and friends have said they would get the COVID-19 booster vaccine once it's available to them	5-point scale (strongly disagree – strongly agree)	0.75 (3 items) / 0.79 (4 items)
Perceived control •I feel in total control as to whether I will have a COVID-19 booster vaccine	5-point scale (strongly disagree – strongly agree)	n/a
Anticipated regretHow much would you regret that you did not get a COVID-19 booster vaccine if it was recommended you have one? •You were hospitalised and admitted to the intensive care unit as a result of a COVID-19 infection •You caught COVID-19 and passed it on to a friend •You caught COVID-19 and passed it on to a family member	5-point scale (not at all – a great deal)	0.97
OTHER FACTORS
Vaccine knowledge •I know enough about the safety of a COVID-19 booster vaccine, to make an informed decision about whether or not to get the vaccine booster •I know enough about how the COVID-19 booster vaccine will help reduce the spread of the virus •I know enough about how effective the COVID-19 booster vaccine will be, to make an informed decision about whether or not to get the vaccine booster	5-point scale (strongly disagree – strongly agree)	0.89
Trust •Information from the Government about the COVID-19 booster vaccine can be trusted	5-point scale (strongly disagree – strongly agree)	n/a
Fear of needles •I am scared of needles	Yes / no / don’t know	n/a