Beliefs about and confidence in new medical treatments: Lessons from COVID-19 vaccines, the follow-up trial

Last registered on February 21, 2023


Trial Information

General Information

Beliefs about and confidence in new medical treatments: Lessons from COVID-19 vaccines, the follow-up trial
Initial registration date
February 13, 2023

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published
February 21, 2023, 10:08 AM EST

First published corresponds to when the trial was first made public on the Registry after being reviewed.



Primary Investigator

The University of Tokyo

Other Primary Investigator(s)

PI Affiliation
The University of Tokyo
PI Affiliation
University of Tsukuba

Additional Trial Information

In development
Start date
End date
Secondary IDs
Prior work
This trial is based on or builds upon one or more prior RCTs.
1. Background and study aims.
Public health depends on both the technological developments of medical treatments and the public's willingness to take up these treatments. The public’s willingness to take up novel treatments depends on the public's beliefs about the costs and benefits of the treatments. By tracing updates on public beliefs about COVID-19 vaccines before and after they began to be administered to the general public in Japan, we investigate the relationships between the evolution of public beliefs about COVID-19 vaccines and the public willingness to take up the vaccines. This study is a follow-up study of AEARCTR-0010145 ( implemented in November 2022.

2. Who can participate?
We recruit a non-probability sample of 15,000 respondents in Japan through a survey company, Rakuten Insight, Ltd.

3. What does the study involve?
1) We ask whether respondents have taken COVID-19 vaccines and, if so, whether they experienced side effects.
2) We ask whether the respondents have taken a second, third, and fourth doses of COVID-19 vaccines and whether they want to take a further dose of COVID-19 vaccine.
3) We ask the respondents to choose the preferred hypothetical conditions for vaccination between the two, which are fully randomly generated by our conjoint experimental design. We allow the respondents to answer to deny either condition and not to take up COVID-19 vaccines.
4) We ask the respondents questions about their demographic and socioeconomic backgrounds.

4. What are the possible benefits and risks of participating?
As a benefit, respondents will receive a certain number of points to be used for shopping. Since the survey is administered through the internet, respondents will face no substantial risk.

5. Where is the study run from?
The survey will be conducted through the internet.

6. When is the study starting and how long is it expected to run for?
We plan to begin the survey on Februrary 27, 2023 and continue until we obtain responses from 15,000 respondents approximated on March 13, 2023. The duration is expected to be two weeks.

7. Who is funding the study?
The Japan Society for the Promotion of Science funds the study (SPS Grant-in-Aid KAKENHI JP18H03630).
External Link(s)

Registration Citation

Fukai, Taiyo, Keisuke Kawata and Masaki Nakabayashi. 2023. "Beliefs about and confidence in new medical treatments: Lessons from COVID-19 vaccines, the follow-up trial." AEA RCT Registry. February 21.
Sponsors & Partners


Experimental Details


We investigate whether belief updates about the costs and benefits of medical treatments affect confidence in medical treatments, taking an example from vaccinations for COVID-19. Testable hypotheses are:
1) Unexpected side effects, which are part of the costs, affect confidence in vaccination.
2) Changes in the financial and time costs of vaccination affect willingness to take up vaccination.

Our fully randomized conjoint design generates various conditions for taking up COVID-19 vaccines. Since attributes of the hypothetical conditions are fully randomized, we can identify a change in attributes and a change in willingness to take up COVID-19 vaccines as a causal effect. Additionally, we investigate whether the causal channel is affected by past experience of "COVID arm" symptoms.
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
Our primary outcome measures are self-reported subjective cost of/confidence in vaccination. In addition, we also measure self-reported history of taking up COVID-19 vaccines.

Additionally, we measure any impact of side effects on beliefs about vaccines, and through them, confidence in vaccination, and how long the change in confidence in vaccination sustains. Our previous study demonstrated that experiencing symptoms of "COVID arm" after a first or second dose of COVID-19 vaccine adversely affected confidence in vaccination and willingness to take a third dose of COVID-19 vaccines. If the causal effect from belief updates to confidence in vaccination is long-lived, the adverse effects are expected to also be observed in this round. If the adverse effects of "COVID arm" symptoms by March 2022 are not observed or attenuate in November 2022, Februrary to March 2023, the effects of belief updates are considered to have been short-lived, and the risk of once unexpected side effects are considered to have been factored in.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
Furthermore, we generate hypothetical vaccination conditions under which the costs of taking up vaccines vary and ask respondents which conditions are preferred. Additionally, if the adverse effects of belief updates about “COVID arm” are long-lived and related to other dimensions of costs for vaccination, the results of our conjoint experiment will be associated with experiences of “COVID-19 arm” symptoms.
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
This study, along with the study implemented in November 2022 (AEARCTR-0010145 is the final stage of a longitudinal interventional study that began in the first wave in February 2021, when vaccines for COVID-19 were not available to the general public in Japan, followed by the second wave in March 2022, when COVID-19 vaccines had been available to the general public.

We have asked whether respondents had experienced the delayed localized hypersensitivity reactions to COVID-19 vaccines, which are referred to as "COVID arm" and were mostly unexpected to most of the public, conditional on vaccines take up, as the vaccines were novel. We use the "COVID arm" symptoms as our first treatment that could have updated the beliefs about the costs of medical treatment.

1) In this wave, we ask respondents whether they took a third dose of COVID-19 vaccine and investigate whether the probability of taking the third dose was associated with having experienced the "COVID arm" symptoms after taking up first or second dose of a COVID-19 vaccine.
2) Additionally, we design a fully randomized conjoint experiment to generate two hypothetical conditions of vaccination against COVID-19, where we allow respondents to accept or decline vaccines, and we ask respondents under which conditions they might more likely take up COVID-19 vaccines.
Experimental Design Details
Randomization Method
Randomization will be done in the office by a computer.
Randomization Unit
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
15000 individuals.
Sample size: planned number of observations
Since we assign 5 conjoint experimental tasks to each indvidual, we plan to obtain 15,000X5=450,000 observations.
Sample size (or number of clusters) by treatment arms
Since our treatment is a fully randomized conjoint design, all 15,000 respondents are included in the treatment arms.
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Institutional Review Boards (IRBs)

IRB Name
Ethical Review Board, Institute of Social Science, The University of Tokyo
IRB Approval Date
IRB Approval Number


Post Trial Information

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Is the intervention completed?
Data Collection Complete
Data Publication

Data Publication

Is public data available?

Program Files

Program Files
Reports, Papers & Other Materials

Relevant Paper(s)

Reports & Other Materials