Despite their established benefits as public measures, vaccines continue to be treated with suspicion by many people, in the US and other parts of the world (Larson et al. 2014; Olive et al. 2018; Lazarus et al. 2020). Since the success of vaccines depends on their high uptake level (Anderson and May, 1985; Fine et al. 2011; Fontanet and Cauchemez, 2020), identifying factors that influence low trust and decision-making in relation to vaccines is essential in order to combat diseases such as the novel Coronavirus (COVID-19).

The investigators study factors that could potentially influence public's trust in COVID-19 vaccines through a large-scale online field experiment. The investigators conduct an online survey of 32,400 subjects in nine countries (USA, Brazil, Mexico, China, India, Indonesia, Russia, Germany, and UK).

The investigators study how willingness to receive the COVID-19 vaccine is affected by (1) the "novelty" of the vaccine technology (conventional vs. RNA vaccines), and (2) the adoption rate of the new vaccine in the country. That is - the impact of controversial science and the force of conformity on the rates of adoption. The latter will also allow us to calculate the "tipping point" adoption rate for each country that will allow the country to achieve herd immunity from COVID-19.

The investigators have four hypotheses, below.

H1 (Conformity): People are more willing to receive a vaccine as the cumulative adoption rate in their community increases.

H2 (Novelty): People are less willing to get a COVID-19 vaccine that uses the new RNA technology, compared to a conventional vaccine

H3 (Interaction between H1 and H2): As the cumulative adoption rate in a community increases, the difference between people's willingness to adopt conventional rather than RNA vaccines decreases.

H4 (Tipping Point): Each country will have a different "tipping point". This is the cumulative adoption rate after which unvaccinated people are significantly more willing to get the vaccine. Countries that have a higher "honesty index" will have the tipping point appear at a lower cumulative adoption rate.

Please note that this study is not a clinical trial. This study is a randomized controlled trial in the form of an online survey.

The investigators study the role of trust in COVID-19 vaccine through a global survey. Specifically, the investigators vary (1) the degree of vaccine innovation and (2) the adoption rate of the vaccine to test if these factors influence the individuals' willingness to receive the vaccine.

An online survey is conducted across 9 countries with 3,600 subjects per country. About 15 questions will be asked for each subject and the survey is expected to take about 10-15 minutes to complete.

The 9 countries are USA, Brazil, Mexico, China, India, Indonesia, Russia, Germany and the UK. The three most populous countries from three major continents (Americas, Asia and Europe) are chosen. The target subjects are healthy adults over 18 years of age.

The survey instrument will consist of two parts. In the first part, the survey will ask 10 questions regarding their demographics and the general perception on the safety, effectiveness, and importance of vaccines. This part of the survey allows us to explore the relationship between the individual factors (such as age, gender or income status) and trust in COVID-19 vaccine, as well as to normalize the attitude towards COVID-19 vaccines over the overall vaccine-related confidence.

The 3 key questions of interest are:

"Suppose that a conventional vaccine for COVID-19 receives approval for human use and is available for the public.

A3. How much do you think the government should subsidize the conventional COVID-19 vaccine? [10%, 25%, 50%, 75%, 100%]

A4. If the conventional COVID-19 vaccine were provided to you for free, how likely are you to accept the vaccination? [Very likely, somewhat likely, neither likely nor unlikely, somewhat unlikely, very unlikely]

A5. Suppose the conventional COVID-19 vaccine is endorsed by your Government, free, but no one in your country has received the vaccine. How likely would you be willing to be vaccinated with this new vaccine? [Very likely, somewhat likely, neither likely nor unlikely, somewhat unlikely, very unlikely]"

The investigators manipulate two dimensions of information between the subjects. The two dimensions are:

• 2 vaccine types: 50% of subjects will be asked for "conventional vaccine"; 50% of subjects will be asked for "RNA vaccine"
• 5 different levels of adoption rate in the country: 0% (i.e., no one in your country has received the vaccine), 20%, 40%, 60%, and 80%

Within each country, there will be 2 (vaccine type) x 5 (different adoption rates) = 10 between-subject treatments.

In the second part of the survey, the investigators ask participants to complete a simple economic dice-rolling task, and predict outcomes of the same task in their own country. The investigators will study how individual performance in these tasks correlate with their trust in COVID-19 vaccines.

Inclusion Criteria:

• Healthy adults over 18 years old

Exclusion Criteria:

• None

Investigators will not be storing or sharing any personal identifiers. All individual level data will be anonymized, and only anonymized data will be shared with other researchers, upon request.

After completion of all analyses. It will be made available in the supporting documentation.

It will be made available in the supporting documentation.