Increasing the Efficacy & Diffusion of Covid-19 Messaging for Vaccination: Evidence from France

Last registered on March 14, 2022


Trial Information

General Information

Increasing the Efficacy & Diffusion of Covid-19 Messaging for Vaccination: Evidence from France
Initial registration date
February 01, 2022

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 01, 2022, 4:58 PM EST

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

Last updated
March 14, 2022, 10:45 PM EDT

Last updated is the most recent time when changes to the trial's registration were published.



Primary Investigator


Other Primary Investigator(s)

PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation

Additional Trial Information

In development
Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
The goal of this study is to provide accurate information on COVID-19 vaccination to a large number of people. Since COVID-19 vaccines became available in France in early 2021, rates of vaccination have increased rapidly but have reached a point where the binding constraint is no longer the supply of vaccines, but rather demand from people to get vaccinated. On September 15, 2021, 70% of the population was fully vaccinated and 75% had received at least one dose. While these numbers are good relative to other countries, they likely fall short of the vaccination rates required to prevent the development and spread of new variants of the virus. The study will also include a few videos on the importance of flu vaccination.

In this study, we plan to conduct a randomized controlled trial with France-based adults initially contacted through Facebook. The aim of the study is to distribute videos recorded by health care professionals along with other resources that encourage COVID-19 and flu vaccination to a large sample of Facebook users. We will test the most effective ways to maximize diffusion of vaccine-related content and increase vaccination rates.

External Link(s)

Registration Citation

Banerjee, Abhijit et al. 2022. "Increasing the Efficacy & Diffusion of Covid-19 Messaging for Vaccination: Evidence from France." AEA RCT Registry. March 14.
Sponsors & Partners

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Experimental Details


There will be two treatment groups and one control group (the units of randomization will be at the geographical level of "code postal" and "EPCI" in France).
• Treatment Group 1 will receive ads with videos encouraging them to get vaccinated
• Treatment Group 2 will receive ads with a link to vaccine-related resources which they are encouraged to share with friends; while these ads will not be targeted, the message will be designed to appeal to people who are vaccinated or intend to be, so they can share resources with friends. Individuals interested in sharing these resources can enroll as “vaccine ambassadors.”
• Finally, there will be a control group to which no ads will be served.

Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
1. Changes in Covid-19 vaccination at the level of postal code or EPCI
Measured by publicly available postal code- or EPCI-level vaccination data. The study will examine total vaccinations as well as vaccinations by age group. The study will also examine people 1) getting their first dose of vaccine, 2) completing their vaccine series, and 3) getting booster shots.

2. Social shares of vaccine-related content
Number of social media shares of vaccine-related videos and messaging that the study team has created, disaggregated at the county or zip code level.

3. Number of views of vaccine-related content
Number of views of the website and vaccine-related videos, disaggregated at the units of randomization.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
1. COVID-19 cases
Measured by publicly available aggregated data, (possibly split by vaccinated vs unvaccinated status)

2. COVID-19 symptoms prevalence
Measured by aggregated data at the EPCI and/or postal code level, if publicly available.

3. COVID-19 hospitalization and mortality
Measured by data on patients admitted to hospitals or who have been recorded to have died from Covid-19, in some areas also split by vaccinated vs unvaccinated status, if available

4. Vaccine hesitancy
Measured by surveys, if available

5. Movement and Travel
Measured using mobile phone location data of Facebook users. Vaccinated people may be more likely to take risks, including travel.
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
The main study will have two treatment arms and a control group. Each of the treatments will be randomized at the postal code level in Marseille, Paris, and Lyon, and randomized at the EPCI level in other parts of France. The treatments will be initiated through a Facebook ad campaign.

Experimental Sample: The experimental sample includes where vaccinated rates remained low in fall 2021.

The following procedures will be used:

Treatment Group 1 ("Direct" messaging): The investigators will conduct a Facebook ad campaign to show content about COVID-19 vaccination to a large number of Facebook users. Facebook will allocate ad credits across users in treatment areas. Like for any Facebook ad, individuals can choose to whether or not to watch the video and can close the ad at any time. If individuals wish, they can also share any of the content with others.

Treatment Group 2 ("Friends" messaging): For this group, the Facebook ad campaign will encourage ad viewers to help spread the word about vaccination to their friends, families and communities. These ads will be disseminated in a similar manner to the content in T1. Individuals will be able to share the ad with others. Individuals interested in learning more can click through a link in the ad to the study website, where they can watch other videos about vaccination, share these videos with friends, and sign up to be a vaccine ambassador. Participants can consume or share any of the information on the webpage. They can share their contact information with the study as a vaccine ambassador to a) be entered into prize lotteries, b) receive reminders to share content with others, c) be contacted in the future for possible follow-up surveys. They will be asked to give their informed consent at this time and will be screened based on whether they live in a treated postal code and whether they are 18 years or older.

Control: individuals in these EPCIs and postal codes will receive no intervention and will never be contacted by members of the study team.

Treatment and control status at the EPCI and postal code level will be merged with aggregated and de-identified datasets to measure COVID vaccinations, COVID symptoms, and cases, and engagement with the vaccine ambassador content.

The study team will create a series of websites with materials that the vaccine ambassadors can share with friends, family and others in their communities. This same content will be disseminated through sponsored Facebook ads in T1.
Experimental Design Details
Randomization Method
Randomization was done using Stata on a computer
Randomization Unit
France postal code for Lyon, Marseille, and Paris
France EPCI for other areas of France
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
251 postal codes
1,030 EPCIs
Sample size: planned number of observations
251 postal codes 1,030 EPCIs
Sample size (or number of clusters) by treatment arms
Postal code randomization: 83 in control, 84 in direct, 84 in friends
EPCI randomization: 344 in control, 343 in direct, 343 in friends
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Institutional Review Boards (IRBs)

IRB Name
Massachusetts Institute of Technology
IRB Approval Date
IRB Approval Number
Analysis Plan

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Post Trial Information

Study Withdrawal

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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