Understanding Who Joined COVID-19 Vaccine Trials and Why

During the COVID-19 pandemic, clinical trial participation became not only a scientific necessity, but also a social barometer. A study published in Human Vaccines & Immunotherapeutics in June 2025 offers perspective into some of these Phase 2/3 COVID-19 vaccines trials and the motivations and perceptions of their adult participants.

Conducted at the Johns Hopkins Center for Immunization Research in Baltimore, Maryland, the study surveyed 163 participants from the Pfizer-BioNTech and AstraZeneca vaccine trials to examine who joined, why they did so, and how their understanding of trial risks evolved over time.

Here’s a look at what the study found.

Participant Demographics

The findings reveal a volunteer pool that was overwhelmingly White and highly educated. More than half of respondents had graduate degrees and nearly half earned over 99,999 dollars annually. These demographics diverge from the general population of Baltimore, where the trials were conducted.

Motivations for Participation

The study identified four main reasons for participation. They include:

• Desire for personal protection against COVID-19, cited by 42.3 percent of respondents,
• The wish to help others or contribute to science, reported by 38.7 percent,
• Desire to be part of a historic vaccine effort, reported 12.3 percent,
• And financial compensation, cited by 4.3 percent.

This research shows that for many, the immediate threat of infection, combined with a sense of urgency and civic responsibility, made trial participation both a personal safeguard and a meaningful contribution to public health.

Perceptions of Risk

Before joining the study, nearly 70 percent of respondents retrospectively perceived some degree of risk. Concerns included potential illness from the vaccine and uncertainty stemming from the novel technologies being used, such as mRNA platforms and adenovirus vectors.

However, these perceptions shifted after over one and a half years of participation. Upon study completion, 74.2 percent of participants rated the trials as “not at all risky,” and only two individuals felt the experience had been riskier than they initially expected.

Trial Experience and Satisfaction

The experience of participants in the AstraZeneca trial, which was not ultimately licensed in the United States, reinforces the notion that trial participation was valued even when the outcome differed from expectations. Among respondents, 97.4 percent of AstraZeneca participants and 100 percent of Pfizer-BioNTech participants indicated they would enroll again, and only six percent and 4.3 percent respectively expressed any regret or dissatisfaction.

Demographic Shifts Compared to Previous Trials

Compared to previous early-phase vaccine trials at the same site, which predominantly enrolled low-income, minority participants, the COVID-19 vaccine studies drew a wealthier, more educated demographic. These findings may underscore systemic barriers that limit trial diversity. Underrepresentation of racial and ethnic minorities in vaccine research can undermine the global generalizability of findings and misses a critical opportunity to promote equity in access to new medical technologies.

Study Limitations and Future Implications

While the study is limited by its single-site sample and the potential for recall bias, it offers valuable insights into how perceptions of risk shift over time and how trust in clinical research can be built through experience.

These findings point to the need for tailored, proactive communication strategies that clearly explain the safety and purpose of vaccine trials, particularly in the early phases when uncertainty is high. As vaccine development accelerates in response to emerging global threats, inclusive and ethically grounded research will depend on early outreach that engages diverse communities and fosters informed participation from the outset.

Reference
Wunderlich B, Feijoo B, Limaye R, Moulton LH, Talaat K. Participant motivators and perceptions of risks and benefits in adult Phase 2/3 COVID-19 vaccine clinical trials. Hum Vaccin Immunother. 2025;21(1):2521192.