On April 13, 2021, the Centers for Disease Control and Prevention (CDC) and Food and Drug Administration recommended a pause in use of the Johnson & Johnson (J&J)–Janssen COVID-19 vaccine due to 6 reports of cerebral venous sinus thrombosis in recently vaccinated individuals [1]. The announcement of the pause required development of a coordinated communication strategy under extreme time pressure and careful messaging by stakeholders to mitigate reduced public confidence in COVID-19 vaccines [2]. Moreover, official communication efforts had to consider the potential influence of already widespread misinformation about the vaccines on attitudes toward the pause [3,4]. In this survey study, we evaluated understanding and impressions of the CDC’s public web-based information about the J&J-Janssen pause among unvaccinated US adults.

A total of 271 and 286 participants were included in cohorts A and B, respectively (demographic characteristics listed in Table 1). Across participants, the median number of correct responses to the comprehension questions was 6 in both cohort A (IQR 1.5; range 0-7) and cohort B (IQR 1.0; range 1-7). The total number of correct responses was negatively associated with intention not to seek vaccination in both cohort A (odds ratio 0.61, 95% CI 0.45-0.82; P=.001) and cohort B (odds ratio 0.48, 95% CI 0.31-0.74; P=.001; Multimedia Appendix 4). Although a majority of participants rated the passages as “clear and easy to read” (cohort A: n=229, 84.5%; cohort B: n=243, 85%), fewer indicated that they would be likely to share them on social media (cohort A: n=53, 19.6%; cohort B: n=75, 26.3%).

The web page mentioned “a small number of reports” of cerebral venous sinus thrombosis in individuals who received the J&J-Janssen vaccine. When asked to guess a specific number, 188 (69.4%) and 133 (46.5%) respondents in cohorts A and B, respectively, estimated 100 or more cases, at least an order of magnitude higher than the actual value; 176 (64.9%) and 128 (44.8%) respondents in cohorts A and B, respectively, estimated 10 or more deaths after vaccination (Figure 1).

Responding to a counterfactual question, 127 (46.9%) and 139 (48.6%) participants in cohorts A and B, respectively, indicated that the pause reduced their confidence in the J&J-Janssen vaccine’s safety (Figure 1). Most participants reported no change in their confidence in COVID-19 vaccines’ safety in general (cohort A: n=182, 67.2%; cohort B: n=194, 67.8%) or intention to receive the Pfizer-BioNTech or Moderna vaccine (cohort A: n=206, 76%; cohort B: n=211, 73.8%).

In our web-based survey about the CDC’s messaging around the J&J-Janssen vaccine pause, many respondents overestimated the number of case reports that prompted the pause, often by several orders of magnitude. Since verbal descriptors are elastic concepts that can be misinterpreted, grounding them with numbers can reduce variability in risk perception and promote informed decision-making [5].

Respondents also expressed reduced confidence in the safety of the J&J-Janssen vaccine, highlighting the potential danger of conveying piecemeal information about risk during a pandemic response [3]. Encouragingly, the reduced confidence did not extend to mRNA COVID-19 vaccines, consistent with previous findings that overall vaccine hesitancy remained stable following the pause [6]. These results were obtained using the counterfactual format, which is less susceptible to overestimating shifts in beliefs than the change format (Multimedia Appendix 1). The negative association between understanding of the passage and self-reported vaccine hesitancy suggests that more targeted messaging may have been useful to promote vaccine confidence [7,8].

Consistent with uncertainty management theory [9], individuals likely viewed the pause in different ways, leading to a spectrum of emotional responses and changes in behavior. Despite being a safety precaution, the pause introduced new uncertainties requiring effective management through clear and consistent messaging, highlighting the balance that must be maintained between fostering trust and preventing unnecessary alarm [10]. Given the limitations of the deficit model of scientific communication [11], just providing technically correct information is insufficient for effective communication during public health crises. Instead, attention should be given to the accessibility of information across diverse socioeconomic groups, in line with the knowledge gap hypothesis [12], and to countering misinformation by providing easy-to-use official guidance [6,7].

The study is limited by the convenience sampling strategy; the participants recruited were not representative of the US population as a whole, and the findings should not be generalized to other contexts. Since the study was conducted on the web, individuals with lower internet and health literacy may have been excluded.