The COVID-19 pandemic is emerging as one of the greatest public health threats in history, with over 140 million infections and 3 million deaths worldwide attributed to the SARS-CoV-2 virus as of April 2021 [1]. As transmission of COVID-19 continues around the globe, a COVID-19 vaccine is an important and valuable tool to reduce the spread of infection. Due to the critical need, the speed of vaccine development, production, and mass rollout has been faster than ever seen before, leading to concerns about vaccine efficacy and safety [2,3]. The intricacy surrounding COVID-19 vaccine hesitancy appears to be further reaching and fundamentally different than other immunizations [2]. Vaccine production cannot meet demand, requiring rollout plans for targeted subpopulations, and there are several types of COVID-19 vaccines being used within one country [2]. This has led to increased concern, mistrust, and confusion surrounding COVID-19 vaccination.

Despite the massive undertaking of vaccine development, a vaccination program is only as successful as its uptake. Vaccine hesitancy, defined as a delay in acceptance or refusal of vaccines despite their availability [4], was mentioned by the World Health Organization as one of the top 10 threats to public health in 2019 [5]. The effective rollout of a COVID-19 vaccine strategy may be obstructed by the beliefs and attitudes of vaccine-hesitant individuals worldwide [6]. A recent survey of US adults in April 2020 found that 23% of persons would not be willing to get vaccinated against COVID-19 [7]. In Canada, a survey in March 2021 demonstrated that 76.9% of Canadians were very or somewhat willing to receive a COVID-19 vaccine [8].

Vaccine hesitancy is a “multifaceted, deeply complex construct that may be rooted in the moral composition that guides our daily decision making” [8,9]. Studies have identified that vaccination decisions are shaped by multiple complex interactions between individual, community, cultural, historical, political, and societal factors [2,10]. There have been several metrics and scales developed and used to measure vaccine confidence and hesitancy [11-13]. The Vaccine Confidence Index (VCI) survey tool was developed to measure individuals’ perceptions on the safety, importance, effectiveness, and religious beliefs toward vaccines, which were identified as key drivers of public confidence in vaccination [11]. Another validated score measuring the psychological antecedents of vaccination is known as the 5C scale [13]. The 5C scale includes confidence (trust and attitudes), complacency (not perceiving diseases as high risk), constraints (structural and psychological barriers), collective responsibility (willingness to protect others), and calculation (information searching) [13].

Negative beliefs about vaccines may prevent the implementation of provaccination policies. Public health officials need to prioritize implementing strategies to help reduce these negative perceptions [14]. Traditional approaches to promoting immunization have assumed that inadequate knowledge of the associated risks and benefits drive hesitancy; however, this stance has proven to be ineffective as an intervention [15]. The source of the hesitancy is often both from lack of information and from lack of trust in institutions such as the government, physicians, and pharmaceutical companies [16,17]. Rather than just trying to enhance knowledge, a different approach to overcoming vaccine hesitancy for COVID-19 may be to focus on changing personal attitudes [15].

Prior studies have demonstrated that social media can help understand attitudes and behaviors during public health crises and promote health messaging [18-20]. Particularly, Twitter has been used for public health research and more specifically regarding COVID-19 [18,21]. A recent study aimed to characterize the main topics of Twitter conversations related to COVID-19 and identified four main themes including the origin of the virus, its sources, the impact on people/countries/economy, and lastly methods of mitigating the risk of infection [18].

Attitudes toward vaccination are, in large part, shaped by information and ideas individuals encounter through social media [22,23]. Social media is a principal informational forum for vaccination uptake with large proportions of content involving antivaccination messaging [22,24]. Reliable and accurate information on social media is often mixed with inaccurate, conspiratorial, incomplete, or biased messages [22]. A recent study evaluating vaccine hesitancy through content analysis of tweets in Canada identified major themes including safety, suspicion of economic or political motivation, knowledge deficit, opinions of authority figures, and lack of liability from pharmaceutical companies [25]. This study demonstrates the significant utility of using Twitter to better understand vaccine hesitancy at a population level [25]. However, a broader reaching and deeper understanding of current patterns of communication that characterize the immunization debate on social media platforms is needed to inform public health interventions aimed toward influencing attitudes on immunization [15,17,22,23].

This study used social network analysis (SNA) and unsupervised machine learning to characterize COVID-19 vaccine content on Twitter, which allows researchers to access the application programming interface (API). The use of machine learning in social sciences is expanding and has generated interesting methodological conversations on different approaches to study COVID-19 sentiment, attitude, and emotion [26-28]. With this study, we aimed to use social media content analysis to provide a further understanding of societal attitudes, communication trends, and barriers to COVID-19 vaccine uptake, at a critical time during the COVID-19 pandemic, just following reporting of initial vaccination trials. We hypothesize that these data will be critical globally for developing targeted outreach programs and intervention strategies on Twitter to impact COVID-19 vaccine hesitancy.

In total, 636,516 tweets were collected from 428,535 accounts, for an average of 79,564 tweets per day. Figure 2 presents the cluster map of COVID-19 immunization conversations on Twitter between November 19 and November 26, 2020. We found a polarized conversation about immunization on social media.

During this observation period, a large proportion of accounts debating COVID-19 immunization on Twitter were connected and exposed to social media conversations promoting vaccination narratives. The largest cluster (green), comprising approximately 49.4% (n=211,549) of Twitter accounts, revealed a vaccine acceptant point of view. Based on degree centrality, the cluster seemed to overlap with more progressive-leaning political leaders, such as Hillary Clinton and US Representative Alexandria Ocasio-Cortez (D-14-NY), and mainstream news media, such as the NY Post, the Hill, ABC, and Reuters. A second provaccine cluster (orange) centered on the Indian COVID-19 immunization online debate, with political leaders, such as Rahul Gandhi and Press Trust of India (the largest news agency in India), leading the conversation.

As Figure 2 indicates, we also found two clusters opposite to these vaccine acceptant clusters exhibiting more vaccine hesitant narratives. There were 23.4% (n=146,191) of conversations on Twitter during this period of observation that can be directly attributed to vaccine hesitancy. First, in red, our study identified a large cluster comprising 88,892 Twitter handles accounting for 18.4% of all accounts in the cluster. These interactions from the Twittersphere in English originated mostly from the United States, the United Kingdom, and Canada, and gravitated around accounts of prominent antivaccine physicians and organizations, right-wing activists, show hosts, such as Rush Limbaugh in the United States and Simon Dolan or Michael J Blair in the United Kingdom, and some alternative news organizations, such as Breitbart News. In this conversation cluster, we found an overlap between ideologically leaning advocates, especially those associated at the margins of right-wing or Conservative parties, and antivaccine online conversations. We found considerable cross-pollination between accounts originating from all across the English Twittersphere, demonstrating the internationalization of COVID-19 vaccination conversations on social media. Second, in blue, a smaller cluster representing 2.7% (n=11,509) of accounts appeared to be shaped around Francophone (from France and Québec) vaccine hesitant conversations.

Examining the COVID-19 vaccine hesitancy narrative during this observation period provided key insights on what drives attitudes. As shown in Table 1, our unsupervised machine learning analysis identified 12 specific archetypical vaccine hesitancy tweets (we tested a different number of clusters between 9 to 20). Social media content highlighted concerns over the efficacy and the safety of a possible COVID-19 vaccine fitting into the 5C scale domain of confidence. Five broad categories focused on vaccine efficacy in our data set. The first topic (topic 3 in Table 1) regrouped tweets suggesting that attempts to produce a COVID-19 vaccine, especially using mRNA technology, remain tentative. These tweets highlighted our failure to develop an HIV vaccine with mRNA and that existing mRNA vaccines for COVID-19 have not been tested enough to demonstrate efficacy. The second category of vaccine hesitancy tweets concentrating on efficacy (topic 2) focused on a comment made by Sir John Irving Bell on November 20, 2020, that existing vaccines were unlikely to completely rid the population of COVID-19. The third and fourth vaccine efficacy topics (topics 7 and 8) grouped together tweets from prominent physicians arguing that existing polymerase chain reaction testing is flawed and that there are no indications of long-term T-cell immunity for COVID-19. The last topic highlighted (topic 10), which obtained subsequent media attention, suggested that as much as 25% of the population would have contracted COVID-19 by the time a vaccine would be rolled out, and consequently, the vaccine would be unnecessary to reach herd immunity.

As for the concern of vaccine safety, we found two broad groups of tweets. The first (topic 5) highlighted a classic antivaccine story where an emergency medical technician/fire rescue was required to get TDAP (tetanus, diphtheria, and pertussis) boosters and had complications. Although not specifically addressing the issue of a COVID-19 vaccine, these tweets emphasize how existing antivaccine narratives have created a baseline from which some individuals frame a COVID-19 vaccine. The second topic (topic 9) centered on tweets in French framing a COVID-19 vaccine as a poison and suggesting that mRNA technology has not been tested yet and would be harmful.

Although safety and efficacy concerns remain a major component explaining COVID-19 vaccine hesitancy, it is only half of the story. As we examined the content of tweets of users integrated in the vaccine hesitancy online cluster, we found a large proportion of those interactions that emphasized a concern for personal freedom and/or some form of mistrust of institutions. In the 5C scale of psychological antecedents of vaccination, confidence in vaccines includes trust in the system that delivers the vaccines and the motivation of policy makers who decide on the need of vaccines [13]. Tweets were framed in three directions. First, a large percentage of such tweets (n=45,628, 31.2%) expressed some criticism toward the government’s handling of the COVID-19 crisis, especially decisions to curb individual freedom. COVID-19 immunization is framed as the right of individuals to decide for themselves whether or not to be vaccinated. Any indication that governmental authorities might require vaccination is perceived as a direct attempt to limit individual freedom. Two main topics (topics 1 and 12) highlighted mistrust in government policy, with one condemning the Johnson government in the United Kingdom for proposing that individuals vaccinated for COVID-19 could receive “freedom passes” and one criticizing Denmark’s decision to cull its mink population to halt the spread of a coronavirus variant. Some topics (topics 6 and 11) expressed mistrust in multinational corporations, most notably airline companies, such a Qantas, who suggested that vaccination should be made compulsory for international travel. This decision is framed as a direct restriction of personal freedom by multinational corporations, and some users made a direct reference to populist narratives suggesting that these policies would only affect commercial flights and elites would be able to avoid vaccination while using private flights.

A final residual category (topic 4) in the vaccine hesitancy conversation singled out a tweet by radio host Rush Limbaugh mentioning that two COVID-19 vaccines were now approved and praising President Trump’s management of the crisis. This tweet was widely circulated in the vaccine hesitant conversation cluster and underlines the reality that any conversation on COVID-19 immunization, and to some degree, vaccine hesitancy clusters on social media, intersect with broader clusters structured by current political polarization.

Figure 3 presents the approximate distribution of these inferred topics in COVID-19 vaccine hesitancy clusters identified during this observation period and allows us to understand what shapes an individual’s attitude toward a COVID-19 vaccine. First, as this data set was collected right after the news of positive trials of COVID-19 vaccines by Pfizer and Moderna, more than half of the tweets (n=76,531, 52.4%) mentioned vaccine efficacy and raised suspicions on whether creating vaccines using mRNA technology was achievable or whether immunization would be long-lasting. With respect to vaccine safety, only 8.5% (n=12,401) of the social media debate in vaccine hesitancy clusters doubted its safety. Second, nearly one-third (n=45,628, 31.2%) of conversations on vaccine hesitancy clusters on Twitter expressed concerns for freedom or mistrust of institutions (either the government or multinational corporations). These results suggest that one key determinant of vaccine hesitancy is trust in institutions. It suggests that vaccine confidence building is a problem of shaping attitudes toward COVID-19 vaccines, which falls into the realm of health policy, as well as promoting social/political trust, which falls more into the realm of politics.