The debate around vaccines has been going on for decades, but the COVID-19 pandemic showed how crucial it is to understand and mitigate anti-vaccine sentiments. While the pandemic may be over, it is still important to understand how the pandemic affected the anti-vaccine discourse, and whether the arguments against non-COVID vaccines (e.g., Flu, MMR, IPV, HPV vaccines) have also changed due to the pandemic. This study attempts to answer these questions through a large-scale study of anti-vaccine posts on Twitter. Almost all prior works that utilized social media to understand anti-vaccine opinions considered only the three broad stances of Anti-Vax, Pro-Vax, and Neutral. There has not been any effort to identify the specific reasons/concerns behind the anti-vax sentiments (e.g., side-effects, conspiracy theories, political reasons) on social media at scale. In this work, we propose two novel methods for classifying tweets into 11 different anti-vax concerns -- a discriminative approach (entailment-based) and a generative approach (based on instruction tuning of LLMs) -- which outperform several strong baselines. We then apply this classifier on anti-vaccine tweets posted over a 5-year period (Jan 2018 - Jan 2023) to understand how the COVID-19 pandemic has impacted the anti-vaccine concerns among the masses. We find that the pandemic has made the anti-vaccine discourse far more complex than in the pre-COVID times, and increased the variety of concerns being voiced. Alarmingly, we find that concerns about COVID vaccines are now being projected onto the non-COVID vaccines, thus making more people hesitant in taking vaccines in the post-COVID era.

翻译：围绕疫苗的争论已持续数十年，但新冠疫情凸显了理解并缓解反疫苗情绪的关键性。尽管疫情可能已经结束，了解其对反疫苗言论的影响，以及针对非新冠疫苗（如流感疫苗、MMR疫苗、IPV疫苗、HPV疫苗）的反对论点是否因疫情而改变，仍具有重要意义。本研究通过大规模分析推特上的反疫苗帖子，试图回答这些问题。此前几乎所有利用社交媒体理解反疫苗观点的研究，仅将立场分为"反疫苗""支持疫苗"和"中立"三大类，尚未有研究在大规模社交媒体中识别反疫苗情绪背后的具体原因或关切（如副作用、阴谋论、政治因素等）。本文提出两种将推文分类为11种反疫苗关切的新方法——基于蕴含的判别式方法和基于大语言模型指令微调的生成式方法——这些方法在多个强基线模型上表现更优。我们将该分类器应用于2018年1月至2023年1月五年间的反疫苗推文，分析新冠疫情如何影响公众的反疫苗关切。研究发现，疫情使反疫苗言论比疫情前时代复杂得多，且被表达的关切种类显著增加。令人警惕的是，对新冠疫苗的关切正被投射到非新冠疫苗上，导致疫情后时代更多人质疑是否接种疫苗。