This claim has been circulating on X and making headlines on online tabloid news platforms such as the Daily Mail. According to the claim, a 19-year-old woman experienced an abnormal increase in breast size – going from a B to a triple G cup over the course of 6 months  – a week after receiving the Covid-19 vaccine from Pfizer. These claims and social media posts assert that the vaccine directly caused breast growth. They are often accompanied by images of the woman and have been re-posted with further speculation about the Covid-19 vaccine’s negative impacts.

These claim posts cite a publication titled: “The Pfizer Boob Job”: A Case of Unexplained Gigantomastia published in December 2024 that is available in full online.

More specifically, the linked article is a case report published in a plastic and reconstructive surgery journal which specifically reports on a single occurrence of unexplained breast growth. However, upon reading the case report, we noticed discrepancies between the actual content and how it is being presented in the circulating social media posts and headlines.

According to the case report, its subject, a 19-year-old woman in Canada, did experience abnormal growth in her breast size. This began a week after she received her Covid-19 vaccine. The report describes her condition as gigantomastia (when breasts become abnormally large) caused by “pseudoangiomatous stromal hyperplasia,” also known as PASH. PASH is a rare condition characterised by an overgrowth of myofibroblastic cells in the breast that typically presents as benign lesions or lumps. In this case, PASH causing bilateral gigantomastia (that is, both breasts increasing abnormally in size) is even more rare – the report points out that only 20 other recorded cases exist. According to the report, the patient underwent surgical reduction, and there has not been a recurrence of breast growth.

While the report notes that the patient in question does believe the vaccine to be the cause, it goes on to conclude that despite this case being noteworthy for occurring shortly after the administration of a vaccine, it represents a temporal association – not a causal one. This means that while the vaccine and gigantomastia overlapped in time period, a direct cause-effect link has not been established. The ultimate cause of abnormal breast growth remains unclear. However, the author establishes the need for future investigation to “comprehend the spectrum of reactions to the vaccine.”Therefore, while this isolated and highly uncommon case of abnormal breast growth did happen, claims which directly describe it as a side effect of the Pfizer vaccine are omitting the full story. There is a possibility that the vaccine triggered the occurrence of PASH, however, this has not been proven – nor has any other direct cause been identified. We give this claim a rating of Unproven.

The presentation of this case in claim posts and the headlines of tabloid articles highlights how the removal of nuance can easily lead to misinformation spreading and serve as a springboard for other agendas or narratives. The omission of the uncertainties clearly described in the report is a good example of how we should always fact-check startling medical claim posts – particularly if the posts are in a short-form format.