After the announcement that a hantavirus outbreak had killed three people on a cruise ship in May 2026, a rumor circulated that hantavirus was a side effect of Pfizer's mRNA vaccine against COVID-19. People spreading the rumor said their source was Pfizer's own internal documents.
A post on X showed a screenshot of an alleged list of side effects next to an image of a glass vial supposedly containing Pfizer's COVID-19 vaccine, saying "Hantavirus is listed in Pfizer's 38-page document. Page 33. It's one of 1,233 listed side effects" (archived):
Other posts on X and Instagram made that claim, and Snopes readers emailed asking whether it was accurate that Pfizer's COVID-19 vaccine causes hantavirus.
The claim was false. While the document is authentic, hantavirus appeared not in a list of side effects, but in a pre-determined list of so-called adverse events of special interest — unwanted medical events that occur after vaccination, which are not necessarily caused by the vaccination. Regulatory bodies ask that pharmaceutical companies closely monitor the adverse events in these lists.
The document people cited was an adverse event report, which governments require when a company rolls out new medications, vaccines or devices, as well as cosmetics and food. This one, authored by Pfizer/BioNTech, reported on adverse events following its mRNA COVID-19 vaccine rollout, which started in late 2020 in the U.S. The report included adverse events recorded up to Feb. 28, 2021. (The U.S. Food and Drug Administration made the document public following a Freedom of Information Act lawsuit.)
Hantavirus does not appear in the list of recorded adverse events that actually followed Pfizer's COVID-19 vaccine rollout, however. Instead, it appears in the above-mentioned list of pre-specified adverse events of special interest.
In the document, Pfizer says it built its list of AESIs based on the lists of various "expert groups and regulatory authorities," such as the U.S. Centers for Disease Control and Prevention and the U.K.'s Medicine and Healthcare Products Regulatory Agency.
The list, which appears as an appendix as a reference, includes "Hantavirus pulmonary infection" on Page 38. In other words, by adding this list in the document, Pfizer said that if any of these adverse events happened, it would monitor the situation more closely. At no point does the document suggest that hantavirus was one of the adverse events that actually happened to a person who had received the vaccine.
There is no evidence that the COVID-19 either causes hantavirus or makes people more vulnerable to it, neither in this adverse events report nor in any of the independent studies published after. Several studies have since found the COVID-19 mRNA vaccine was safe, including one of 28 million people in France published in 2025. Another, of 1.5 million people in Denmark with a special focus on 29 adverse events, showed no increased risk after vaccination.
For further reading, Snopes covered a 2025 study that showed the COVID-19 mRNA vaccine provided an immunity boost that helped to extend survival of certain skin and lung cancers.