Up to 98 million Americans received polio vaccines contaminated with SV40, a cancer-causing monkey virus

SV40 in the Polio Vaccine: What Regulators Knew and When

For nearly a decade, millions of American children received polio vaccines containing a virus that health officials knew could cause cancer in laboratory animals. The question of how this happened—and what it says about institutional accountability—remains unresolved more than 60 years later.

The story begins with a seemingly straightforward problem. In the 1950s, polio vaccines were being manufactured using kidney cells from rhesus monkeys. The process worked well enough, but it carried an unexpected risk: the monkey cells themselves were infected with Simian Virus 40, or SV40, a pathogen unknown to medical science at the time. Between 1955 and 1963, this contaminated vaccine was distributed across the United States. The CDC would later acknowledge that between 10 and 30 million Americans received doses containing SV40, though some estimates place the figure as high as 98 million.

What makes this a "they knew" story isn't the initial contamination—that was genuinely accidental. What matters is what happened after officials discovered it. Manufacturers became aware of SV40's presence in monkey kidney cultures around 1960. Animal studies conducted during this period showed the virus could cause tumors in laboratory animals. Yet the contaminated vaccine continued to be administered for several more years while authorities debated how to respond.

The official position from health agencies was reassuring: there was no evidence the virus was harming humans. Regulators argued that a virus causing cancer in lab animals didn't necessarily translate to human disease. The problem was minimized in public statements, and the vaccine campaign continued without interruption. Parents were never informed of the contamination.

The evidence emerged slowly over decades. Researchers began finding SV40 in human tumors—specifically in mesotheliomas, brain tumors, and bone cancers—at rates suggesting more than random chance. Studies from the National Cancer Institute documented SV40 DNA in cancer samples from patients who received the contaminated vaccine. The virus had survived in human bodies for years, sometimes decades, after vaccination.