Date: Oct. 22, 2002
Contacts: Christine Stencel, Media Relations Officer
Cory Arberg, Media Relations Assistant
(202) 334-2138; e-mail <>

More Data Needed to Determine if Contaminated Polio Vaccine
From 1955-1963 Causes Cancer in Adults Today

WASHINGTON -- Scientific evidence is insufficient to prove or disprove the theory that exposure to polio vaccine contaminated with a monkey virus between 1955 and 1963 has triggered cancer in humans, says a new report from the Institute of Medicine of the National Academies.

The vast majority of population studies, which carry the most weight in establishing causal relationships, have found no increased rates of cancer in people who received the vaccine contaminated with simian virus-40 (SV40). However, a possible link cannot be completely ruled out because of limitations in the available data and in the way the studies were conducted.

Moreover, while there is a strong body of biological evidence that SV40 is capable of causing cancer, it is not clear that exposure to the virus through the tainted polio vaccine could cause certain cancers suspected of being associated with SV40 -- mesothelioma, osteosarcoma, ependymoma, and non-Hodgkin's lymphoma -- said the committee that wrote the report.

"Biological data can help shape research directions, but cannot prove causality on their own," said committee chair Marie McCormick, professor and chair, department of maternal and child health, Harvard School of Public Health, Boston. "Given the uncertainties raised by all the studies, our report offers a research strategy and suggests a process for handling contamination of vaccines should it ever occur again."

In the early years of polio vaccine production, the tissue cultures that were used to grow poliovirus for the vaccine came from the kidneys of rhesus and cynomolgus monkeys. Researchers discovered in 1960 that these tissues could be infected with SV40, a previously unknown virus that commonly causes a harmless infection in certain species of Asian macaques, particularly the rhesus monkey. When SV40 was detected in polio vaccine, health officials began taking steps to remove the virus. Following the implementation of altered production techniques and more stringent screening, the polio vaccine has been free of SV40 since 1963. Through large-scale vaccination efforts, polio was eliminated from the Western Hemisphere by 1994, and strides to eradicate the disease worldwide are under way.

Researchers estimate that 10 percent to 30 percent of the polio vaccine given to adults and children in the United States between 1955 and 1963 was contaminated with SV40, potentially exposing between 10 million and 30 million Americans to the virus. However, as with all viruses, not everyone who comes into contact with SV40 will become infected; and those who are infected may never suffer symptoms or adverse health consequences. In the absence of sensitive and specific blood tests for SV40, it is impossible to determine how many people actually became infected with the virus.

The committee does not recommend a review of polio vaccination policy on the basis of concerns over cancer risks from SV40 contamination, since the polio vaccine is no longer contaminated. However, it does recommend that the U.S. Department of Health and Human Services coordinate and supplement existing efforts to develop a comprehensive plan for addressing vaccine contamination, in case similar problems ever arise again.

The bulk of studies to date that have examined the potential link between the vaccine and cancer in human populations indicate no increased risk of cancer in vaccine recipients. However, the committee found substantial statistical and design limitations in the 13 population studies. In many cases, year of birth was used instead of individual vaccination records to determine who received the polio vaccine during the period of contamination. Furthermore, there is no way for researchers to know now which individuals received the contaminated polio vaccine decades ago. The rarity of most of the tumors thought to be associated with exposure to SV40 makes it difficult to do statistically sound studies.

In laboratory studies, rodents that were exposed to SV40 developed the same type of tumors as those human cancers suspected of being associated with the virus. The incidence of some of these cancers has risen, but this could be attributable to better detection techniques and other possible causes. Studies also show that the DNA of SV40, like that of other viruses, has been detected in human tumors. However, the committee noted that not all the tumors studied contained the viral genetic material, and some studies have detected SV40 in normal tissues from healthy subjects. The presence of viral material in a human tumor does not by itself demonstrate a causal link.

Future research efforts should focus on developing sensitive blood tests and standardized techniques to more definitively detect SV40 in people who may have been infected, the committee said. Once the best detection methods and protocols are determined, they should be used to assess the incidence of infection in humans prior to the introduction of polio vaccine in 1955 and after the elimination of the contaminated supplies in 1963. These analyses would help to reveal how much of the existing SV40 infection in humans can be attributed to contaminated polio vaccine as opposed to other potential sources. The committee advised against conducting more epidemiological studies of people potentially exposed to the contaminated vaccine until the technical issues of detection and study design have been resolved.

Polio infection reached epidemic levels in the United States during the first half of the 20th century, peaking at more than 20,000 cases of paralysis in 1952. The most widely recognized form of polio is an infection of the central nervous system that results in paralysis of limbs or respiratory muscles. The first polio vaccine was introduced in 1955, and within a decade, the number of cases reported in the United States had dropped to 61. By 1994, the entire Western Hemisphere was declared free of the poliovirus.

This study is the fifth in a series of eight on vaccine safety sponsored by the Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Diseases. The Institute of Medicine is a private, nonprofit institution that provides health policy advice under a congressional charter granted to the National Academy of Sciences. A committee roster follows.

The report Immunization Safety Review: SV40 Contamination of Polio Vaccine and Cancer is available on the Internet at Copies will soon be available for purchase from the National Academies Press; tel. (202) 334-3313 or 1-800-624-6242. Reporters may obtain a pre-publication copy from the Office of News and Public Information (contacts listed above).

Board on Health Promotion and Disease Prevention

Immunization Safety Review Committee

Marie C. McCormick, M.D., Sc.D.1 (chair)
Professor and Chair
Department of Maternal and Child Health
Harvard School of Public Health

Ronald Bayer, Ph.D. 1,2
Division of Sociomedical Sciences
Joseph L. Mailman School of Public Health
Columbia University
New York City

Alfred O. Berg, M.D., M.P.H.1,2
Professor and Chair
Department of Family Medicine
University of Washington School of Medicine

Rosemary Casey, M.D. 2
Associate Professor of Pediatrics
Jefferson Medical College, and
Lankenau Faculty Pediatrics
Wynnewood, Pa.

Joshua Cohen, Ph.D.
Senior Research Associate
Harvard Center for Risk Analysis
Harvard School of Public Health

Betsy Foxman, Ph.D.
Department of Epidemiology
School of Public Health
University of Michigan
Ann Arbor

Constantine Gatsonis, Ph.D.
Professor of Medical Science and Applied Mathematics, and
Director, Center for Statistical Sciences
Brown University
Providence, R.I.

Steven N. Goodman, M.D., M.H.S., Ph.D.
Associate Professor
Department of Oncology
Division of Biostatistics
School of Medicine
Johns Hopkins University

Ellen J. Horak, M.S.N. 2
Education and Nurse Consultant
Public Health Certification Program
Public Management Center
University of Kansas

Michael Kaback, M.D.1
Professor of Pediatrics and Reproductive Medicine
University of California
San Diego

Gerald Medoff, M.D. 2
Professor of Medicine and Microbiology and Immunology, and Senior Adviser to the Chairman of the Internal Medicine Department
Washington University School of Medicine
St. Louis

Rebecca T. Parkin, Ph.D.
Associate Research Professor
Department of Occupational and Environmental Health
School of Public Health and Health Services
George Washington University Medical Center
Washington, D.C.

Bennett A. Shaywitz, M.D.1
Professor of Pediatrics and Neurology and Chief of Pediatric Neurology
Yale University School of Medicine, and
Yale Center for the Study of Learning and Attention
New Haven, Conn.

Christopher Wilson, M.D.
Professor and Chair
Department of Immunology
University of Washington


Kathleen Stratton, Ph.D.
Study Director

1 Member, Institute of Medicine
2 Did not attend the meeting on the topic of this report