Simian virus 40 (SV40) is a virus that infects several species of monkeys and typically does not cause symptoms or disease in them. The virus was discovered in 1960 in rhesus macaque monkey kidney cells that were used in the production of the original Salk and Sabin polio vaccines (1). Since the mass immunization program for polio began in 1955 before the discovery of the virus, contaminated vaccine lots were inadvertently used for the first few years of the program.

     When reports appeared in 1961 that injection of SV40 into hamsters could induce tumors (2–5), the United States government instituted a screening program requiring all new lots of poliovirus vaccine be free of SV40 because of concerns about possible adverse effects on human health. (Earlier lots were not withdrawn from the mass immunization program.) No SV40 has been found in the polio vaccine lots tested after 1963. The polio vaccine currently used in the United States is produced under carefully regulated conditions designed to ensure that contamination with SV40 does not occur. As a result of the earlier contamination, however, it is estimated that 10 million to 30 million people vaccinated in the United States from 1955 through early 1963 were inadvertently exposed to live SV40 (6).

Epidemiology Studies

     Over the last four decades, an intense research effort has been made to determine whether this route of exposure to SV40 has caused health problems in people, including cancer. Epidemiology studies involving decades of observations in the United States and Europe have failed to detect an increased cancer risk in those likely to have been exposed to the virus. These include a long-term Swedish study, which followed 700,000 people who received SV40-contaminated vaccine (7), a German study with 22 years of follow-up of 886,000 persons who received the contaminated vaccine as infants (8), a 20-year study of 1,000 people in the United States inoculated during the first week of life with contaminated vaccines (9), and a 30-year follow-up of approximately 10 percent of the entire U.S. population (using data from the National Cancer Institute's Surveillance, Epidemiology, and End Results registry) (10). The Centers for Disease Control and Prevention finds no evidence that SV40-contaminated vaccine lots cause cancer (11).

Laboratory Studies and Additional Epidemiology Reports

     The issue of SV40 resurfaced in the last few years when an increasing number of laboratories, using an extremely sensitive molecular biology technique, the polymerase chain reaction (PCR), found traces of the virus in some rare human tumors including pleural mesothelioma (a cancer of the lining of the lung), osteosarcoma (a type of bone cancer), ependymoma and choroid plexus tumors of the brain, and recently non-Hodgkin's lymphoma (12–29). Other studies reported that SV40 T-antigen, a viral protein, binds to human tumor suppressor proteins such as p53 and RB (30–32), suggesting a possible carcinogenic mechanism. Not all studies, however, find that SV40 plays a significant role in human cancer (33–46).

     In order to resolve why some laboratories detect traces of SV40 in mesothelioma while others do not, an International SV40 Working Group, which included the majority of laboratories studying SV40 in human tissues, was formed in 1997. Nine laboratories from the working group agreed to participate in a study, funded and organized by the National Cancer Institute (NCI). Each group was given 25 paired-duplicate samples of human mesotheliomas, a single set of 25 normal lung tissue samples, and positive and negative control samples. All the samples were blinded (labeled so that the human tumors and controls could not be distinguished) and each laboratory used its particular assay for detecting SV40, many of which had been used to detect SV40 previously. The results, published in the May 2001 issue of Cancer Epidemiology, Biomarkers and Prevention (36), showed that none of the mesothelioma specimens was consistently positive for SV40. Although methods in the study appeared to perform well on the control samples, additional methods that can be used widely and easily to detect the presence of SV40 DNA in human tissues are needed.

Recent Research

     To further explore the relationship between poliovirus vaccine exposure and the incidence of pleural mesotheliomas, a recent study compared vaccine exposure and subsequent rates of these tumors in the United States (37). The authors found that incidence rates for pleural mesotheliomas increased the most among males who were age 75 or older, the age group least likely to have been exposed to the contaminated poliovirus vaccine. Incidence rates among males in the age groups most heavily exposed to SV40-contaminated poliovirus vaccine (between ages 25 and 54) remained stable or decreased from 1975 through 1997. Similar trends were seen among women whose over-all rates of pleural mesothelioma are six times lower than the rates for men. In fact, even though women had similar or greater exposure to SV40-contaminated vaccine as men, pleural mesotheliomas were persistently rare (an average of less than 38 cases each year among the more than 13 million women in SEER), and the few female cases that did occur were mainly in age groups unlikely to have ever received any poliovirus vaccine. Sophisticated “age-period-cohort models” were used to obtain a comprehensive statistical assessment of trends in pleural mesothelioma incidence, and failed to detect any increases in rates that could be attributed to SV40-contaminated poliovirus vaccine in males or females.

    The authors concluded that after almost 40 years of follow-up, U.S. cancer incidence data have not shown an increased incidence of pleural mesothelioma among the age groups that were exposed to SV40-contaminated poliovirus vaccine. However, they noted that continued surveillance of people who received the vaccine is needed, in view of conflicting reports on the detection of SV40 genomic DNA sequences in mesothelioma tumor samples.

     Another epidemiology study involved analyzing cancer incidence data from Denmark following exposure to SV40-contaminated poliovirus vaccine (38). A vigorous vaccination campaign in Denmark, beginning in the spring of 1955 and extending through the early 1960s, resulted in nearly 100 percent vaccination rate in targeted groups, particularly children and young adults. In addition, there is good evidence that most, if not all, of the poliovirus vaccine lots used during this time contained live SV40. From 1963 on, new production techniques eliminated SV40 from lots of Danish poliovirus vaccine.

     Using data from the Danish Cancer Registry covering the period from 1943 through 1997, the researchers compared the cancer incidence in people vaccinated with SV40-contaminated poliovirus vaccine as infants (i.e., those born from 1955–1961) or children (i.e., those born from 1946–1952), to those not exposed to SV40 (i.e., those born from 1964–1970). Those exposed to SV40 had a lower overall cancer risk than those not exposed. Furthermore, they did not have an increased incidence of mesothelioma, brain tumors (including ependymoma and choroid plexus tumors), osteosarcoma and other bone tumors, non-Hodgkin's lymphoma, or testicular cancer, compared to those not exposed to the virus.

    To look at the short-term effects of SV40 exposure on cancer risk, the authors also compared cancer incidence among children ages 0 to 4 years who were exposed or unexposed to SV40-contaminated poliovirus vaccine. They found that the incidence of cancers combined, intracranial tumors, or leukemia was not elevated in years when these children had received SV40-contaminated poliovirus vaccine. Although the incidence of one type of brain tumor, ependymoma, was significantly higher in the exposed vs. the unexposed group, the authors noted that because the peak incidence of ependymoma occurred in 1969, well after SV40 was removed from the Danish vaccine, the effect probably was not due to SV40 exposure.

     A study involving a population uniquely exposed to SV40—people living in northern India—was also published recently (39). It is unclear whether humans can be infected with SV40 but, if this occurs, human infection might be especially common in northern India, where contacts between humans and SV40-infected monkeys frequently occur. NCI researchers and collaborators tested for the presence of SV40 in 47 archived samples of choroid plexus tumors and ependymomas, which are rare human brain tumors reportedly linked with SV40 (15, 17), from the All India Institute of Medical Sciences in northern India. They did not find SV40 in any of the tumors.

     In a case/control study in Spain involving 520 lymphoma cases and 587 controls (45), researchers tested blood samples from cases and controls for the presence of antibodies to SV40. If SV40 circulates in human populations and is implicated in lymphomas, SV40 serum antibodies might be detected at high levels in lymphoma cases. However, the researchers found no increased antibody levels to SV40 detected in lymphoma cases vs. the controls.

     Overall, SV40 antibody levels were low in both cases and controls. Additional testing suggested that a large part of these antibodies may be antibodies to the human virus BK, and not to SV40. Because the DNA of the SV40 virus is nearly 70 percent identical to the BK virus, it is difficult to distinguish between antibodies to the two viruses. Most humans carry antibodies to BK in their blood, since the virus commonly infects humans as children. BK, however, is not associated with any disease in healthy people.

     In another case/control study involving 782 brain tumor cases and 799 controls (46), the risk of developing glioma, meningioma, or acoustic neuroma was not associated with having received either injected or oral poliovirus vaccine during the time period (1955–1963) when vaccines were contaminated with SV40. Exposure to the potentially contaminated poliovirus vaccine was based on self-reporting. Although some participants may not have been able to recall vaccinations they received as young children, the high percentage (85 percent) of reported vaccinations among controls less than 20 years of age in 1961 was similar to the values reported during the same period for the general population.

On-Going Research

     The NCI is continuing to evaluate the possible link between SV40 infection and human cancers through intramural research conducted by the Division of Cancer Epidemiology and Genetics (DCEG), which is still monitoring populations known to have been exposed to SV40-contaminated vaccines. A number of additional extramural grant-supported studies funded by NCI to evaluate the possible relationship of SV40 to cancer are underway (http://fundedresearch.cancer.gov/). Other institutes at the National Institutes of Health are also funding a wide range of studies related to SV40 (http://crisp.cit.nih.gov/).

IOM Report

     The Institute of Medicine (IOM) issued a report in October 2002, which concluded that the scientific evidence was insufficient to prove or disprove the theory that exposure to poliovirus vaccine contaminated with SV40 resulted in cancer in humans (http://www.iom.edu/?id=6014). The IOM is one of the National Academies to which the nation’s leaders often turn for conducting independent scientific studies.

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Glossary Terms

Having to do with sound or hearing.

An unexpected medical problem that happens during treatment with a drug or other therapy. Adverse effects do not have to be caused by the drug or therapy, and they may be mild, moderate, or severe. Also called adverse event.

A protein made by plasma cells (a type of white blood cell) in response to an antigen (a substance that causes the body to make a specific immune response). Each antibody can bind to only one specific antigen. The purpose of this binding is to help destroy the antigen. Some antibodies destroy antigens directly. Others make it easier for white blood cells to destroy the antigen.

Any substance that causes the body to make a specific immune response.

A laboratory test to find and measure the amount of a specific substance.

Primary bone cancer is cancer that forms in cells of the bone. Some types of primary bone cancer are osteosarcoma, Ewing sarcoma, malignant fibrous histiocytoma, and chondrosarcoma. Secondary bone cancer is cancer that spreads to the bone from another part of the body (such as the prostate, breast, or lung).

The growth of abnormal cells in the tissues of the brain. Brain tumors can be benign (not cancer) or malignant (cancer).

A term for diseases in which abnormal cells divide without control and can invade nearby tissues. Cancer cells can also spread to other parts of the body through the blood and lymph systems. There are several main types of cancer. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Sarcoma is a cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue. Leukemia is a cancer that starts in blood-forming tissue such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the blood. Lymphoma and multiple myeloma are cancers that begin in the cells of the immune system. Central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord. Also called malignancy.

The individual unit that makes up the tissues of the body. All living things are made up of one or more cells.

A thin layer of tissue that is part of the middle layer of the wall of the eye, between the sclera (white outer layer of the eye) and the retina (the inner layer of nerve tissue at the back of the eye). The choriod is filled with blood vessels that bring oxygen and nutrients to the eye.

A group of individuals who share a common trait, such as birth year. In medicine, a cohort is a group that is part of a clinical trial or study and is observed over a period of time.

The molecules inside cells that carry genetic information and pass it from one generation to the next. Also called deoxyribonucleic acid.

A type of brain tumor that begins in cells lining the spinal cord central canal (fluid-filled space down the center) or the ventricles (fluid-filled spaces of the brain). Ependymomas may also form in the choroid plexus (tissue in the ventricles that makes cerebrospinal fluid). Also called ependymal tumor.

The study of the patterns, causes, and control of disease in groups of people.

The study of genes and heredity. Heredity is the passing of genetic information and traits (such as eye color and an increased chance of getting a certain disease) from parents to offspring.

A cancer of the brain that begins in glial cells (cells that surround and support nerve cells).

A technique used to cause an immune response that results in resistance to a specific disease, especially an infectious disease.

The number of new cases of a disease diagnosed each year.

Invasion and multiplication of germs in the body. Infections can occur in any part of the body and can spread throughout the body. The germs may be bacteria, viruses, yeast, or fungi. They can cause a fever and other problems, depending on where the infection occurs. When the body’s natural defense system is strong, it can often fight the germs and prevent infection. Some cancer treatments can weaken the natural defense system.

Use of a syringe and needle to push fluids or drugs into the body; often called a "shot."

A tumor that occurs in the brain.

One of a pair of organs in the abdomen. Kidneys remove waste from the blood (as urine), produce erythropoietin (a substance that stimulates red blood cell production), and play a role in blood pressure regulation.

Research done in a laboratory. These studies may use test tubes or animals to find out if a drug, procedure, or treatment is likely to be useful. Laboratory studies take place before any testing is done in humans.

Cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of blood cells to be produced and enter the bloodstream.

One of a pair of organs in the chest that supplies the body with oxygen, and removes carbon dioxide from the body.

Cancer that begins in cells of the immune system. There are two basic categories of lymphomas. One kind is Hodgkin lymphoma, which is marked by the presence of a type of cell called the Reed-Sternberg cell. The other category is non-Hodgkin lymphomas, which includes a large, diverse group of cancers of immune system cells. Non-Hodgkin lymphomas can be further divided into cancers that have an indolent (slow-growing) course and those that have an aggressive (fast-growing) course. These subtypes behave and respond to treatment differently. Both Hodgkin and non-Hodgkin lymphomas can occur in children and adults, and prognosis and treatment depend on the stage and the type of cancer.

A type of slow-growing tumor that forms in the meninges (thin layers of tissue that cover and protect the brain and spinal cord). Meningiomas usually occur in adults.

A benign (not cancer) or malignant (cancer) tumor affecting the lining of the chest or abdomen. Exposure to asbestos particles in the air increases the risk of developing malignant mesothelioma.

The National Cancer Institute, part of the National Institutes of Health of the United States Department of Health and Human Services, is the Federal Government's principal agency for cancer research. The National Cancer Institute conducts, coordinates, and funds cancer research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer. Access the National Cancer Institute Web site at http://www.cancer.gov. Also called NCI.

NCI, part of the National Institutes of Health of the United States Department of Health and Human Services, is the Federal Government's principal agency for cancer research. It conducts, coordinates, and funds cancer research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer. Access the NCI Web site at http://www.cancer.gov. Also called National Cancer Institute.

A tumor that arises in nerve cells.

Any of a large group of cancers of lymphocytes (white blood cells). Non-Hodgkin lymphomas can occur at any age and are often marked by lymph nodes that are larger than normal, fever, and weight loss. There are many different types of non-Hodgkin lymphoma. These types can be divided into aggressive (fast-growing) and indolent (slow-growing) types, and they can be formed from either B-cells or T-cells. B-cell non-Hodgkin lymphomas include Burkitt lymphoma, chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), diffuse large B-cell lymphoma, follicular lymphoma, immunoblastic large cell lymphoma, precursor B-lymphoblastic lymphoma, and mantle cell lymphoma. T-cell non-Hodgkin lymphomas include mycosis fungoides, anaplastic large cell lymphoma, and precursor T-lymphoblastic lymphoma. Lymphomas that occur after bone marrow or stem cell transplantation are usually B-cell non-Hodgkin lymphomas. Prognosis and treatment depend on the stage and type of disease. Also called NHL.

By or having to do with the mouth.

A cancer of the bone that usually affects the large bones of the arm or leg. It occurs most commonly in young people and affects more males than females. Also called osteogenic sarcoma.

A tumor suppressor gene that normally inhibits the growth of tumors. This gene is altered in many types of cancer.

A laboratory method used to make many copies of a specific DNA sequence. Also called polymerase chain reaction.

A thin layer of tissue that covers the lungs and lines the interior wall of the chest cavity. It protects and cushions the lungs. This tissue secretes a small amount of fluid that acts as a lubricant, allowing the lungs to move smoothly in the chest cavity while breathing.

A laboratory method used to make many copies of a specific DNA sequence. Also called PCR.

A molecule made up of amino acids that are needed for the body to function properly. Proteins are the basis of body structures such as skin and hair and of substances such as enzymes, cytokines, and antibodies.

A person who has studied science, especially one who is active in a particular field of investigation.

Checking for disease when there are no symptoms. Since screening may find diseases at an early stage, there may be a better chance of curing the disease. Examples of cancer screening tests are the mammogram (breast), colonoscopy (colon), Pap smear (cervix), and PSA blood level and digital rectal exam (prostate). Screening can also include checking for a person’s risk of developing an inherited disease by doing a genetic test.

The clear liquid part of the blood that remains after blood cells and clotting proteins have been removed.

A virus that infects some types of monkeys. It may also infect humans, and was found in some polio vaccines tested in the early 1960s. Although the virus has been shown to cause cancer in laboratory animals, there is no evidence that it causes cancer in people. Also called SV40.

In medicine, the ongoing collection of information about a disease, such as cancer, in a certain group of people. The information collected may include where the disease occurs in a population and whether it affects people of a certain gender, age, or ethnic group.

Simian virus 40. A virus that infects some types of monkeys. It may also infect humans, and was found in some polio vaccines tested in the early 1960s. Although the virus has been shown to cause cancer in laboratory animals, there is no evidence that it causes cancer in people. Also called simian virus 40.

An indication that a person has a condition or disease. Some examples of symptoms are headache, fever, fatigue, nausea, vomiting, and pain.

Cancer that forms in tissues of the testis (one of two egg-shaped glands inside the scrotum that make sperm and male hormones). Testicular cancer usually occurs in young or middle-aged men. Two main types of testicular cancer are seminomas (cancers that grow slowly and are sensitive to radiation therapy) and nonseminomas (different cell types that grow more quickly than seminomas).

A group or layer of cells that work together to perform a specific function.

An abnormal mass of tissue that results when cells divide more than they should or do not die when they should. Tumors may be benign (not cancer), or malignant (cancer). Also called neoplasm.

A type of gene that makes a protein called a tumor suppressor protein that helps control cell growth. Mutations (changes in DNA) in tumor suppressor genes may lead to cancer. Also called antioncogene.

Treated with a vaccine.

Treatment with a vaccine.

A substance or group of substances meant to cause the immune system to respond to a tumor or to microorganisms, such as bacteria or viruses. A vaccine can help the body recognize and destroy cancer cells or microorganisms.

In medicine, a very simple microorganism that infects cells and may cause disease. Because viruses can multiply only inside infected cells, they are not considered to be alive.