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Vaccine Safety > Issues of Interest > Contamination
Concerns about 
Vaccine Contamination

What evidence is there that vaccines are contaminated with agents that can transmit infectious diseases?

There is no reliable evidence that currently available vaccines are contaminated with infectious agents that cause disease. Today, manufacturers are required to test cell lines used in the production of vaccines for the presence of a variety of infectious agents, to prevent contamination of vaccine products. The following issues have been proposed by some as potential infectious agents:

Reverse transcriptase (RT)

Vaccines produced using chick embryo may contain minute quantities of an avian enzyme necessary for a retrovirus to reproduce. This enzyme does not confirm the presence of a retrovirus. In addition, this enzyme
is not derived from the human immunodeficiency virus (HIV), a well-known retrovirus.

Reverse transcriptase (RT) is an enzyme necessary for retroviruses to reproduce. Retroviruses are found in many different species. RT is not infectious in humans or animals, and it has not been shown to cause any
adverse health effects in people. Using a highly sensitive polymerase chain reaction (PCR) based assay, RT activity has been detected in minute quantities in vaccines manufactured with chick embryo fibroblasts. The source of the enzyme is probably a partial viral genome coding for RT, believed to be integrated into chick cells hundreds or thousands of years ago. Avian retroviruses that produce this RT are not known to affect humans. While the human immunodeficiency virus (HIV, the virus that leads to AIDS), is a retrovirus, the RT activity detected in vaccines is definitively not derived from HIV. Furthermore, the presence of RT does not confirm the presence of a retrovirus.

Among vaccines with evidence of RT are the measles, mumps, influenza, and yellow fever vaccines. The presence of RT does not necessarily indicate the presence of a retrovirus that could cause infection or lead to illness in humans. In fact, investigations of the vaccines and the chick cells from which they were produced, conducted by the Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA), and others, have found no evidence of an infectious, transmissible retrovirus.


"Stealth" virus

There is no evidence that polio vaccine, or any other vaccine, has been contaminated with a "stealth" virus. To our knowledge, existence of this virus as suggested by one researcher has not been confirmed by other

"Stealth" virus is the label that one researcher has given to a cytopathic virus he has described. He has suggested that such viruses may reside within the body without being detected by the immune system. There is no evidence that polio vaccine, or any other vaccine, has been contaminated with a "stealth" virus. To our knowledge, existence of this virus has not been confirmed by other investigators.

The researcher reports that the "stealth" virus or a related virus has been cultured from the tissues of patients with chronic fatigue syndrome and other chronic neuropsychiatric and autoimmune conditions. He further suggests that the "stealth" virus could be the cause of these illnesses. These findings have not been confirmed by any other investigators. Moreover, it has been stated that the same researcher found similar rates of viral detection among persons with chronic fatigue syndrome and control persons without these illnesses in a blinded study. If a "stealth" virus were a cause of these illnesses, higher rates of infection would be expected among ill persons, compared with healthy control subjects.

He has reported that a partial sequence of this virus is like that of simian cytomegalovirus (SCMV) and suggests that the source of the SCMV-like virus is the African Green monkey. Because African Green monkey kidney cells are used in production of live oral polio vaccine, he hypothesized that the SCMV-like virus ("stealth" virus) may be an adventitious agent in polio vaccines. The researcher's results have not been confirmed in independent laboratories (the usual way to confirm scientific findings) to date.

At a workshop in July 1996, the FDA and National Institutes of Health scientists reviewed some of this scientist's observations. The results presented were considered inconclusive. The need for sharing among scientists of methods and reagents was emphasized so that other workers might independently evaluate the methods and conclusions. FDA considers that the methods applied by the manufacturer to test for SCMV in polio vaccine would detect replicating SCMV.


Simian virus 40 (SV 40)

This virus has no relationship to HIV or simian immunodeficiency virus (SIV). In the early 1960s, some lots of polio vaccines were discovered to contain SV 40. Since 1961, manufacturers have been required by the Food and Drug Administration to test for SV 40. There is limited evidence that SV 40 can infect humans, but there is no evidence it causes human health problems.

Simian virus 40 was discovered in the early 1960s to be present in some lots of polio vaccines It was found in the inactivated (injectable) polio vaccine and some experimental lots of oral polio vaccine. The polio vaccine in those lots had been manufactured in kidney cells from simians (monkeys) that harbored SV40. The control methods used before this time did not identify this adventitious agent. The formalin inactivation process used to kill the poliovirus was found not to inactivate the SV40 completely. Beginning in 1961, manufacturers have been required by FDA to test for SV40, using modified methods and different cell cultures, and lots positive for SV40 are not released. A few years later, the source of monkeys used for production was changed to species that do not harbor SV40.

SV40 is a polyomavirus. It has no relationship to HIV or simian immunodeficiency virus (SIV; the virus that causes AIDS in monkeys). SV40 can cause cancer in laboratory animals. Limited evidence suggests that SV40 can infect humans, but there is no evidence that it causes human health problems. Recent publications have identified SV40 DNA sequences in samples of tissue from rare human tumors, including choroid plexus tumors, mesotheliomas, ependymomas, and osteosarcomas, as well as in apparently normal (non-neoplastic) human tissue. However, this finding has not been confirmed by some investigators. The entire SV40 genome has been isolated from one human choroid plexus tumor.

People who received polio vaccine between 1954 and 1962 may have received a dose that contained SV40. As many as 10 to 30 million persons in the U.S. could have received SV40-contaminated injectable polio vaccine. The participants of several clinical trials (approximately 10,000 persons) may have received oral polio vaccine that was contaminated with SV40. Studies in the U.S. have shown no increased rates of cancer among persons who would have received polio vaccine between 1954 and 1962; similarly, studies in Sweden have not shown any increased risk for cancers among people exposed to vaccines potentially containing SV40, compared with people who were not exposed to these vaccines. Moreover, there is no evidence that polio vaccine administered since the early 1960s contained SV40. For decades, manufacturers have been required by FDA to test for SV40 using infectivity assays.


Fetal tissue

No new fetal tissue is used to produce vaccines. Cell lines generated from a single fetal tissue source are sometimes used for some vaccines. Vaccine manufacturers obtain human cell lines from FDA-certified cell banks.

Fetal Tissue and Vaccines. Some vaccines such as rubella and varicella are made from human cell-line cultures, and some of these cell lines originated from aborted fetal tissue, obtained from legal abortions in the 1960s. No new fetal tissue is needed to produce cell lines to make these vaccines, now or in the future.

In 1964, there were an estimated 20,000 babies born with congenital rubella syndrome (CRS) in the United States. When women contract rubella disease during early pregnancy, this often causes miscarriage or
neurological damage to the unborn child, resulting in the child suffering blindness, deafness and retardation. In 1993, the number of CRS cases was seven, reflecting a 99.9% reduction in this preventable disease since 1964.

In 1996, CDC, the American Academy of Pediatrics and the American Academy of Family Physicians jointly recommended routine use of varicella vaccine to protect children from chicken pox. In March 1995, this vaccine was licensed for use in the United States by the FDA and was shown to be safe and effective. Widespread use of varicella vaccine--which took more than 20 years for development, testing, and licensing--can help prevent thousands of children from being hospitalized each year because of complications from chicken pox. Complications of chickenpox include secondary infections such as group A streptococcal infections, which can lead to necrotizing fasciitis, multiple organ failure, shock, and death. In the United States each year, from 50 to 100 previously healthy children or adults die from complications associated with their chickenpox illness.


Precautions Against Contamination

Vaccines are safe and effective. The FDA requires manufacturers to adhere to numerous procedures to protect its safety when producing vaccines. All vaccines undergo rigorous clinical trials for safety and effectiveness before being licensed by the FDA for use in the United States.

FDA has many procedures in place, designed to detect and prevent the presence of adventitious agents in vaccines. These procedures include characterizations of the cell substrates used for vaccine production as well as testing the products by specified methods. For example, manufacturers today are required to test the cell lines (monkey kidney cells) used for the production of poliovirus vaccines for a variety of infectious agents: tuberculosis, SV40, herpes viruses (including simian cytomegalovirus), Coxsackie virus, lymphocytic choriomeningitis virus, measles, and others. Cell (tissue) culture tests must show an absence of cytopathogenic effects. Adult and suckling animals are also used to screen for viable microbial agents. A neurovirulence safety test is conducted for live oral poliovirus vaccine. The cell substrates used for measles/mumps/rubella (MMR) vaccine are derived from flocks free of avian leukosis virus. MMR vaccine also undergoes extensive testing for adventitious viral activity.

As technology improves, the FDA may require the application of new methods to screen for infectious agents in vaccines. Experimental assays have been applied by FDA on an ad hoc basis. For example, RT and PCR methods have been applied to some monopools(1) of polio vaccine to assure the absence of SIV and HIV. Other vaccines have been examined using other highly sensitive PCR methods.


For more information

For more information about vaccines and vaccinations, contact CDC's Information Contact Center: 1-800-CDC-INFO (1-800-232-4636; or visit NIP's website:

1. There are three types of polio: Type 1, Type 2, and Type 3. During vaccine production, vaccines for the three types are produced separately, each being a monopool. The three monopools are then combined to produce the polio vaccine, which is a trivalent vaccine.

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This page last modified on September 28, 1997


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