Safety > Issues of Interest > 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:
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
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.
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
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.
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
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
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
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: http://www.cdc.gov/nip
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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