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What is the Vaccine Adverse Events Reporting System (VAERS)?

The Vaccine Adverse Events Reporting System (VAERS) is one of several vaccine monitoring programs

that are used to monitor vaccine safety in the United States. The U.S. Food and Drug Administration

(FDA) and the Centers for Disease Control and Prevention (CDC) established VAERS in 1990 to record

and track reactions or adverse events that occur after a vaccine is given to patients.

Anyone can file a VAERS report if they think they or someone they know has had a reaction to a

vaccine. VAERS receives reports from health care professionals, parents, and patients.

Doctors and other medical research experts from FDA and CDC analyze the VAERS reports to monitor

vaccine safety. Potential safety problems that are detected during regular reviews of VAERS reports are

considered to be only a "signal" of a possible safety problem. When the VAERS data "signals" a

potential safety problem, the vaccine is thoroughly investigated through other research methods. This

is because adverse events reports to VAERS are self-reported and not routinely verified before being

entered into the system, thus great caution must be used in interpreting patterns found in the data.

Verification studies have shown that most adverse events reported in the VAERS database are not

actually caused by an immunization. Rather, the "adverse events" that are shown to be unrelated to

an immunization were the result of some other cause, such as a naturally occurring illness.

The rotavirus vaccine is a recent example of how VAERS works. Analysis of VAERS data in 1999

identified that 15 cases of a rare but serious bowel obstruction were reported in association with

the rotavirus vaccine (after approximately 1.5 million doses of the newly licensed vaccine had been

given to patients). Medical experts are now conducting further studies to better understand the

relationship between the bowel obstructions and the vaccine. In the meanwhile, the vaccine has

been withdrawn from use in the United States.

The VAERS telephone number is 1-800-822-7967; the Web site is; and the

email address is

Are there "hot lots" of vaccines that have been associated with more adverse events than

others have? Should parents find the numbers of these lots and make sure their children don't

receive vaccines from them?

The answer to both of these questions is no. Manufacturers produce and distribute vaccines in

quantities known as "lots." Lot sizes vary widely between different types of vaccines and different

manufacturers. Samples of each lot are sent to the FDA for tests of safety, potency, and purity

before the lot may be given to patients.


Source: The National Network for Immunization Information (

Updated: October 2000 * Page 1 of 5


Common Questions about


Adverse Events That Follow Vaccination


VAERS data can be used to monitor how many adverse events have been reported for each vaccine

"lot" approved for use. However, because vaccine lots are not all of the same size, nor distributed

and used at the same rate, differences in the numbers of adverse events reported must be

interpreted with great caution. Some people have misinterpreted the difference between the

number of adverse events in some lots versus other lots as meaning that some lots, i.e. "hot lots,"

are more dangerous than others.

FDA officials routinely monitor vaccine lots using VAERS data and other information. With the

exception of an early lot of polio vaccine in 1955, which was not fully inactivated, there has never

been a "hot lot."

Members of the public can also view the adverse reaction data in the VAERS database online at Currently, however, it is difficult for members of the public to interpret this

information because the number of doses created in a given vaccine lot and the actual use, or

market share, is considered to be the proprietary information of the manufacturer, and as a result,

is not made available to the public. FDA officials do have access to this information, and they use it

in their vaccine safety monitoring, but they are not allowed to make proprietary information public.

The perspective of the National Network for Immunization Information is that vaccine manufacturers

should release to the public information on the number of doses created and used. Releasing this

information to the public, or allowing the FDA to do so, will have an important benefit and is

unlikely to put any vaccine manufacturer at a competitive disadvantage in the marketplace. The

benefit of releasing the data is that interested members of the public will be able to prove to

themselves that there are no "hot lots."

Are some people more susceptible to adverse reactions than others?

Yes, but it is important to understand that serious adverse reactions to vaccines are very rare.

Some people are allergic to a substance present in a vaccine, such as an antibiotic or gelatin

stabilizer. These people may experience a severe allergic reaction to the vaccine (that can cause

difficulty in breathing, a drop in blood pressure, and sometimes shock); this occurs very rarely

approximately (1 out of 500,000 doses).




The American Academy of Pediatrics, the Centers for

Disease Control and Prevention, and the American Academy of Family Physicians recommend that

doctors avoid immunizing a person who has had an anaphylactic reaction to a component of a

vaccine. However, people who have had less serious reactions to a vaccine might choose to be

vaccinated to avoid the risk of illness or death from vaccine-preventable diseases.

Also, children with a personal or family history of seizures may be at greater risk of having seizures

after being vaccinated with the DTP (diphtheria, tetanus, pertussis) vaccine.




However, this vaccine

is no longer recommended for use in the United States. The newer DTaP vaccine (which includes the

greatly purified acellular pertussis vaccine [aP] as a component) which is now used is far less likely

to cause seizures in any children. DTaP is safe and recommended even for children with a family

history (siblings or parents) of seizures.


Source: The National Network for Immunization Information (

Updated: October 2000 * Page 2 of 5


Common Questions about


Adverse Events That Follow Vaccination



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If one person in a family has an adverse reaction to a vaccine, will other members of the family

also have the same reaction? Is there a laboratory test that can identify whether a person

might have an adverse reaction to a vaccine before being vaccinated?

There is no single test that can determine whether a person will have an undesired reaction to a

vaccine. However, if a person has a serious allergic reaction after a vaccination, he or she may be

referred to an allergist who can attempt to determine which component of the vaccine may have

caused the reaction. If an allergy to a vaccine component is found in one person, siblings, and

children of that person can also be tested for the allergy. However, most reactions are not likely to

occur in two members of the same family.

Can vaccines cause permanent adverse events, such as a long-lasting impairment, or death?

Yes, however it is important to understand that the risk of serious adverse events is extremely small

(approximately 1 serious event occurs for each 100,000 doses of vaccine given). Most adverse

events associated with vaccines are minor and short-lived. Of those few serious adverse events

which do occur, only a small proportion result in long-lasting impairment or death. The diseases

that vaccines prevent are far more dangerous than the vaccines that effectively prevent them.

Some people believe that certain immunizations can lead to a number of chronic diseases. Many

scientific studies have been conducted to investigate these concerns. The results of this research

repeatedly point to the conclusion that vaccines do not cause chronic diseases. For example, the

Institute of Medicine, an independent research organization that is part of the National Academy of

Sciences, reviewed all existing evidence on health problems that occur after vaccination. Their

review did not show a cause-and-effect relationship between vaccines and any long-term illness.




G Some scientists, and some parents, are concerned about a possible link between MMR

(measles, mumps, and rubella) vaccine and autism. It is not yet known for certain what causes

autism, but the best available evidence indicates that autism is a condition that begins before

birth (in the first trimester of pregnancy), not after a child is born. The disease is usually

diagnosed when children are 18 to 30 months old, which is the period shortly after they receive

many of the recommended vaccines. Because of this coincidence in timing, some people have

come to believe that the development of autism is somehow associated with the MMR vaccine.

With the exception of one research study, whose findings have now been widely refuted, all of

the scientific evidence has concluded that vaccines are in no way associated with autism.




G Similarly, one investigator has suggested that the onset of diabetes might somehow be linked

to whole-cell pertussis vaccine, to

Haemophilus influenzae

type b (Hib) vaccine, the new

pneumococcal conjugate vaccine, or to the timing of immunizations in general




(also see Scientific research studies and research reviews, however, have all

concluded that vaccines do not cause diabetes.


13 -17


G Other people have been concerned that vaccines may be associated with Sudden Infant Death

Syndrome (SIDS), which typically occurs in infants between 2 and 4 months of age (a period

when many immunizations are given). All scientific studies and review papers have shown that

vaccines do not cause SIDS.




Since 1992, the rate of SIDS in the United States has been

reduced by 40% as a result of an education campaign encouraging parents to put their babies to

sleep on their backs.


Source: The National Network for Immunization Information (

Updated: October 2000 * Page 3 of 5


Common Questions about


Adverse Events That Follow Vaccination



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G Rates of childhood asthma have increased in recent years. This increase occurred during a

period of time when the number of routine childhood immunizations also increased, so some

have speculated that these may be related. As a result, medical researchers have studied

whether vaccines might cause asthma. These studies found no increased risk of asthma after





G There has also been concern that multiple sclerosis (MS) might be associated with use of the

hepatitis B vaccine.




In 1994, the Institute of Medicine, an independent research organization

that is part of the National Academy of Sciences, reviewed all available information and

determined the evidence did not show that the vaccine causes nervous system diseases. More

recently, in 1998, the Viral Hepatitis Prevention Board of the World Health Organization asked a

panel of experts to review scientific data again. These experts also concluded that the hepatitis

B vaccine does not cause multiple sclerosis.



1 Rosenthal S, Chen R, Hadler S. The safety of acellular pertussis vaccine vs. whole-cell pertussis vaccine.

A postmarketing assessment. Arch Pediatr Adolesc Med 1996;150:457-460.

2 Livengood JR, Mullen JR, White JW, Brink EW, Orenstein WA. Family history of convulsions and use of pertussis vaccine.

J Pediatr 1989;115:527-531.

3 Atkinson W, Wolfe C, Humiston S, Nelson R, eds. Epidemiology and Prevention of Vaccine-Preventable Diseases.

6th ed. (The Pink Book.) Atlanta: Centers for Disease Control and Prevention; 2000.

4 Institute of Medicine. Adverse effects of pertussis and rubella vaccines. Washington, DC: National Academy Press; 1991.

5 Institute of Medicine. Adverse events associated with childhood vaccines. Washington, DC:

National Academy Press; 1994.

6 Afzal MA, Minor PD, Schild GC. Clinical safety issues of measles, mumps and rubella vaccines.

Bulletin of the World Health Organization 2000; 78(2): 199-204.

7 Chen RT, DeStefano F. Vaccine adverse events: causal or coincidental? Lancet 1998; 351:611-612.

8 Rodier, P. The early origins of autism. Sci Am. 2000 Feb;282(2):56-63.

9 Taylor B, Miller E, Farrington CP, et al. Autism and measles, mumps, and rubella vaccine: no epidemiological evidence

for a causal association. Lancet 1999;353:2026-9.

10 Wakefield et. al. (1998). Ileal-Lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental

disorder in children. Lancet, 351: 637-41.

11 DeStefano F and Chen R. Autism and measles, mumps and rubella vaccine: No epidemiological evidence for a causal

association. J Pediatrics 2000;136:125-126.

12 Classen DC and Classen JB. The timing of pediatric immunization and the risk of insulin-dependent diabetes mellitus.

Infectious Diseases in Clinical Practice, 1997;6:449-454. (

13 Blom L, Nystrom and Dahlquist G. The Swedish childhood diabetes study. Vaccinations and infections at risk

determinants for diabetes in childhood. Diabetologia, 1991, 34: 176-181.

14 Graves PM, Barrige KJ, Norris JM, Hoffman MR, Yu L, Eisenbarth GS, Rewers M. Lack of association between early

childhood immunizations and B-cell autoimmunity. Diabetes Care, 1999, 22:1694-1697.


Source: The National Network for Immunization Information (

Updated: October 2000 * Page 4 of 5


Common Questions about


Adverse Events That Follow Vaccination



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15 Heijbel H, Chen RT, Dahlquist G. Cumlative incidence of childhood-onset IDDM is unaffected by pertussis immunization.

Diabetes care, 1997, 20:173-175.

16 Karvonen M, Cepaitis Z, Tuomilehto J. Association between type 1 diabetes and Haemophilus influenzae type b

vaccination: birth cohort study. BMJ 1999;318:1169-1172.

17 The Institute for Vaccine Safety Diabetes Workshop Panel. Childhood immunizations and type 1 diabetes: summary of

an Institute for Vaccine Safety Workshop. Pediatr Infect Dis J. 1999;18:217-22.

18 Bouvier-Colle MH, Flahaut A, Messiah A, Jougla E, Hatton F. Sudden infant death and immunization: an extensive

epidemiological approach to the problem in France - Winter 1986. Int J Epidemiol 1989;18:121-6.

19 Cherry JD, Brunell PA, Golden GS, Karzon DT. Report of the Task Force on Pertussis and Pertussis Immunization -1988.

Pediatrics 1988;81(suppl):939-84.

20Hoffman HS, Hunter JC, Damus K, et al. Diphtheria-tetanus-pertussis immunization and sudden infant death: results of

the National Institute of Child Health and Human Development Cooperative Epidemiological Study of Sudden Infant

Death Syndrome Risk Factors. Pediatrics 1987;79:598-611.

21 Griffin MR, Ray WA, Livengood JR, Schaffner W. Risk of sudden infant death syndrome (SIDS) after immunization with

the diphtheria-tetanus-pertussis vaccine. N Engl J Med 1988;319: 618-23.

22Henderson J, North K, Griffiths M, Harvey I, Golding J. Pertussis vaccination and wheezing illnesses in young children:

prospective cohort study. The Longitudinal Study of Pregnancy and Childhood Team. BMJ 1999;318:1173-6.

23 Kemp T, Pearce N, Fitzharris P, Crane J, Fergusson D, St. George I, Wickens K, Beasley R. Is infant immunization a risk

factor for childhood asthma or allergy? Epidemiology 1997;8:678-80.

24Meeting Report. Multiple sclerosis and hepatitis B vaccine. Vaccine 1999;17:2473-2475.

25 Halsey et. al. Hepatitis B vaccine and central nervous system demyelinating diseases. Pediatr Infect Dis J 1999;18:23-4.


Recommended books and Web sites on this topic:


Offit PA and Bell LM. Vaccines: What every parent should know, revised edition. New York: IDG Books; 1999.

Humiston SG and Good C. Vaccinating your child: Questions & answers for the concerned parent.

Atlanta: Peachtree Publishers; 2000.

The Centers for Disease Control and Prevention (

National Alliance for Autism Research (NAAR) Web site (

National Multiple Sclerosis Society Web site (


Source: The National Network for Immunization Information (

Updated: October 2000 * Page 5 of 5


Common Questions about


Adverse Events That Follow Vaccination