Information Resources

What Do I Need To Know About Vaccine Reactions?

Updated December 28, 2023

Q: Have specific reactions been associated with specific vaccines?

A: Like with prescription drugs and other pharmaceutical products, reactions to vaccines can be mild or severe. There can be complete recovery or long lasting health problems after vaccine reactions. Rarely, vaccine reactions can be fatal. 1 2

As a consumer vaccine safety advocacy organization, NVIC worked on the National Childhood Vaccine Injury Act of 1986 and was responsible for urging the creation of safety provisions in that historic law. This included a centralized vaccine reaction reporting system, the Vaccine Adverse Event Reporting System (VAERS), that is jointly operated by the federal Food and Drug Administration and Centers for Disease Control.

If you are making a decision about vaccination for yourself or your child, it is important to first become fully informed about each vaccine’s reported side effects, precautions and contraindications, which means reasons for why a person may be at risk for having a serious reaction. 3 4

It is especially important for parents to become informed about vaccine complications and how to recognize the symptoms of a vaccine reaction. 5 When a person collapses within seconds, minutes or hours of vaccination, it may be anaphylactic shock – a life threatening allergic reaction like can happen after bee stings, or exposure to certain drugs, chemicals or foods like peanuts. 6 7 Signs of anaphylaxis include problems with breathing and swallowing, wheezing, chest or abdominal pain, vomiting, diarrhea, hives and rashes, swelling, weakness, mental confusion and loss of consciousness.

Depending upon the vaccine, most vaccine reactions take longer than a few minutes to develop. For example, brain inflammation reactions to pertussis (whooping cough) vaccine in the DPT or DTaP shots usually occur within 72 hours or a week of vaccination, 8 while it may take between one and two weeks for signs of brain inflammation to develop after getting live virus measles, mumps and rubella vaccine. 9

There should be careful monitoring of signs and symptoms of vaccine reactions for at least a week or two after vaccination. Look for symptoms like:

  • High fever (over 103F);
  • Skin redness or discoloration, swelling or hardened lump and pain at the site of the injection; 10 11 12
  • High pitched screaming, which is shrill inconsolable crying by babies that lasts for hours 13 14
  • Collapse/Shock, which is known as a hypotonic/hyporesponsive episode, when a child suddenly goes limp and may have pale skin or blue tinged lips 15
  • Excessive sleepiness, when a child sleeps deeply for an unusually long period of time and does not wake to eat
  • Convulsion, also known as a seizure, which may include rolling of eyes, blank staring for long periods, twitching, trembling, shaking, stiffness and jerking of the body 16 17 18 19 20
  • Brain inflammation, also known as encephalitis or encephalopathy, is the most serious vaccine complication. Depending upon the age, symptoms can include agitation, high-pitched screaming with arching of the back or inconsolable crying, convulsions, muscle weakness, paralysis, double vision, slurred speech, mental confusion and unconsciousness. 21 22 23 24 25 26 27
  • Behavior Changes and Mental or Physical Regression after vaccination can be signs of brain or immune system damage, such as loss of muscle control and developmental milestones, sleep disturbances, personality changes, onset of severe allergies and intestinal disorders or other dramatic changes in health. 28 29 30 31 32 33 34

If your, or your child, experiences serious health problems following vaccination, regressed physically, mentally or emotionally after vaccination or has experienced acute symptoms within hours, days or weeks of vaccination it should be reported to the federal Vaccine Adverse Event Reporting System (VAERS) website, or by calling 1-800-822-7967. Your doctor is required by law to report adverse reactions to vaccination within 30 days of vaccination. You, a nurse or health department official may also report serious health problems following vaccination to VAERS.

If there is a question about whether or not brain inflammation or another type of serious reaction is occurring after vaccination, contact the person who administered the vaccine(s) and go to a hospital emergency room immediately. The details of the health problems experienced after vaccination should be entered into the permanent medical record and reported to the federal Vaccine Adverse Event Reporting System. 35

If a doctor or medical worker will not make a vaccine reaction report to the government, you can make a report yourself. 36

NVIC also provides as a public service our Ask Eight, Before You Vaccinate webpage that gives and overview of reaction symptoms and injury compensation.

Although it has been the law since 1986 for doctors and other vaccine providers to report hospitalizations, injuries, deaths and serious health problems following vaccination to VAERS, it is estimated that less than 10 percent, perhaps less than one percent of all vaccine-related health problems are ever reported to VAERS. It is very important that ALL serious health problems, which develop after vaccination, be reported to the federal health authorities at VAERS. NVIC has acted as a watchdog on VAERS and we encourge consumers to also report vaccine reactions to NVIC's Vaccine Reaction Registry, operated since 1982. We also host MedAlerts, the service that allows the public to easily search the federal VAERS database.

As a society we need to know how many people who get vaccinated are getting sick after vaccination and how many go on to die or are permanently injured. In addition, information provided to the VAERS may also help identify common factors, which affect certain individuals who have adverse responses to vaccination. More information can lead to safer vaccines, safer vaccine policies and medical research to develop screening techniques to help reduce vaccine reactions.

Q: I'm trying to get a list that identifies the adverse reaction rates of all the childhood vaccines. In other words, which vaccines have the highest rate of adverse reactions?

A:  Although this information should be available, it either does not exist or is not available to the public. Theoretically, it should be possible to calculate for each vaccine how many adverse reactions are reported to the federal reporting system, VAERS, every year. However, we know that only between one and ten percent of reactions are ever reported even though there has been a federal law in place since 1986 that requires serious health problems following vaccination to be reported to the government.

Even if all the vaccine-related adverse health events were reported, it wouldn't necessarily mean that every adverse health event that has been reported to VAERS after vaccination has been solely caused by the vaccine(s) recently given. Another complicating factor is that, today, children tend to get many different vaccines simultaneously at every office visit. As a result, it is often difficult to attach a specific health problem following vaccination to a specific vaccine.

Another major limitation to evaluating "adverse reaction rates" is that "rates" can be computed by dividing the numerator (number of adverse reactions) by the denominator (number of vaccines given). There isn't a valid publicly available source of data regarding the numbers of doses of each vaccine and numbers of children who receive doses of each vaccine annually in the U.S.

Other than the VAERS database, information on adverse reaction rates must come from large, prospective studies that are conducted over a long period of time to evaluate the health outcomes of children who receive multiple vaccines from birth. An ideal clinical trial would compare the health of those children who receive many vaccines to those who received far fewer and no vaccines. No such clinical trial has ever been conducted.


1 Centers for Disease Control (CDC). Possible Side Effects from Vaccines.

2 Food and Drug Administration (FDA). Vaccines Licensed for Use in the United States (Manufacturer Product Information Package Inserts).

3  CDC. Vaccine Recommendations of the ACIP: Contraindications and Precautions.

4 Food and Drug Administration (FDA). Vaccines Licensed for Use in the United States (Manufacturer Product Information Package Inserts).

5 National Vaccine Information Center. If You Vaccinate, Ask 8.

6  U.S. National Library of Medicine. Anaphylaxis (Anaphylactic Shock). MedLine Plus.

7  CDC. Preventing and Managing Adverse Vaccine Reactions.  Feb. 21, 2018.

8  Institute of Medicine Committee to Study New Research on Vaccines. DPT Vaccine and Chronic Nervous System Dysfunction: A New Analysis. Executive Summary (pp.1-2) Washington, D.C. The National Academies Press 1994.

9  Millichap JG. Measles Vaccine and Encephalopathy. Pediatric Neurology Briefs 1998; 12(3): 22.

10 Pineau A, Durand C et al. Role of aluminum in skin reactions after diphtheria-tetanus-pertussis-poliomyelitis vaccination: an experimental study in rabbits. Toxocology 1992; 73(1): 117-125.

11 Pichichero ME, Edwards KM et al. Safety and Immunogenicity of Six Acellular Pertussis Vaccines and One Whole-Cell Pertussis Vaccine Given as a Fifth Dose in Four to Six Year old Children. Pediatrics 2000; 105(1).

12  Donaldson S, Canavan T et al. Injection Site Necrosis and Ulceration Following Vaccination in an Adult Patient. JDD 2018; 17(3).

13 Markel H, Oski JA et al. The Crying Infant. In: The Portable Pediatrician 1992; pp.70-72.

14 Neuroimmunolgy Clinic, KK Women’s and Children’s Hospital. Encephalitis in Children: Symptoms, Complications and Treatment. Health Xchange 2016.

15 DuVernoy TS, Braun MM, the VAERS Study Group. Hypotonic-Hyporesponsive Episodes Reported to the Vaccine Adverse Event Reporting System (VAERS), 1996-1998. Pediatrics 2000; 106(4).

16  Wheless JW, Sirven JI. Seizures in Newborns. Epilepsy Foundation Aug. 27, 2013.

17  Sun Y, Christensen J et al. Risk of febrile seizures and epilepsy after vaccination with diphtheria, tetanus, acellular pertussis, inactivated poliovirus and Haemophilus influenzae type B. JAMA 2012; 307(8): 523-531.

18  Duffy J, Weintraub E et al. Febrile Seizure Risk After Vaccination in Children 6 to 23 Months. Pediatrics 2016; 138(1).

19  Wolf SM, Forsythe A. Epilepsy and mental retardation following febrile seizures in childhood. Acta Pediatr Scand 1989; 78(2): 291-295.

20 MacDonald BK, Johnson AL et al. Febrile convulsions in 220 children – neurological sequelae at 12 years follow-up. Eur Neurol 1999; 41(4): 179-186.

21 LaRoche SM. Seizures and Encephalopathy. Semin Neurol 2011; 31(19): 194-201.

22 HRSA. Encephalopathy, Encephalitis, Acute Dissemination Encephalomyelitis. Vaccine Injury Compensation Program Vaccine Injury Table. Mar. 21, 2017.

23 Pellegrino P, Carnovale C, Perrone V et al. Acute Disseminated Encephalomyelitis Onset: Evaluation Based on Vaccine Adverse Events Reporting System. PLOS One Oct. 18, 2013.

24 Alicino C, Infante MT et al. Acute disseminated encephalomyelitis with severe neurological outcomes following virosomal seasonal influenza vaccine. Hum Vaccin Immunother 2014; 10(7).

25 Rao S, Elkon B et al. Long-Term Outcomes and Risk Factors Associated with Acute Encephalitis in Children. J Ped Infect Dis Soc 2015; 1(1): 20-27.

26  Iro MA, Sadarangani M et al. ImmunoglobuliN in the Treatment of Encephalitis (IgNiTE): protocol for a multicenter randomized controlled trial. BMJ Open 2016; 6(11).

27 Burton KLO, Williams TA et al. Long-Term Neuropsychological Outcomes of Childhood Onset Acute Disseminated Encephalomyelitis (ADEM): A Meta-Analysis. Neuropsychology Rev 2017; 27(2): 124-133.

28 Institute of Medicine Committee to Study New Research on Vaccines. DPT Vaccine and Chronic Nervous System Dysfunction: A New Analysis. Executive Summary (pp.1-2) Washington, D.C. The National Academies Press 1994.

29 Thompson NP, Montgomery SM. Is measles vaccination a risk factor for inflammatory bowel disease? Lancet 1995; 345(8957): 1071-1074.

30 Hurwitz E, Morgenstern H. Effects of diphtheria-tetanus-pertussis or tetanus vaccination on allergies and allergy-related respiratory symptoms among children and adolescents in the United States. J Manipulative Physiol Ther 2000; 23(2): 81-90.

31 Enriquez R, Addington W et al. The relationship between vaccine refusal and self reports of atopic disease in children. J Allergy Clin Immunol 2005; 115(4): 737-744.

32 Mawson AR, Ray BD, Bhuiyan AR, Jacob B. Pilot comparative study on the health of vaccinated and unvaccinated 6-to-12-year-old US children. J Trans Sci 2017; 3(3): 1-12.

33 Rismanchi N, Gold JJ et al. Neurological Outcomes After Presumed Childhood Encephalitis. Pediatr Neurol 2015; 53(3): 200-206.

34 Matsubara Y, Osaka H et al. Long-term outcomes in motor and cognitive impairment with acute encephalopathy. Brain Dev 2018; 40(9): 801-812.

35 NVIC. Report Vaccine Reactions: It’s the Law!

36 DHHS. Vaccine Adverse Event Reporting System (VAERS). Two Ways to Submit an Online Report to VAERS. Also Online VAERS Reporting Demonstration. July 2017.

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