Last updated 22 April 2022
A vaccine is a medical product. Vaccines, though designed to protect from disease, can cause side effects, just as any medication can.
Most side effects from vaccination are mild, such as soreness, swelling, or redness at the injection site. Some vaccines are associated with fever, rash, and achiness. Serious side effects are rare, but may include seizure or life-threatening allergic reaction.
A possible side effect resulting from a vaccination is known as an adverse event.
Each year, American babies (1 year old and younger) receive more than 10 million vaccinations. During the first year of life, many babies suffer serious, life-threatening illnesses and medical events, such as Sudden Infant Death Syndrome (SIDS). Additionally, congenital conditions may become evident during the first year. Therefore, due to chance alone, many babies will experience a medical event in close proximity to a vaccination. This does not mean, though, that the event is actually related to the immunization. The challenge is to determine when a medical event is directly related to a vaccination.
The Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) have set up systems to monitor and analyze reported adverse events and determine whether they are likely related to vaccination.
Types of Side Effects
To understand the range of possible vaccination side effects events, it is useful to compare a vaccine with relatively few associated side effects, such as the vaccine for Haemophilus influenza type B, with a vaccine known to have many potential side effects, such as the infrequently used smallpox vaccine (given to military personnel and others who might be first responders in the event of a bio-terror attack).
Haemophilus influenza type B is a bacterium that can cause serious infections, including meningitis, pneumonia, epiglottitis, and sepsis. The CDC recommends children receive a series of Hib vaccinations starting when they are two months old.
Smallpox is a serious infection, fatal in 30% to 40% of cases, and caused by the Variola major or Variola minor virus. Since the 1970s, no wild smallpox cases have been reported. The World Health Organization has declared it eradicated.
The information below about side effects of Hib and smallpox vaccination is from the Centers for Disease Control and Prevention.
Hib Vaccine Side Effects
- Redness, warmth, or swelling where the shot was given (up to 1 out of 4 children)
- Fever over 101°F (up to 1 out of 20 children)
No serious side effects have been related to the Hib vaccine.
Smallpox (Vaccinia) Vaccine Side Effects
Mild to Moderate Problems:
- Mild rash, lasting 2-4 days.
- Swelling and tenderness of lymph nodes, lasting 2-4 weeks after the blister has healed.
- Fever of over 100°F (about 70% of children, 17% of adults) or over 102°F (about 15%-20% of children, under 2% of adults).
- Secondary blister elsewhere on the body (about 1 per 1,900).
Moderate to Severe Problems:
- Serious eye infection, or loss of vision, due to spread of vaccine virus to the eye.
- Rash on entire body (as many as 1 per 4,000).
- Severe rash on people with eczema (as many as 1 per 26,000).
- Encephalitis (severe brain reaction), which can lead to permanent brain damage (as many as 1 per 83,000).
- Severe infection beginning at the vaccination site (as many as 1 per 667,000, mostly in people with weakened immune systems).
- Death (1-2 per million, mostly in people with weakened immune systems).
For every million people vaccinated for smallpox, between 14 and 52 could have a life-threatening reaction to the smallpox vaccine.
How Do I Find Out the Side Effects for Different Vaccines?
When you or a child gets a vaccine, the health care provider gives you a handout known as the Vaccine Information Statement (VIS). The VIS describes common and rare side effects, if any, of the vaccine. Your health care provider will probably discuss possible side effects with you. VIS downloads are also available through the CDC’s website.
Package inserts produced by the vaccine manufacturer also provide information about adverse events. Additionally, these inserts usually show rates of adverse events in experimental and control groups during pre-market testing of the vaccine.
How Are Adverse Events Monitored?
The CDC and FDA established in 1990. According to the CDC, VAERS is used “to detect possible signals of adverse events associated with vaccines.” (A signal in this case is evidence of a possible adverse event that emerges in the data collected.) About 30,000 events are reported each year to VAERS. Between 10% and 15% of these reports describe serious medical events that lead to hospitalization, life-threatening illness, disability, or death.
VAERS is a voluntary reporting system. Anyone, such as a parent, a health care provider, or friend of the patient, who suspects an association between a vaccination and an adverse event, may report that event and information about it to VAERS. The CDC then investigates the event and tries to find out whether the vaccination actually caused the adverse event.
The CDC states they monitor VAERS data to
- Detect new, unusual, or rare vaccine adverse events
- Monitor increases in known adverse events
- Identify potential patient risk factors for particular types of adverse events
- Identify vaccine lots with increased numbers or types of reported adverse events
- Assess the safety of newly licensed vaccines
Not all adverse events reported to VAERS are actually caused by a vaccination. The two occurrences may be related only in time. And, it is probable that not all adverse events resulting from vaccination are reported to VAERS. The CDC states that many adverse events, such as swelling at the injection site, are underreported. Serious adverse events, according to the CDC, “are probably more likely to be reported than minor ones, especially when they occur soon after vaccination, even if they may be coincidental and related to other causes.”
VAERS has successfully identified several rare adverse events related to vaccination. Among them are
- An intestinal problem after the first vaccine for rotavirus was introduced (the vaccine was withdrawn in 1999)
- Neurologic and gastrointestinal diseases related to yellow fever vaccine
According to Plotkin et al., VAERS identified a need for further investigation of MMR association with a blood clotting disorder, encephalopathy after MMR, and syncope after immunization (Plotkin SA et al. Vaccines, 5th ed. Philadelphia: Saunders, 2008).
Vaccine Safety Datalink
The CDC established this system in 1990. The VSD is a collection of linked databases containing information from large medical groups. The linked databases allow officials to gather data about vaccination among the populations served by the medical groups. Researchers can access the data by proposing studies to the CDC and having them approved.
The VSD has some drawbacks. For example, few completely unvaccinated children are listed in the database. The medical groups providing information to VSD may have patient populations that are not representative of large populations in general. Additionally, the data come not from randomized, controlled, blinded trials but from actual medical practice. Therefore, it may be difficult to control and evaluate the data.
Rapid Cycle Analysis is a program of the VSD, launched in 2005. It monitors real-time data to compare rates of adverse events in recently vaccinated people with rates among unvaccinated people. The system is used mainly to monitor new vaccines. Among the new vaccines monitored in Rapid Cycle Analysis are the conjugated meningococcal vaccine, rotavirus vaccine, Tdap vaccine, and the HPV vaccine. Possible associations between adverse events and vaccination are then further studied.
Vaccine Injury Compensation
For information on systems for compensating individuals who have been harmed by vaccines, see our article on .