The Center for Disease Control (“CDC”) defines “adverse reaction” as “an untoward (unfavorable) effects caused by a vaccine that are extraneous to (not in keeping with) the vaccine's primary purpose of production of immunity.”[1] Adverse reactions are also known as “side effects.” Relatedly, but more general in nature, is an “adverse event,” defined as “any medical event that occurs following vaccination. An adverse event could be a true adverse reaction (vaccine side effect) or just a coincidental event, with further research needed to distinguish between them.”[2]

The CDC maintains a page on its website containing information regarding the known side effects to vaccines licensed for use in the United States.[3] The information contained on the website differentiates between mild, moderate, and severe problems. It also details the probability of certain side effects. Finally, in some cases it lists suggested preventative measures. For example, with the HPV—Cervarix vaccine, people report mild to moderate side effects, which includes about 9 in 10 people reporting pain at the site of the injection, and about 1 in 4 people reporting nausea, vomiting, diarrhea, or abdominal pain.[4] It is suggested that one may experience symptoms of fainting (e.g. dizziness, vision change) with any vaccine, and if this happens, lying down for 15 minutes can help prevent any injuries/falls.[5]   

Additionally, along with the Food and Drug Administration, the CDC maintains the Vaccine Adverse Event Reporting System (VAERS).[6] The purpose of this website is to monitor and gather information regarding the broad concept of adverse events related to vaccinations. Anyone who gives or receives a vaccination is encouraged to report any adverse events.[7] This is especially useful with newer vaccines, such as the HPV vaccines, which are being monitored for the occurrence of unusual or severe problems. This information collection and monitoring is an important tool for public health, as it helps identify and warn of problems with vaccines, however, the CDC recognizes that VAERS data has strengths and a few weaknesses:

Strengths

• VAERS collects national data from all U.S. states and territories.
• VAERS accepts reports from anyone.
• The VAERS form collects information about the vaccine, the person vaccinated, and the adverse event.
• Data are publicly available.
• VAERS can be used as an early warning system to identify rare adverse events.
• It is possible to follow-up with patients to obtain health records, when necessary.

Limitations

• It is generally not possible to find out from VAERS data if a vaccine caused the adverse event.
• Reports submitted to VAERS often lack details and sometimes contain errors.
• Serious adverse events are more likely to be reported than mild side effects.
• Rate of reports may increase in response to media attention and increased public awareness.
• It is not possible to use VAERS data to calculate how often an adverse event occurs in a population.[8]

[1] http://www2a.cdc.gov/nip/isd/ycts/mod1/scripts/showglos.asp?back=http://www2a.cdc.gov/nip/isd/ycts/mod1/courses/genrec/10905.asp?student_id=&new=true#A

[2] Id.

[3] http://www.cdc.gov/vaccines/vac-gen/side-effects.htm

[4] Id.

[5] Id.

[6] United States. Center for Disease Control. (2015, August 28). Vaccine Adverse Event Reporting System (VAERS), Retrieved on Sept. 27, 2016, from the Center for Disease Control Web site: http://www.cdc.gov/vaccinesafety/ensuringsafety/monitoring/vaers/

[7] Id.

[8] Id.