How It Works
The vaccinia vaccine helps the body develop immunity against smallpox, a potentially deadly infection caused by the variola virus. The vaccine is made from live vaccinia, a virus related to—but milder than—smallpox. The vaccine cannot cause smallpox infection.
A health professional gives a small amount of vaccinia vaccine through several quick punctures on the upper arm with a two-pronged needle. Then the vaccine site is covered with gauze to prevent the vaccinia virus from being spread to another part of the body or to another person. Over about 21 days, the site develops a series of normal skin reactions similar to the stages of smallpox rash. These reactions are a sign that the body is building immunity to the virus:footnote 2
- On or about day 3, a raised red bump (papule) develops on the vaccination site.
- On or about day 5, the raised area fills with clear fluid (vesicle).
- By day 7, the fluid becomes cloudy and white (pustule), growing to its largest size by about day 10.
- The dried pustule scabs and falls off 14 to 21 days after vaccination, leaving a scar.
See a picture of a typical smallpox vaccine reaction.
Why It Is Used
Smallpox is considered a potential bioterrorist weapon. Most people in Canada have little or no immunity to the virus that causes smallpox. Mass vaccinia vaccination in Canada and the U.S. ended in 1972 because the risks associated with the vaccine were greater than the risk of smallpox coming into the country. In 1980, health experts declared smallpox eradicated. But there is concern that smallpox could be used as a terrorist weapon. As a result, people at high risk of infection in a bioterrorist attack—certain health care workers, public health personnel, and members of the military—are being vaccinated. The vaccine is not available to the public at this time.
The vaccine can protect against smallpox infection by building the body's immunity before exposure to the variola virus. The vaccine also can prevent or significantly lessen the severity of symptoms after exposure if given within 3 days of the exposure. Vaccination 4 to 7 days after exposure may offer some protection from the disease or may lessen its severity.footnote 3
How Well It Works
The vaccinia vaccine prevents smallpox infection in more than 95% of those vaccinated. Immunity after a first-time vaccination is likely to last 3 to 5 years. Revaccination may extend immunity for 10 to 20 years or more.footnote 4
The vaccinia vaccine does not contain smallpox (variola) virus and therefore cannot cause smallpox infection. But vaccinia can cause side effects. Historically, about 1,000 people in every 1 million vaccinated for the first time had serious, though not life-threatening, reactions; between 14 and 52 people in every 1 million vaccinated for the first time experienced life-threatening reactions.footnote 3
Mild to moderate reactions after vaccinia vaccination include:
- Fever. About 10% of people vaccinated for the first time reported fever of 38°C (100°F) or higher.footnote 5
- Spread of the virus. In the past, more than half of all smallpox vaccine complications were caused by touching a vaccine site (one's own or another person's), then spreading the virus to other areas. Commonly infected areas included the face, nose, mouth, genitalia, rectum, and eyelid. If you are vaccinated, avoid touching the site for three weeks. If you accidentally touch the site, wash your hands with soap and water. Make sure to cover the site with a bandage and wear a long-sleeved shirt (even at night) until the scab falls off.
- Generalized vaccinia. A rash can develop on the body if the vaccinia virus spreads through the bloodstream to the skin.footnote 6 In otherwise healthy people, the skin lesions usually go away on their own and don't leave a scar.
Severe reactions to vaccinia vaccination
Among the more than 14 million people given the smallpox vaccination in the United States in 1968, the three most serious, though rare, reactions were:footnote 6
- Eczema vaccinatum, which is the spread of vaccine virus on the skin of people who have active or healed eczema (or atopic dermatitis). In the past, this condition was usually mild, but it can be severe, or fatal.footnote 6 Eczema vaccinatum could occur because of vaccination or because of transmission to the unvaccinated close contact of a vaccinated person.
- Progressive vaccinia (vaccinia necrosum), which affected people with impaired immune systems. With progressive vaccinia, the virus continues to multiply from the vaccination site, breaking down tissue as it progresses. Progressive vaccinia was often fatal, despite intensive treatment.
- Post-vaccinal encephalitis, which was often fatal or resulted in permanent brain damage. There was no known treatment.
Vaccinia immune globulin (VIG) is used to treat some reactions to the vaccine. VIG is a product containing antibodies made from the blood of people who have received the smallpox vaccine several times. Cidofovir is an antiviral medicine that might be used, but its use for smallpox vaccine reactions has not been approved by Health Canada's Therapeutic Product's Directorate (TPD) or the U.S. Food and Drug Administration (FDA). Both of these medicines are given into the vein (intravenous, or IV) in a hospital.
Certain people may have an increased risk of developing complications from the vaccine, including:
- People with skin conditions, such as active or healed eczema, or atopic dermatitis.
- Children younger than age 1.
- People with impaired immune systems.
- Pregnant women, because of the risk of infecting the fetus.
- Breastfeeding women, because of the risk of infecting the baby.
- People with serious, life-threatening allergies to the antibiotics polymyxin B, streptomycin, tetracycline, or neomycin.
- People with heart problems or three or more risk factors for heart disease.
People should not get the smallpox vaccine if they live in the same house or have close physical contact with a person who has active or healed eczema or a weak immune system (from cancer, for example), because of the risk of spreading the virus to that person.
Women who are pregnant should not have close contact with anyone who has received the smallpox vaccine within the past 28 days.
But if you are directly exposed to smallpox, you should be vaccinated even if you fall into one of these groups.footnote 1, footnote 2 In such a situation, the risk of dying from smallpox is greater than the risk of having an adverse reaction to the vaccine.
See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)
What To Think About
For accurate, up-to-date information, visit the Public Health Agency of Canada (PHAC) website at www.phac-aspc.gc.ca/ep-mu/smallpox-eng.php or the U.S. Centers for Disease Control and Prevention (CDC) website at www.bt.cdc.gov/agent/smallpox.
- National Advisory Committee on Immunization (NACI) (2002). Statement on smallpox vaccination. Canada Communicable Disease Report, 28(ACS-1): 1–12.
- Centers for Disease Control and Prevention (2001). Vaccinia (smallpox) vaccine recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR, 50(RR-10): 1–25.
- Centers for Disease Control and Prevention (2007). Smallpox fact sheet: Vaccine overview. Available online: http://emergency.cdc.gov/agent/smallpox/vaccination/facts.asp.
- Breman JG, Henderson DA (2002). Diagnosis and management of smallpox. New England Journal of Medicine, 346(17): 1300–1308.
- Centers for Disease Control and Prevention (2004). Side effects of smallpox vaccination. Available online: http://www.bt.cdc.gov/agent/smallpox/vaccination/reactions-vacc-public.asp.
- Henderson DA, et al. (1999). Smallpox as a biological weapon: Medical and public health management. JAMA, 281(22): 2127–2137.
Primary Medical Reviewer John Pope, MD - Pediatrics
Brian D. O'Brien, MD - Internal Medicine
W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Specialist Medical Reviewer William Atkinson, MD, MPH -
Thomas Emmett Francoeur, MD, MDCM, CSPQ, FRCPC - Pediatrics
Current as ofJune 18, 2018
Current as of: June 18, 2018