Global Vaccine Safety

Safety of smallpox vaccine: questions and answers1

1. How safe is the smallpox vaccine?

Most people experience normal, usually mild reactions that include a sore arm, fever, and body aches. In recent tests, one in three people felt bad enough to miss work, school, or recreational activity or had trouble sleeping after receiving the vaccine. However, the vaccine does have some risks. In the past, about 1,000 people for every 1 million people vaccinated for the first time experienced reactions that, while not life-threatening, were serious. These reactions include a vigorous (toxic or allergic) reaction at the site of the vaccination and spread of the vaccinia virus (the live virus in the smallpox vaccine) to other parts of the body and to other people. These reactions typically do not require medical attention. Rarely, people have had very bad reactions to the vaccine. In the past, between 14 and 52 people per 1 million vaccinated experienced potentially life-threatening reactions, including eczema vaccinatum, progressive vaccinia (or vaccinia necrosum), or postvaccinal encephalitis. Based on past experience, it is estimated that between 1 and 2 people out of every 1 million people vaccinated may die as a result of life-threatening reactions to the vaccine.

2. How can the risks of adverse reactions be avoided?

Careful screening of potential vaccine recipients is essential to ensure that those at increased risk do not receive the vaccine. People most likely to have side effects are those who have, or even once had, skin conditions (especially eczema or atopic dermatitis), and people with weakened immune systems, such as those who have received a transplant, are HIV positive, or are receiving treatment for cancer. Anyone who falls within these categories, or lives with someone who falls into one of these categories, should NOT get the smallpox vaccine unless they are exposed to the disease. Pregnant women should not get the vaccine because of the risk it poses to the fetus. Women who are breastfeeding should not get the vaccine. Children younger than 12 months of age should not get the vaccine.

3. Do individuals who have had the smallpox vaccine in the past have less severe reactions to the current vaccine?

Historically re-vaccinees have had less severe adverse events, but since routine vaccination was stopped globally 20 years ago, most people may be assumed to have low to no protective immunity to smallpox. Therefore, re-vaccinees may have similar rates of reactions as first-time vaccinees.

4. Is a history of no adverse reactions in childhood to smallpox vaccine a predictor of no or minor reactions to revaccination in adulthood?

No. Simply because a person did not experience an adverse reaction to the vaccine in childhood does not mean that they will not experience adverse reactions as an adult. Many of the conditions that increase the likelihood of serious adverse reactions may not have been present in childhood (e.g. skin conditions, taking medication that suppresses the immune system).

5. Have heart problems such as cases reported recently in the US been previously associated with smallpox vaccination?

Myocarditis has been reported previously following smallpox vaccination in Europe, but had not been a well-accepted complication following vaccination with the US-licensed New York City Board of Health vaccine. Coronary events, including angina or myocardial infarction, have not been previously associated with smallpox vaccine. The current smallpox vaccination program may differ from historical experience because a greater number of older patients with underlying heart disease and cardiac risk factors such as hypertension and diabetes mellitus may be receiving vaccinations. Moreover, because current diagnostic tests, including cardiac enzymes and echocardiography, are more sensitive for diagnosing myocardial infarction, more events may be detected than were historically observed.

6. What is the risk of death due to cardiac problems as a result of receiving the smallpox vaccination?

Cardiac-associated death following smallpox vaccination, although extremely rare, has been reported in Europe and Australia and has been thought to be associated with myocarditis. However, a death certificate study of vaccinia-associated deaths in the U.S. over a 20-year period did not identify any deaths associated with cardiac complications. The US Centres for Disease Control and Prevention (CDC) reports that it is not clear whether the number of events that have recently occurred in the US is greater than would be expected. A certain number of cardiac events and deaths following vaccination would be expected to occur by chance alone, given both how common cardiac problems are and the numbers of people vaccinated in the US civilian program.

7. What is WHO currently doing to assure safe and quality smallpox vaccines at the global level?

WHO has convened 2 expert consultations to review the scientific basis for guidance on production and quality control of smallpox vaccines. As a result, revised WHO recommendations for production and control of smallpox vaccines have been established in 2003. These technical specifications are designed to assure that newly manufactured smallpox vaccines meet all current standards for safety and efficacy. Such technical specifications do not, however, provide assurance against serious adverse events in known risk groups. In addition, the Global Advisory Committee on Vaccine Safety reviewed historical and recent safety data for the smallpox vaccine and concluded that there is a real risk of serious adverse events following immunization with smallpox vaccine, including safety issues that have not previously been recognized, and that implementation of immunization would require significant capacity and resources for safety monitoring. The Committee will continue to monitor the safety of smallpox vaccines and advise WHO.

1Several of the questions and answers are adapted from original material prepared by the US Centres for Disease Control.

Page last reviewed: 7 January 2009