The University Record, May 6, 2002

Study examines the potential risks of smallpox vaccination

By Kara Gavin
Health System Public Relations

An immunization campaign to vaccinate Americans against smallpox would be protective against a bioterror attack, but the smallpox vaccination itself poses a risk of death or serious illness. So, a new study recommends that any decision to resume smallpox vaccination should be based on the likelihood of an attack, the effectiveness of other smallpox control measures and the potential harm of the vaccine. These conclusions are based on historical evidence from prior smallpox vaccination campaigns, adjusted to the current American population.

Smallpox is deadly, killing as many as 30 percent of those infected. Immunization against smallpox using vaccinia, the live cowpox virus, was successful in eradicating smallpox globally by 1980. But routine vaccination in the United States was stopped in 1972 when it was determined that the risk posed by the vaccine outweighed the likelihood of contracting smallpox.

It is not known how much immunity against smallpox remains in those people vaccinated prior to 1972. The recent anthrax attacks have raised concern about the nation’s vulnerability to smallpox attack, and recent polls found that many Americans would be interested in a vaccination, if available.

UMHS researchers—led by Alex Kemper, assistant professor of pediatrics and communicable diseases—hope their results may help guide public officials as they develop plans to prepare for a possible bioterror attack with smallpox. Their research group, the Child Health Evaluation and Research (CHEAR) Unit, focuses on issues of vaccine delivery and preventive health.

Historical evidence indicates that children less than a year old, and people with compromised immune systems or eczema, are most at risk for side effects from smallpox vaccination and should not receive it. And, because the smallpox vaccine is a live virus, a recently vaccinated person could pass the smallpox vaccine virus to a vulnerable unvaccinated person, potentially leading to death or serious illness. To prevent this, the researchers recommend that people in close contact with high-risk individuals should not be vaccinated.

As a result of these precautions, the U-M researchers estimate that 25 percent of the U.S. population would need to be excluded from any smallpox vaccination campaign because they are high risk for a side effect or have close contact with someone who is at high risk.

Kemper and his colleagues estimate that an immunization campaign targeting young people 1–29 year(s) of age could lead to the vaccination of 82.5 million individuals, of which 190 people would be expected to die from vaccine complications. A more comprehensive campaign, for those from 1–65 year(s) of age, could lead to vaccination of 178.5 million individuals, of which 285 people would be expected to die.

The number of people suffering serious, but survivable, side effects would be 1,600 in the campaign targeted toward children and young adults, and 4,600 in the more comprehensive campaign.

According to Kemper, “These numbers need to be interpreted cautiously, because they are based on historical data. Improvements in healthcare over the past three decades may decrease the number of fatal or serious side effects.”

But, he continued, “In contrast, the number of serious side effects may be greater due to the increased number of individuals who are immuno-compromised. In any case, the risk posed by smallpox vaccination is greater than the risk from other recommended vaccines, such as the MMR vaccine or the varicella vaccine against chicken pox. The public should be aware of these risks.”

In addition to Kemper, the authors of the study are pediatrics researcher Matthew Davis, lecturer in pediatrics and communicable diseases, and Gary Freed, professor of pediatrics and communicable diseases and chief of the Division of General Pediatrics and director of the CHEAR Unit.