History shows smallpox could be contained

Ring approach alternative to mass vaccination

Reviewing two historical smallpox outbreaks, researchers in the United Kingdom say it may be possible to contain transmission of the virus without resorting to mass pre-event immunization. Indeed, unless there is a smallpox outbreak, the individual risks of pre-event smallpox vaccination may outweigh the potential benefits.

The so-called ring containment strategy used to eradicate smallpox in the wild has been criticized as a response to terrorism. Instead, many have argued for mass pre-event vaccination. But the lessons of history suggest that smallpox could indeed be contained even in the event of an intentional release within a susceptible population.

Targeted surveillance and containment interventions have been successful in the past and should be explored as alternatives to mass vaccination, says lead researcher Emma Kerrod, MD, an epidemiologist at the Centre for Emergency Preparedness and Response at the Health Protection Agency in Salisbury, England.

"Containment of an outbreak today, even in a population that is susceptible, would most likely be practicable — assuming good contact tracing, case isolation, and surveillance," she explains to Bioterrorism Watch. "This is a reasonable assumption in any developed country. Mathematical modeling of smallpox outbreaks has done much to support this view, although interventions would ultimately depend on the size of the outbreak."

Kerrod and colleagues reviewed historical data from smallpox outbreaks in Liverpool in 1902-03 and Edinburgh in 1942.1 In both outbreaks, extensive contact tracing, quarantine, and staged vaccination campaigns were initiated, and the outbreaks were controlled within 15 months and three months, respectively.

In Edinburgh, the number of fatalities associated with vaccination exceeded number of deaths from the disease. In Liverpool, ambulatory, vaccine-modified cases and misdiagnosis as chickenpox resulted in problems with outbreak control. Active surveillance, vaccination of contacts, and prompt hospital isolation of patients were important aspects of disease control in both outbreaks, the authors said. The relatively slow spread of smallpox, particularly in Liverpool, allowed for effective implementation of targeted intervention methods.

"The Liverpool outbreak was ultimately contained even though the disease was endemic, social conditions were arguably worse, and disease importations frequent," Kerrod says. "The outbreak in Edinburgh [1942] lasted only three months before the situation was contained. And this was a situation in which smallpox was not endemic and social conditions were more likely to have been better than in Liverpool. Targeted vaccination was employed in both instances."

In Edinburgh, 360,000 people were vaccinated after the outbreak, leading to 10 vaccine-related deaths — two more than died of actual smallpox. "[That serves as] a poignant reminder when weighing up the pros and cons of pre-emptive mass vaccination against smallpox when an outbreak is only a possible, not a probable." she notes.

Analysis of the Edinburgh and Liverpool outbreaks suggests that outbreaks after deliberate release of smallpox virus may "evolve over time," the study said. Therefore, sufficient opportunity exists for targeted enhanced surveillance measures to be put in place, for additional staff to be mobilized for an effective follow-up, and for a containment strategy to be implemented, it added.

In contrast, mass immunization is problematic because the smallpox vaccine poses a health threat to the millions of people who have atopic dermatitis or are immune-compromised due to HIV infection, chemotherapy, immunity disorders, and transplantations. Kerrod cites another study that suggests a smallpox outbreak could be contained in less than six months by contact tracing and case isolation alone in a population with only 20% immunity.2 "Another author suggests that ring vaccination, can be successful in containing a small [e.g., number of index cases] outbreak, if infectious cases are rapidly diagnosed and intervention measures are very effective," she adds.3 "[Our] argument for targeted vs. mass vaccination echoes the views of many authors."

References

1. Kerrod E, Geddes AM, Regan M, et al. Surveillance and control measures during smallpox outbreaks. Emerg Infect Dis 2005; 11(2). Web site: www.cdc.gov/ncidod/EID/vol11no02/04-0609.htm.

2. Eichner M. Case Isolation and Contact Tracing can prevent the spread of smallpox. Am J Epidemiol 2003; 158:118-128.

3. Kretzschmar M, van den Hof S, Wallinga J, et al. Ring vaccination and smallpox control. Emerg Infect Dis 2004; 10(5):832-841. Web site: www.cdc.gov/ncidod/EID/vol10no5/03-0419.htm.