Research shows simple policies could control a smallpox epidemic

See also...

-Infectious Disease Epidemiology

External Sites:
-Centers for Disease Control and Prevention - smallpox

-PNAS

(Imperial College is not responsible for the content of these external internet sites)

For immediate release
Thursday 27 July 2006

A series of simple public health policies would be able to effectively contain the spread of smallpox if it were released into a population, according to research published in PNAS.

Computer models developed by scientists at Imperial College London and the University of Hong Kong show that a strategy of isolation, contact tracing and vaccination would control an outbreak even in a high transmission scenario.

Dr Steven Riley, from the University of Hong Kong, and one of the researchers said: "Although there hasn't been a release of smallpox, it still causes significant public concern and if there was even a single case, it could cause substantial levels of social disruption. Fortunately this study shows we could control even a large outbreak through simple public health policies."

The researchers looked at three scenarios where the number of additional infections generated by one infected person varied between 1.5 for a low transmission scenario to 2.6 for a medium transmission scenario and 5.4 for a high transmission scenario.

Using computer models, they tested the three main public health policies used to control epidemics, and found different but high levels of effectiveness in all.

The first policy the team looked at was rash-motivated case isolation, in which infected individuals with a rash sought medical care, were identified as smallpox cases and subsequently isolated. They found this intervention was highly effective in controlling a low transmission scenario provided the average delay to getting a patient isolated was two days. If this was reduced to half a day, it could also control the medium transmission scenario.

The second policy the researchers looked at was combining contact tracing with vaccination to create a policy of 'ring' vaccination. This works by contacting everyone an infected individual had been in touch with, who is then contacted and vaccinated. The modelling found this to be effective in all three transmission scenarios, even if the contact tracing was only 65 percent effective.

The third policy combined regional mass vaccination with isolation and contact tracing with vaccination. The mass vaccination would result in all individuals within a certain distance of an infected individual being vaccinated. While this policy did prove more effective than the first two it was not effective in terms of the number vaccinated compared with the number of infections stopped. For a medium transmission scenario, an additional nine million doses stopped only six infections, while for a high transmission scenario13.7 million doses prevented only 77 infections.

Dr Riley added: "Not only does the study show these simple policies are effective, they are probably much safer than a policy of mass vaccination. When nearly 40,000 were vaccinated in the United States, three deaths, two permanent disabilities and ten life threatening illnesses were attributed to vaccination, showing that the risks may well outweigh any potential benefits."

For further information please contact:

Tony Stephenson
Press Officer
Communications Division
Tel: +44 (0)20 7594 6712
Mobile: +44 (0)7753 739766
E-mail: at.stephenson@imperial.ac.uk

Notes to editors:

1. 1. About smallpox - Smallpox was a highly contagious viral disease unique to humans, responsible for an estimated 300-500 million deaths in the 20th century. As recently as 1967, the World Health Organization (WHO) estimated that 15 million people contracted the disease and that two million died in that year. After successful vaccination campaigns, the WHO in 1979 certified the eradication of smallpox, though cultures of the virus are kept by the Centers for Disease Control and Prevention (CDC) in the United States and at the Institute of Virus Preparations in Siberia, Russia. Although only these two laboratories are recognised as storing the virus, it is likely that an unknown number of samples are held covertly. For this reason, contingency planning for the possible deliberate reintroduction of smallpox has become a priority for many national public health organisations.

2. Smallpox transmission and control: spatial dynamics in Great Britain, PNAS.

3. Consistently rated in the top three UK university institutions, Imperial College London is a world leading science-based university whose reputation for excellence in teaching and research attracts students (11,000) and staff (6,000) of the highest international quality.
Innovative research at the College explores the interface between science, medicine, engineering and management and delivers practical solutions that enhance the quality of life and the environment - underpinned by a dynamic enterprise culture.
Website: www.imperial.ac.uk