Simple policies could control a smallpox epidemic

Jul 27 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."