Researchers call for better ways to identify highly infectious individuals.
Epidemiologists are warning that we need to focus more attention on the influence of superspreaders, the highly infectious individuals who pass on disease to scores of other people. Their data illustrate just how widespread the influence of such people is.
The team hopes to prompt further study of superspreaders and help policy-makers to focus plans for vaccination or quarantine in the face of possible future pandemics.
The idea that one person can infect a disproportionately large number of people is not new itself. The most famous case is that of Irish immigrant Mary Mallon, who worked as a cook in New York in the early 1900s and subsequently infected dozens of people with typhoid. She even refused to switch her profession after authorities demanded she do so. Typhoid Mary became "the patron saint of superspreaders", says Sebastian Schreiber of the College of William and Mary in Williamsburg, Virginia.
The concept hit headlines more recently during the spread of the fatal respiratory illness SARS in 2003. One SARS superspreader, who was a patient at Hong Kong's Prince of Wales Hospital, ended up directly infecting more than 90 people.
Until now, experts have applied the concept of superspreading mainly to sexually transmitted diseases and infections spread through parasitic worms. They have also relied on a general 20/80 rule, which states that 20% of infected individuals are responsible for 80% of transmission of a disease.
To check on the real numbers across a wide range of diseases, Schreiber and James Lloyd-Smith at the University of California, Berkeley, along with colleagues, modelled the transmission of eight infectious diseases, including SARS, smallpox and pneumonic plague. They report their findings in Nature1.
Some degree of superspreading was found in all eight types of disease, but it varied considerably. SARS roughly follows the 20/80 rule, with the most infectious 20% of individuals responsible for roughly 85% of transmission. For Ebola that top 20% were responsible for less than 35% of transmission.
One factor that helped to determine whether someone would infect huge numbers of people was the presence of a second illness. This was the case with one smallpox patient quarantined at a hospital in Western Germany in 1970, for example. He managed to infect more than a dozen others because he also suffered from bronchitis, which caused him cough out large amounts of infectious material.
"This pattern of one disease helping another is quite common," says Lloyd-Smith.
He and his co-authors hope that health officials will take the findings of the study to heart and implement more rigorous monitoring of disease transmission, as well as taking steps to identify and treat superspreaders.
That might require the development of new tools, says Mark Nicas, a scientist at the University of California, Berkeley, who has modelled disease transmission. He suggests that medical engineers develop instruments that can rapidly measure the amount of infectious material in a person's sneeze, for example, in order to help detemine what qualities make for a superspreader, and to beef up quarantine measures for these people.
Lloyd-Smith also adds that public health officials might also consider asking people who come in contact with many others, such as teachers, to wear masks in the face of a pandemic to prevent the spread of disease. "I think some of the more extreme measures may be appropriate" he says, should, for example, an easily transmissible form of avian flu appear.
Lloyd-SmithJ.O, SchreiberS.J., KoppP.E& GetzW.M.. Nature, 438. 355 - 359doi:10.1038/nature04153 (2005).
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Khamsi, R. Could sneeze meters combat pandemics?. Nature (2005). https://doi.org/10.1038/news051114-7