Mechanics of contagion: Most carriers won’t spread virus, but a few infect many

Chennai

In Italy, for example, scientists looked at stored samples of wastewater for the earliest trace of the virus. Last week they reported that the virus was in Turin and Milan as early as Dec. 18. But two months would pass before northern Italy’s hospitals began filling with victims of Covid-19. So those December viruses seem to have petered out. As strange as it may seem, these reports don’t contradict each other. Most infected people don’t pass on the coronavirus to someone else. But a small number pass it on to many others in so-called superspreading events.

“You can think about throwing a match at kindling,” said Ben Althouse, principal research scientist at the Institute for Disease Modeling in Bellevue, Wash. “You throw one match, it may not light the kindling. You throw another, it may not. But then one match hits in the right spot, and all of a sudden the fire goes up.”

Understanding why some matches start fires while many do not will be crucial to curbing the pandemic, scientists say. “Otherwise, you’re in the position where you’re always one step behind the virus,” said Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine. When the virus first emerged in China, epidemiologists scrambled to understand how it spread from person to person. One of their first tasks was to estimate the average number of people each sick person infected, or what epidemiologists call the reproductive number.

The new coronavirus turned out to have a reproductive number somewhere between two and three. It’s impossible to pin down an exact figure, since people’s behavior can make it easier or harder for the virus to spread. By going into lockdown, Massachusetts drove its reproductive number down from 2.2 at the beginning of March to 1 by the end of the month; it’s now at .74.

This averaged figure can also be misleading because it masks the variability of spread from one person to the next. If nine out of 10 people don’t pass on a virus at all, while the 10th passes it to 20 people, the average would still be two. In some diseases, such as influenza and smallpox, a large fraction of infected people pass on the pathogen to a few more. These diseases tend to grow steadily and slowly. “Flu can really plod along,” said Kristin Nelson, an assistant professor at Emory University.

But other diseases, like measles and SARS, are prone to sudden flares, with only a few infected people spreading the disease. Epidemiologists capture the difference between the flare-ups and the plodding with the dispersion parameter. It is a measure of how much variation there is from person to person in transmitting a pathogen. But James Lloyd-Smith, a UCLA disease ecologist who developed the dispersion parameter 15 years ago, cautioned that just because scientists can measure it doesn’t mean they understand why some diseases have more superspreading than others. “We just understand the bits of it,” he said. When COVID-19 broke out, Dr Kucharski and his colleagues tried to calculate that number by comparing cases in different countries. If COVID-19 was like the flu, you’d expect the outbreaks in different places to be mostly the same size.

— Carl Zimmer is an Op-Ed columnist with NYT© 2020