Transmissible Strains, Population Density Drive SARS-CoV-2 Spread

With nearly 100 million cases globally, including 25 million in the United States, the question remains: What drives SARS-CoV-2 transmission?

According to an analysis published on January 5th by Communications Biology, the answer may be as simple as the R0 number of specific strains and population density. Unfortunately, with the highly transmissible B.1.1.7. strain set to become the “predominant” one in circulation across much of the world, again including the US, this isn’t exactly good news—and it likely should influence how vaccines are distributed.

“Because we were looking at the early phase of the outbreak, the B.1.1.7 strain hadn’t yet been seen,” co-author Anthony R. Ives, PhD, Steenbock Professor of Biological Sciences at the University of Wisconsin-Madison, told Contagion®. “However, we did find an association between strains containing the G614 spike mutation and the early spread rates of COVID-19. This is indirectly relevant for the B.1.1.7 variant, because it shows that a strain with higher transmissibility can demonstrably increase the spread rate and hence the mortality rate caused by COVID-19.”

For their analysis, Dr. Ives and his colleague Claudio Bozzuto, a researcher with Zurich-based Wildlife Analysis, focused on the spread of COVID-19 in the US at the start of the pandemic, before people changed their behavior (eg, mask-wearing, social distancing) to avoid the disease—assuming they did, of course. Using county-level data from 39 states through May 23rd of last year, they found that the higher a county’s population density, the more readily SARS-CoV-2 spreads from person to person.

This likely explains, at least in part, why places such as New York City (in the spring of 2020) and Los Angeles (currently) have experienced such crushing outbreaks of COVID-19.

Conversely, counties with low or moderate population densities did not show high rates of virus spread. Notably, regions in proximity with one another had similar transmission rates, perhaps due in part at least to similar public health responses being implemented in neighboring counties.

However, they also found that regions hosting more cases involving SARS-CoV-2 strains containing a mutation called G614 saw increased viral spread. This conclusion is supported by other research showing that this strain is more transmissible than others, the researchers said. The low number of strains with G614 mutations in the Pacific Northwest and southeastern US was associated with lower transmission rates, they said.

Based on their findings, the researchers suggest that directing available COVID-19 vaccine supplies toward densely populated counties would help to interrupt virus transmission—an approach that, at least to date, hasn’t been explored.

“The national-level strategies have focused on who to prioritize for vaccination—healthcare personnel, the elderly, essential workers, etc—[and] we very much agree with this approach,” Ives said. “The implications of county-level variation in R0 are orthogonal to this issue. Our results argue that for long-term control of COVID-19, levels of immunity (from vaccination or acquired immunity) will need to be higher in densely populated areas like New York City than sparsely populated areas. This shouldn't be a surprise from an epidemiological point of view. Thus, for long-term control, it would make sense to distribute more vaccine doses to areas with high R0. Once in these areas, it would make immanent sense to prioritize healthcare personnel, the elderly, essential workers, etc.”