A scientific study supporting this suspicion—which would assign #SARS-CoV-2 a reproduction number comparable to smallpox and just slightly below that of the common cold—is a paper from Los Alamos National Laboratory that will appear in the July issue of Emerging Infectious Diseases, a journal published by the U.S. Centers for Disease Control and Prevention. The paper is the kind of hard scientific work that is prompting concerns about the virus’s ability to spread even with partial shutdowns. Senior author Ruian Ke and colleagues at Los Alamos used data from Wuhan, China, to derive a calculation of the SARS-CoV-2 reproduction number, concluding that the R0 of SARS-CoV-2 is likely to be about 5.7, with a 95 percent probability that it lies in a range between 3.8 and 8.9.
IF TRUE, this has several important implications, beginning with the concept of herd immunity. Herd immunity occurs when a sufficiently large percentage of a population becomes immune to a disease, either through infection or vaccination, with the result that transmission no longer occurs readily, thus protecting even those who are not immune. In the case of SARS-CoV-2, reports suggesting that herd immunity will be achieved when 55 percent of the population is immune assume that the R0 of the virus is 2.2. If the R0 is 5.7 instead, then 82 percent of the population must acquire immunity—because the higher the reproductive number, the larger the immune population must be to halt the spread of the disease. An #R0 of 5.7 rather than 2.2 would require billions more doses of vaccine globally—or, if herd immunity is achieved only through infection, millions of excess deaths worldwide.
Another implication of a higher reproduction number is that seasonality—which, if applicable to SARS-CoV-2, is estimated (based on studies of other coronaviruses) to reduce transmission by about 20 percent during the summer—will not lower the R0 to a range anywhere near 1.
HOW LIKELY is the 5.7 estimate to be correct? Epidemiologist Marc Lipsitch, a professor at the Harvard T.H. Chan School of Public Health and director of its Center for Communicable Disease Dynamics, says, “There is a view that [the R0] is that high. It stills seems to be a minority view, but I think it is a credible one.”
