First things first. What is R0 ?

In epidemiology, the basic reproduction number is abbreviated as R0 (pronounced like “R naught”). R0 is the number which describes numerically how an epidemic is expanding or contracting.  Grossly oversimplifying, R0 is the number of people each infected person subsequently infects. When an R0=1, there is one additional case per each identified case and the outbreak is staying steady. If the R0 is >1, the outbreak is expanding. An R0 of <1 indicates that the outbreak is declining (CDC) .

There are many factors which can affect the R0. Not every patient will infect the same number of new individuals, thus the R0 is an average across all cases. Some of the factors:

  • Reservoir. We believe that 2019-nCoV is transmitted in at least two ways: by contact with a reservoir or by person-to-person spread. It has been suggested that the 2019-nCoV is transmitted by bats to a secondary (likely mammalian) reservoir, and from there to humans, but this has not yet been confirmed.  Until we know the reservoir, we cannot accurately calculate an R0 for cases acquired directly from infected reservoirs (though we will be able to do so for disease acquired through person-to-person transmission).
  • Mode of transmission. Airborne infections like measles often have very high R0 (12-18) while infections that require sexual contact like HIV have a lower R0 (2-5).
  • Length of time of asymptomatic infectivity. HIV is a good example here. Infected persons are often asymptomatic for 5-10 years. During this time, they may have sex with multiple people. However, not every sexual interaction would result in a transmission of virus, which is why the R0 of HIV is relatively low.
  • Length of time of symptomatic infectivity. Pertussis, like measles, has an R0 value of 12-17, despite being transmitted via respiratory droplets rather than by droplet nuclei. This is likely due to the very prolonged nature of its associated cough. While those afflicted by the disease might be convinced to isolate themselves initially, to don a procedural mask, or to practice cough etiquette, these measures are difficult to sustain for several weeks.
  • Percent of population susceptible to infection. This can be affected by several things. Immunizing individuals against disease decreases the number of susceptible individuals in a population. Haemophilus influenza B is a great example, and as immunization coverage approached 90 percent, cases of invasive diseased declined to almost zero. In some diseases, like Norovirus gastroenteritis, there is a genetic susceptibility. A minority of the population cannot be infected because they lack a carbohydrate moiety which allows the virus to attach.
  • Medical interventions. For respiratory viruses, including 2019-nCoV, wearing masks, social distancing and increased hand washing can all effect changes in R0.

Do we know the R0 for the 2019-NCoV?

Simply put, no. Although Dr. Tony Fauci, director of the NIH National Institute of Allergy and Infectious Diseases (NIAID), remarked that preliminary reports indicate an R0 ranging from 1.5 to 3.5 (HHS.gov), the R0 is unknown for the 2019 novel coronavirus. Current estimates are based on reported cases and it’s possible that there are more cases than have been confirmed. Additionally, with some infections there can be mild illness or asymptomatic. These are often missed early in epidemics and lead to underestimating the R0.

Furthermore, in a recently released manuscript that has not yet been peer-reviewed, the R0 is estimated using two calculation methods. Both methods yielded an R0=2.9 with the lower end of both 95 percent confidence intervals at 2.3. Additionally, they report an average incubation period of 4.8 days (J. H. Tao Liu). For comparison, the R0 of SARS in 2003 was calculated at approximately 1.8 in the early stages of the outbreak, eventually ranging between 1.1-4.2depending on which calculation method was used (Bauch).  And, as was mentioned above, the R0 can vary widely depending on many factors. For example, the R0 estimates of some common and historic diseases are:

Measles, 12-18

HIV, 2-5

Ebola, 1.5 – 2.5

1918 Flu, 2-3

Moreover, It is important to note that historic estimates of R0 may be based on data obtained from long ago, and re-calculation in today’s environment may produce different results (Delamater PL).

How can we lower the transmission risk?

With respiratory viruses, including the flu and the 2019 novel Coronavirus, frequent hand-washing and social distancing are likely the best methods to stay healthy. There is conflicting data on whether masks are helpful for preventing seasonal flu in the healthcare setting, and it’s even less clear whether they are helpful in a community setting. However, research indicating detectable coronavirus on Personal Protective Equipment (PPE) after 4 hours (Casanova, Rutala and Sobsey) suggest that the careful donning and doffing of PPE is imperative in reducing the transmission of such viruses.

It’s worth noting that early in the course of the 2019-nCoV, it was thought this was a point-source, zoonotic outbreak associated with a fresh seafood market in Wuhan. Point source and zoonotic outbreaks typically have very limited person-to-person spread, thus the R0=0. Unfortunately, it is now clear that this novel coronavirus is passed from person to person. Once a serologic test is developed and accurate estimates of asymptomatic and mildly ill individuals can be made, more accurate estimates of R0 can be calculated.