The first studies initially reported estimates of R0 with lower values. Estimations subsequently increased and then again returned in the most recent estimates to the levels initially reported (Figure 1). A closer look reveals that the estimation method used played a role.
The two studies using stochastic methods to estimate R0, reported a range of 2.2–2.68 with an average of 2.44.1,9 The six studies using mathematical methods to estimate R0 produced a range from 1.5 to 6.49, with an average of 4.2.2,4–6,8,10 The three studies using statistical methods such as exponential growth estimated an R0ranging from 2.2 to 3.58, with an average of 2.67.3,7,11
Our review found the average R0 to be 3.28 and median to be 2.79, which exceed WHO estimates from 1.4 to 2.5. The studies using stochastic and statistical methods for deriving R0 provide estimates that are reasonably comparable. However, the studies using mathematical methods produce estimates that are, on average, higher. Some of the mathematically derived estimates fall within the range produced the statistical and stochastic estimates. It is important to further assess the reason for the higher R0 values estimated by some the mathematical studies. For example, modelling assumptions may have played a role. In more recent studies, R0 seems to have stabilized at around 2–3. R0 estimations produced at later stages can be expected to be more reliable, as they build upon more case data and include the effect of awareness and intervention. It is worthy to note that the WHO point estimates are consistently below all published estimates, although the higher end of the WHO range includes the lower end of the estimates reviewed here.
R0 estimates for SARS have been reported to range between 2 and 5, which is within the range of the mean R0 for COVID-19 found in this review. Due to similarities of both pathogen and region of exposure, this is expected. On the other hand, despite the heightened public awareness and impressively strong interventional response, the COVID-19 is already more widespread than SARS, indicating it may be more transmissible.