Here's a report by the Infectius disease department
COVID-19 R0: Magic number or conundrum?Giulio Viceconte,1Nicola Petrosillo21Department of Clinical Medicine andSurgery, University “Federico II”,Naples; 2National Institute for InfectiousDiseases “L. Spallanzani”, IRCCS,Rome, ItalyThere is an increasing concern aboutCOVID-19 worldwide. This is a newemerging infectious disease caused by anovel coronavirus (SARS-CoV-2), whichrecently broke out from the Chinese city ofWuhan and has quickly spread in China,with sporadic cases in each continent.1At the date of February 20th, 2020,SARS-CoV-2 caused 74 675 infections inChina with 2 121 deaths, and 1 073 infec-tions in 26 countries with 8 deaths outsideChina.1COVID-19 represents the third coron-avirus-associated epidemic to emerge froma species leap from wild animals to humans,after Severe Acute Respiratory Syndrome(SARS) in 2003, and the Middle EastRespiratory Syndrome (MERS) in 2012.2,3SARS-CoV-2 Often causes a respiratorydisease, similar to SARS and MERS, rang-ing from mild upper respiratory illness to asevere interstitial pneumonia, also requiringintensive care.4,5One of the most discussed issues aboutCOVID-19 is its basic reproduction number(R0). Public opinion and mass media areincreasingly focusing on this epidemiologi-cal value, often alarming about the spread-ing potential of this novel infection, defin-ing R0 as a “fatal number”: the more itincreases, the greater is the risk for the pop-ulation, including higher mortality poten-tial.On the other hand, the scientific com-munity has not given a definite and soundresponse about the real epidemiologicalpotential of COVID-19, to date. Scientistsare currently debating about the actualreproductive number of COVID-19 and it isnot hard to find sensationalistic statementsabout the R0 and its impact on the pandemicCOVID-19 potential. Indeed, sinceCOVID-19 broke out, several publishedstudy aiming to forecast its epidemic trend,have estimated different R0 values, oftenmuch higher than that of SARS and MERS. R0 is the average number of secondaryinfections produced by an infectious case ina population where everyone is susceptibleand it is used to measure the transmissionpotential of a communicable disease.6When R0 is >1, it means that each indi-vidual affected by a transmittable disease isexpected to infect a number of subjects thatincrease exponentially with the increase ofthe R0 value and the disease is expected tospread through the susceptible population.Conversely, when R0 is <1 each case trans-mits the disease to one or less than one indi-vidual and the disease is expected to die outin the population.6Although the concept of R0 is very intu-itive, its calculation is based on complexmodels and may lead to misinterpretations,especially for what concerns the real weightthat R0 has on the spreading of an infectiousdisease and on the feasibility of controllingan epidemic.6The basic reproductive number (R0) ofCOVID-19 has been initially estimated bythe World Health Organization (WHO) torange between 1.4 and 2.5, as declared inthe statement regarding the outbreak ofSARS-CoV-2, dated 23th January 2020.7However, several published studiesaimed to precisely estimate the COVID-19R0. A recent review written by Liu et al.compared 12 studies published from the 1stof January to the 7th of February 2020which have estimated the R0 for COVID-19, finding a range of values between 1.5and 6.68.8The authors of the review calcu-lated the mean and the median of R0 esti-mated by the 12 studies and they found afinal mean and median value of R0 forCOVID-19 of 3.28 and 2.79, respectively,with an interquartile range (IQR) of 1.16.8According to these findings, the COVID-19R0 would exceed the reproductive numberestimated for SARS.9The reasons behind a low level ofaccordance between studies attempting toestimate the R0 are complex and can beattributed to 3 possible reasons: i) differentvariables considered; ii) different methodsfor modeling; and iii) different estimationprocedures.9Firstly, R0 is not an intrinsic variable ofthe infectious agent, but it is calculatedthrough at least three parameters: the dura-tion of contagiousness; the likelihood ofinfection per contact between; and the con-tact rate, along with economical, social andenvironmental factors, that may vary amongstudies aimed to estimate the R0.More, the use of different models forthe estimation of R0 may play a role in thediscrepancies observed among the studieson COVID-19. In fact, according to Liu’sfindings, the studies using mathematicalmethods produce estimates that are higherthan stochastic and statistic models in deter-mining COVID-19 R0.8It must be noted that the estimation ofR0 assumes that the number of secondaryinfections produced by a single case has novariations.9However, super-spreadingevents, in which a single individual, notnecessarily strongly symptomatic, mayinfect a wide number of subjects, asoccurred in the past with SARS and MERS,may occur.10 Recently, a British business-man with COVID-19 has been alleged totransmit the infection to 11 people in aFrench chalet.11Therefore, the models used to estimatethe R0 cannot fully consider the large het-erogeneity in space, transmissibility, andsusceptibility of an infection.Additionally, the basic reproductivenumber is constantly modified during anepidemic by the control measures adoptedto reduce the fundamental coefficient of R0,namely: i) the duration of contagiousness;ii) the likelihood of infection per contact;and iii) the contact rate.12One effective measure is quarantine.During SARS epidemic, several countriesintroduced the use of mass quarantine forall individuals suspected of having had con-tact with a confirmed SARS case. Thesecoordinated global efforts were remarkablyeffective at curtailing the spread of the dis-ease, and this strategy was effective, togeth-er with isolation of infected patients andpublic health measures to contain the epi-demic and avoiding SARS reemergence.
