Elimination is the act or process of bringing Rc<1
Olivier Johan Tavai Briët, Swiss TPH
11 October 2010
Dr Cohen and colleagues provide a valuable overview of the historical and current ambiguities in the definition of elimination, and convincingly show that current targets for elimination, varying from a complete absence of local transmission to fewer than three epidemiologically linked cases per year, are not achievable in the presence of "intrinsic transmission potential" Rc>0 and the reality of imported infections. The authors then propose new terminology for 'targets', 'milestones', 'goals' or 'states' of transmission levels. Some of the ambiguity around the term “elimination” arises from differential usage of the word to describe a process, both a process and a state (within the same definition), or a state. The authors opt to use 'elimination' to describe a state. In most English language dictionaries, elimination is described as an ‘act, process or instance’, and only some dictionaries give a second meaning as describing a 'state'. It is thus preferred to describe elimination as an act or process, built on the verb 'eliminate', from latin 'eliminare', which means 'to turn out of doors' or 'to bring beyond a threshold'. In the act/process of disease elimination, it is clear what the threshold for transmission potential is (when local transmission is at an equilibrium close to 0): a) with the goal to achieve a "controlled non-endemic" state: Rc=1, as below this threshold, disease will not be able to re-establish itself in the presence of control; b) with the goal to achieve a "non-endemic" state: R0=1, as below this threshold, disease will not be able to re-establish itself even in the absence of control. The term "controlled non-endemic malaria" proposed by Cohen and colleagues could thus be a suitable name for an equilibrium state of near 0 local transmission where 0=<Rc<1=<R0, thus where in the absence of 'active' control, R0 would be equal or larger than 1. Areas where 'natural transmission' never took place, or where malaria was eliminated (with or without the help of active control), and where at present in the absence of active control the equilibrium state is close to zero transmission with the 'intrinsic transmission potential' 0=<R0 <1, are "non-endemic" for malaria (and they are not 'in a state of elimination'). If in an area an Rc or R0 is from time to time >1, but on average <1, there is a state of epidemic malaria.
Conclusion There is no need to preserve a definition of elimination in line with the secondary meaning that it describes a state. Elimination is the act or process of bringing down local transmission to a stable equilibrium near 0 with Rc (the controlled reproductive number) or R0 (the basic reproductive number, a measure for 'intrinsic transmission potential') below 1. The term "controlled non-endemic malaria" is suitable to describe an equilibrium situation / state with close to 0 local transmission where 0=<Rc<1=<R0, which therefore requires continued control to be stable. The term "non-endemic malaria" is suitable to describe an equilibrium situation / state with close to 0 local transmission where 0=<R0<1 without the need for active control measures.
PS: A suggestion to change the formula for pn in the paper to pn=1-[(1-p1)i]n
Elimination is the act or process of bringing Rc<1
11 October 2010
Dr Cohen and colleagues provide a valuable overview of the historical and current ambiguities in the definition of elimination, and convincingly show that current targets for elimination, varying from a complete absence of local transmission to fewer than three epidemiologically linked cases per year, are not achievable in the presence of "intrinsic transmission potential" Rc>0 and the reality of imported infections. The authors then propose new terminology for 'targets', 'milestones', 'goals' or 'states' of transmission levels. Some of the ambiguity around the term “elimination” arises from differential usage of the word to describe a process, both a process and a state (within the same definition), or a state. The authors opt to use 'elimination' to describe a state. In most English language dictionaries, elimination is described as an ‘act, process or instance’, and only some dictionaries give a second meaning as describing a 'state'. It is thus preferred to describe elimination as an act or process, built on the verb 'eliminate', from latin 'eliminare', which means 'to turn out of doors' or 'to bring beyond a threshold'.
In the act/process of disease elimination, it is clear what the threshold for transmission potential is (when local transmission is at an equilibrium close to 0): a) with the goal to achieve a "controlled non-endemic" state: Rc=1, as below this threshold, disease will not be able to re-establish itself in the presence of control; b) with the goal to achieve a "non-endemic" state: R0=1, as below this threshold, disease will not be able to re-establish itself even in the absence of control. The term "controlled non-endemic malaria" proposed by Cohen and colleagues could thus be a suitable name for an equilibrium state of near 0 local transmission where 0=<Rc<1=<R0, thus where in the absence of 'active' control, R0 would be equal or larger than 1. Areas where 'natural transmission' never took place, or where malaria was eliminated (with or without the help of active control), and where at present in the absence of active control the equilibrium state is close to zero transmission with the 'intrinsic transmission potential' 0=<R0 <1, are "non-endemic" for malaria (and they are not 'in a state of elimination'). If in an area an Rc or R0 is from time to time >1, but on average <1, there is a state of epidemic malaria.
Conclusion
There is no need to preserve a definition of elimination in line with the secondary meaning that it describes a state. Elimination is the act or process of bringing down local transmission to a stable equilibrium near 0 with Rc (the controlled reproductive number) or R0 (the basic reproductive number, a measure for 'intrinsic transmission potential') below 1.
The term "controlled non-endemic malaria" is suitable to describe an equilibrium situation / state with close to 0 local transmission where 0=<Rc<1=<R0, which therefore requires continued control to be stable.
The term "non-endemic malaria" is suitable to describe an equilibrium situation / state with close to 0 local transmission where 0=<R0<1 without the need for active control measures.
PS: A suggestion to change the formula for pn in the paper to pn=1-[(1-p1)i]n
Competing interests
None declared