The terms elimination, eradication, and control are often used loosely, which results in misunderstanding.43
The word eradication has previously been used to describe what we now call elimination,44
but is used only to mean the “permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts. Interventions are no longer needed.”45
Generally, cessation of malaria transmission in a defined geographic area, such as an island or an entire country, is now called elimination. WHO defines malaria elimination as: “interrupting local mosquito-borne malaria transmission in a defined geographical area—ie, zero incidence of locally contracted cases, although imported cases will continue to occur. Continued intervention measures are required.”31
The WHO definition of elimination, taken literally, is achieved almost nowhere. Most countries that have achieved elimination and are maintaining their WHO-certified malaria-free status, such as Italy or the USA, have occasional, small outbreaks of secondary cases that are contracted from primary imported cases. For example, despite 12 outbreaks of malaria in the USA between 1993 and 2009, and 26 secondary cases,46
no one is suggesting that the USA has not eliminated malaria. WHO addressed this issue in 2006 by proposing an operational criterion of elimination as “nationwide per year fewer than three ‘epidemiologically linked’ cases of a malaria infection without an identifiable risk factor other than local mosquito transmission, for 3 consecutive years”.27
Cohen and colleagues47
reviewed the use of the terms control, elimination, and eradication and proposed a new definition of elimination as “a state where interventions have interrupted endemic transmission and limited onward transmission from imported infections below a threshold at which risk of re-establishment is minimised. Both capacity and commitment to sustain this state indefinitely are required.”47
This new definition recognises that maintaining “zero incidence of locally contracted cases”31
is not always an achievable target. Furthermore, as addressed in the third paper20
in this Series, this definition emphasises the maintenance of a highly functional surveillance and outbreak response system that is capable of preventing re-establishment of local transmission.
Although many stages occur between the control and the elimination of malaria, this Series regards the alternative to malaria elimination as a state of malaria epidemiology that is typically achieved before launching an elimination effort, often referred to as sustained control.1
Definition of this state is important because a country that decides not to eliminate is not deciding to do nothing, but is deciding to continue investments and programmatic activities to maintain a state of malaria control indefinitely.
WHO defines malaria control as “reducing the disease burden to a level at which it is no longer a public health problem”,31
which allows for a contextual and disease-oriented endpoint. Cohen and colleagues47
introduced a new term, controlled low-endemic malaria, which they define as “a state where interventions have reduced endemic malaria transmission to such low levels that it does not constitute a major public health burden, but at which transmission would continue to occur even in the absence of importation”.47
Generally, this term applies to areas where malaria-specific mortality and severe disease outcomes are rare and malaria's contribution to all-cause childhood mortality is small.48
These definitions of elimination and controlled low-endemic malaria from Cohen and colleagues47
are adopted in this paper and throughout this Series (panel 1
). WHO categorises countries in four programme phases by use of malaria slide positivity and incidence rates: control, pre-elimination, elimination, and prevention of re-introduction. For example, a country in the elimination phase has an annual parasite incidence of less than one case per 1000 people at risk.49
Panel 1. Definitions used in this Series
“A state where interventions have interrupted endemic transmission and limited onward transmission from imported infections below a threshold at which risk of re-establishment is minimised. Both capacity and commitment to sustain this status indefinitely are required.”47
• A country that has formally declared a national, evidence-based elimination goal, has assessed its feasibility, and has embarked on a malaria elimination strategy.
• A country that is strongly considering an evidence-based national elimination goal, and that has already made substantial progress in spatially progressive elimination (eg, by eliminating malaria from specific islands, provinces, or geographic areas) and in greatly reducing malaria nationwide.
Controlled low-endemic malaria
“A state where interventions have reduced endemic malaria transmission to such low levels that it does not constitute a major public health burden, but at which transmission would continue to occur even in the absence of importation.”47
This Series builds on WHO's categorisations and other work.2,3
We propose a new term, a malaria-eliminating country, which refers to a country in the process of moving from controlled low-endemic malaria towards elimination. There are two proposed categories for malaria-eliminating countries: the first includes countries that have formally declared a national, evidence-based elimination goal, have assessed the feasibility of such a goal, and have embarked on a malaria elimination strategy; the second includes countries that are strongly considering an evidence-based national elimination goal, and that have already made substantial progress in spatially progressive elimination (eg, by eliminating malaria from specific islands, provinces, or geographical areas) and in greatly reducing malaria nationwide.
Ten countries in the WHO elimination phase, eight in the pre-elimination phase,14
and 14 that may or may not have met WHO's epidemiological criteria for pre-elimination or elimination (because data are insufficient or ambiguous, or, in some cases, controversy exists with respect to data interpretation)—such as Cape Verde, Namibia, and Swaziland (webappendix pp 3–11
)—qualify as malaria-eliminating countries under the first definition category. Several countries that have not declared a national elimination goal, but are on a plausible pathway to elimination, and that are participating in active scientific, technical, and financial discussions about how to achieve and sustain elimination (eg, Bhutan, Costa Rica, Dominican Republic, São Tomé and Príncipe, and South Africa; webappendix pp 3–11
qualify as malaria-eliminating countries under the second definition category. Webappendix pp 3–11
provides data for each of the 32 malaria-eliminating countries and clarifies present categorisations.
This categorisation of malaria-eliminating countries is closely aligned with the geographical distribution of elimination feasibility that is described in the second paper of this Series.19
We have identified a third of all malarious countries as malaria-eliminating countries, and thus draw attention to the need for greater support, research, and investment into elimination and the difficult task of preventing reintroduction.
The definitions and categorisations used in this Series address all plasmodium species that infect human beings. However, some countries might choose to set interim elimination targets by parasite species49
in recognition of the added challenge of eliminating Plasmodium vivax
malaria. The second paper19
in this Series, which presents wordwide rankings of malaria elimination feasibility, underscores this challenge and does not rank the technical feasibility of P vivax
elimination because of the poor understanding of its epidemiology and the likely species-specific strategies and interventions that will be needed.