A sticky situation: the unexpected stability of malaria elimination

Debate about the best path for tackling malaria has been ongoing for many years: should countries work toward national elimination of the disease, with the goal of eradicating malaria at a global scale, or should attention instead be focused on deploying interventions to control malaria and reduce it to a minor public health problem? By reviewing every example of malaria elimination since the 1950s, researchers have shown that elimination may be a surprisingly stable state – one that results in long-lasting gains for countries that achieve it.

Models of malaria transmission have generally suggested that even if countries achieve elimination, they will likely need to maintain a high level of coverage of control interventions in order to prevent imported infections driving a resurgence of malaria. This suggests that the implications of choosing to pursue long-term control strategies may not be all that different from committing to elimination. This highlights a key question: if countries will continue to require significant financial and operational resources to maintain elimination, is it sustainable ­– and is it worth the additional risk and cost?

Recent work has shed new light on this critical question by revisiting a natural experiment in malaria control and elimination: the Global Malaria Eradication Program (GMEP), which was launched by the WHO in 1955 and abruptly ended in 1969 when funding collapsed. Assessing the outcomes of the GMEP showed that countries that had made substantial reductions in malaria, but had not eliminated it completely, experienced a resurgence of the disease after the cessation of the program. However, malaria was very unlikely to reemerge in countries that had achieved elimination. In fact, they found that in all but four of the 50 countries that eliminated malaria, transmission declined by at least 98% from its endemic level. This points toward a surprising conclusion: malaria elimination may be a highly stable state.

What accounts for this trajectory? The evidence suggests that elimination might actually reinforce its own stability, by causing changes that make it easier to sustain. This effect could work through a number of different mechanisms. Improvements in health systems that come about as a result of attempts to eliminate malaria may lead to increased capacity to monitor and treat new malaria cases, and so prevent outbreak. In populations where malaria has been eliminated, people also start to lose immunity. This means that when people are infected they are more likely to experience clinical symptoms and seek treatment; by making malaria obvious, a loss of immunity might also make it easier to control. Travel patterns could also play a role. Importation of malaria cases from other countries is generally assumed to be a major threat to sustaining elimination. However, in a country where malaria transmission is very low, it’s unlikely that potentially infected travelers will make their way to the remote, rural areas where mosquito densities – and the potential for transmission – are highest.

These conclusions have substantial implications for the question of whether or not to pursue elimination and eventual eradication of malaria. Achieving and sustaining elimination will still require concerted investment in strengthening country health systems and ensuring high coverage of vector control interventions for a period of time, until immunity has waned. But if elimination is stable, and indeed reinforces its own stability in various ways, then the projected costs of maintaining it may be lower than initially anticipated, particularly in comparison to indefinite efforts at minimizing the burden of malaria but never getting rid of it entirely.

This is not a one-size-fits-all solution, of course. Stable elimination is only possible where malaria cases can be promptly identified and treated, and countries with higher levels of endemic malaria transmission and weak health systems will likely face tougher challenges than those with low to moderate transmission and better health system capacity. However, the stability of elimination also has another very important implication: it means that eradicating malaria, unlike other diseases, is not necessarily an all-or-nothing endeavor. These insights suggest that complete eradication will not be dependent on a globally coordinated effort; instead, individual countries can succeed in maintaining elimination themselves, and as a result elimination can proceed country by country, and region by region, until a world free of malaria is achieved.