**2. Human factors influencing the duration of infection**

The development of asymptomatic and low-density infections is intimately related to an individual's tolerance to parasites [12]. Several host characteristics have been linked to differential clinical expression of malaria infection, as well as to modulation of parasite levels, including genetic factors [13], acquired immunity [14, 15], co-infections with non-falciparum malaria parasites [16], iron status [17], among others. In this section, we discuss two widely prevalent factors that are likely to influence the frequencies of asymptomatic and low-density infections in various settings: haemoglobinopathies, which are genetically determined and consequently whose effects on parasites might remain unchanged with decreases in transmission, and acquired immunity, that varies with cumulative exposure to parasites and will wane as exposure drops or ceases.

Both haemoglobin S (HbS) and haemoglobin C (HbC) mutations are protective against clinical malaria [18], and evidence from a longitudinal study performed in Uganda [19] suggests that HbS reduces progression of infection to disease. This protective effect suggests that these mutations are associated with chronic infections or clinical episodes with delayed onset (**Figure 1**; light blue, green or orange line). Data also suggest that the parasite densities observed in individuals with sickle cell trait [20] are lower compared to densities in HbAA individuals and thus presumably more likely to be sub-patent and not necessarily detected in population surveys. Given the high prevalence of haemoglobinopathies in many malaria endemic countries, particularly those in Africa for HbS [21, 22], and the potential for carriage of sub-patent infections, the contribution of this group of individuals to the transmission reservoir should be considered. Determining how often parasite densities in heterozygous individuals are below the lower limit of detection of standard diagnostics would be informative. This is particularly relevant as haemoglobinopathies have been associated with increased gametocyte positivity and duration of gametocyte carriage [19, 23–25], which could amplify the infectivity of asymptomatic individuals with these mutations. Unlike naturally acquired immunity, these genetic traits will persist for several generations even after reductions in malaria transmission and they have the potential to influence transmission phenotypes in the whole spectrum of endemicities, including in areas approaching malaria elimination.

of transmission from these infections. This will allow an assessment of whether specific individuals with asymptomatic infections need to be targeted and, if so, how this might be done, for example by enhanced coverage efforts or more sensitive infection detection tools targeted

In Sections 2 and 3, we consider factors that influence the establishment of asymptomatic infections and their parasite and gametocyte carriage levels. Specifically, we discuss how different factors might relate to the different archetypes of parasite dynamics described in **Figure 1**: chronic infections with fluctuating patent and sub-patent levels; chronic sub-patent infections; clinical episodes with short incubation period; clinical episodes with long incubation period; and short asymptomatic infections. Additionally, we discuss how blood sampling for parasite detection can influence estimates of prevalence of sub-microscopic infections. In Section 4, we use malariotherapy data and validated mathematical models to assess the benefits of targeting

The development of asymptomatic and low-density infections is intimately related to an individual's tolerance to parasites [12]. Several host characteristics have been linked to differential clinical expression of malaria infection, as well as to modulation of parasite levels, including genetic factors [13], acquired immunity [14, 15], co-infections with non-falciparum malaria parasites [16], iron status [17], among others. In this section, we discuss two widely prevalent factors that are likely to influence the frequencies of asymptomatic and low-density infections in various settings: haemoglobinopathies, which are genetically determined and consequently whose effects on parasites might remain unchanged with decreases in transmission, and acquired immunity, that varies with cumulative exposure to parasites and will wane as

Both haemoglobin S (HbS) and haemoglobin C (HbC) mutations are protective against clinical malaria [18], and evidence from a longitudinal study performed in Uganda [19] suggests that HbS reduces progression of infection to disease. This protective effect suggests that these mutations are associated with chronic infections or clinical episodes with delayed onset (**Figure 1**; light blue, green or orange line). Data also suggest that the parasite densities observed in individuals with sickle cell trait [20] are lower compared to densities in HbAA individuals and thus presumably more likely to be sub-patent and not necessarily detected in population surveys. Given the high prevalence of haemoglobinopathies in many malaria endemic countries, particularly those in Africa for HbS [21, 22], and the potential for carriage of sub-patent infections, the contribution of this group of individuals to the transmission reservoir should be considered. Determining how often parasite densities in heterozygous individuals are below the lower limit of detection of standard diagnostics would be informative. This is particularly relevant as haemoglobinopathies have been associated with increased gametocyte positivity and duration of gametocyte carriage [19, 23–25], which could amplify the infectivity of asymptomatic individuals with these mutations. Unlike naturally acquired immunity, these genetic traits will persist for several generations even after reductions in

at those individuals who have a higher probability of being chronically infected.

**2. Human factors influencing the duration of infection**

the asymptomatic reservoir of parasites.

132 Towards Malaria Elimination - A Leap Forward

exposure drops or ceases.

Another cause of variation in the risk of symptoms and in parasite burden is acquired immunity against asexual blood stage parasites, which develops with cumulative exposure and consequently age. Asymptomatic adults have lower parasitaemias compared to children [4], and a higher proportion of their infections are sub-patent [26]. Adults are also less likely to develop symptoms, especially in highly endemic areas, and when they do, the parasite densities associated with fever are on average lower than the corresponding densities in children [27]. On the other hand, estimates based on clone-specific carriage show that in highly endemic areas, asymptomatic infection duration is higher in schoolchildren compared to adults [8] though the differential detectability of clones may affect observations. Together, these studies suggest that infections in adults most commonly correspond to the archetype parasite dynamics of short duration asymptomatic infection or chronic infections with sub-patent carriage (**Figure 1**, yellow and orange lines). In settings where transmission intensity approaches elimination levels, depending on how fast transmission decreases, acquired immunity in adults would still be effective against parasitaemia and symptoms, while in young children with limited cumulative exposure to falciparum parasites, this might not be the case. In this scenario, the epidemiological differences between these demographic groups could be enhanced. Interestingly, in an area of Papua New Guinea with recent declines in transmission, reductions in parasite prevalence have been associated with an increase in the proportion of infections that are subpatent [28], indicative, perhaps of persisting immune responses that control parasitaemia in a setting where the incidence of super infection is reduced.

Short-term changes in immunity might also be relevant. For example, recent malaria infection might modulate immune responses to subsequent infections [29], which suggests that dynamics of parasitaemia might differ at the start *versus* peak of transmission season, and so might the proportion of infected individuals that remain asymptomatic. Indeed, several epidemiological studies using different methodologies have shown that the risk of clinical symptoms during infection varies during a transmission season: Mueller and colleagues [30] observed that after adjusting for the incidence of new infections, defined by molecular identification of individual clones, the risk of clinical malaria per infection was higher at the beginning of the transmission season. In Mali, the ratio of asymptomatic to symptomatic infections was higher during the low transmission season compared to the rainy season [31]. Whether this is due to modulation of host immune responses or to changes in parasite phenotype, it may result in longer infections at the end of the transmission season that would be advantageous for falciparum parasite populations to persist over the often long dry seasons. Furthermore, shortterm immunological changes might also directly affect infectivity of asymptomatic infections: in Burkina Faso, experimental mosquito infections indicate that short-lived immunity that reduces transmission is boosted after season-long exposure to parasites [32].

Of note, high-density infections in the absence of symptoms have been described, in particular in young children [3, 33, 34]. The relevance of these infections to transmission is unknown, although it could be anticipated that unless commitment to sexual development is reduced, these infections will produce high numbers of gametocytes and be potentially highly infectious.
