A baby usually inherits its mother’s immunity. Does a lady who emigrated from West Africa to the US still have a higher level of immunity to malaria and some other tropical illnesses? And has she been able to pass that level of immunity to her baby?‘
These questions can be re-stated as
- How long is a migrant protected after moving away from a malaria*-endemic region such as West Africa? A few years.
- Do mothers pass protective anti-malaria immunity to babies? How long does such protection last? Yes, ~6 to 9 months.
* Features pertinent to anti-malaria immunity are broadly applicable to other tropical illnesses.
How long is an emigrant protected after moving away from a malaria-endemic region such as West Africa?
To understand how long naturally acquired immunity protects against malaria requires longitudinal migrantthat follow groups of 1st generation migrants who have moved away from malaria-endemic regions and periodically return to visit friends and relatives. Such studies should then assess
- How many caught malaria from their return trip(s).
- How many got the severe form.
- The relationship between length of time away and risk of clinical malaria, especially severe malaria.
Though few malaria studies have examined such issues in depth, a handful permit some general inferences.
- More the frequency and number of prior exposures to malaria, stronger the likelihood a person has developed robust immunity against it. This means more a migrant got exposed to malaria before they left, greater their likelihood of retaining protective immunity for longer after they move away.
- Studies from Papua ( ), Kenya, Gambia ( ), and Kenya again ( ) suggest as much.
- Severe Plasmodium falciparum malaria and even deaths are less frequent among sub-Saharan migrants compared to other travelers to malaria-endemic regions.
- Less severe malaria could be lower parasite density, less frequent presentation of clinically severe malaria, and/or faster clearance of both parasites and fever.
- Studies from Europe ( ), France ( , 6, , ), Italy (9, ), Poland ( ) and UK ( ) suggest as much.
- However, longer a migrant stays away from malaria exposure, more their risk of severe malaria resembles that of a non-exposed traveler. This is because protection by naturally acquired immunity wanes over time. A small study from Sweden found immigrants who had lived in Sweden for >15 years had similar risk of severe malaria as those not previously exposed to it ( ). More such studies would help generalize how long protective natural anti-malaria immunity might last.
Do mothers pass protective anti-malaria immunity to babies? How long does such protection last?
Barring risk factors such as malnutrition and exposure to malaria in utero, babies and infants <6 months of age in malaria-endemic regions rarely show clinical signs of malaria (fever, high levels of parasite in blood).
- This is understood as mother having passed protective IgG antibodies against malaria to baby ( ).
- A handful of studies in the 1960s found dramatic improvement in malaria symptoms in hospitalized children given serum from healthy adults with naturally acquired immunity to malaria (15, 16). However, details of this process remain sketchy and poorly understood even today.
- Amount of protective anti-malaria antibodies passed from mother to baby matters ( , ).
- Thus we’re back to where we started, more exposure of a migrant mother to malaria before they leave, more robust their naturally acquired immunity and higher the levels of protective antibodies they transfer to their baby.
- Protective maternal antibody levels typically wane by 6 to 9 months of age, meaning protection to baby is time-limited.
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