The writer is a science commentator
The hypothesis is loosely known as the “rebound effect”. If babies in malaria-prone areas are shielded from the disease at the start of life, will they fail to build up sufficient immunity into adulthood and become more vulnerable later on?
A study published last week in The New England Journal of Medicine suggests that, for malaria at least, the rebound effect is a myth. Babies and children who regularly slept under insecticide-treated bed nets in rural Tanzania showed a significant survival advantage over their unprotected peers well into adulthood. The finding adds detail to a surprisingly sketchy picture of how children build up immunity in a pathogen-rich world, which now includes the Covid-19 virus. It also highlights the value of stress-testing assumptions in the real world.
The genesis of the bed nets study dates back more than two decades, when researchers from the Ifakara Health Institute in Tanzania collected data on about 6,700 babies born between 1998 and 2000. They found that insecticide-treated nets seemed to cut the risk of children dying from any cause by 27 per cent (malarial infection dents immunity to other diseases).
But they remained troubled by the possibility of the rebound effect, hints of which had emerged elsewhere: children given prophylactic antimalarial drugs, similar to those taken by tourists, or vaccines seemed to be more susceptible to disease later on. That stoked a worry that infants slumbering under bed nets were simply storing up a grim immunological debt to be paid at a future date.
The research team, including epidemiologist Joanna Schellenberg, now at the London School of Hygiene and Tropical Medicine, decided to track down the bed net kids to see if there had in fact been a surge of catch-up deaths later on. They found the opposite. Children who had regularly slept under treated bed nets were 43 per cent more likely to survive into adulthood. That was after accounting for other possible influences, such as parental education.
“It was a very messy data set and we analysed it every which way,” Schellenberg said, delighted at the results but also that 89 per cent of the original cohort were traced. “No matter which way we looked at the data, it all told the same story: that the benefits of early-life malaria control, in this case with treated nets, persist for two decades.”
Petter Brodin, a paediatric immunologist at Imperial College London, said the study suggested bed nets struck a happy balance between protecting very young babies from serious disease while allowing immunity-inducing, low-level exposure from mosquitoes later on. That reflects, he said, the high-wire act that the infant immune system treads in the first few months: being open enough to allow microbes to enter and colonise the body (essential for health) but defensive enough to keep dangerous infections at bay. Additional immune back-up in the early months comes from maternal antibodies.
This tension now haunts the debate over whether to vaccinate younger children against Covid-19. Some suggest that the Omicron variant, less severe than its predecessors, could act like a natural vaccine. Brodin disagrees. Vaccination, he believes, currently offers safer, more controlled exposure than infection and carries a lower risk of unintended consequences, such as the emergence of new variants.
“When asked, I have suggested we should vaccinate five to 11-year-olds, given the small but real risk of MIS-C (multisystem inflammatory syndrome in children) and long Covid,” he said. “Millions of children have been vaccinated in the US and elsewhere; if there were side-effects of the vaccines, we would have seen them by now.” The planned lifting of isolation rules in the UK amid high transmission should focus minds anew on the need to protect children.