Researchers have shown that monetary incentives lead to infants being immunised on time.
Doreen Auma puffs as her stride shortens with every step. The humidity is thick and sticky in western Kenya's rain season, and today there seems to be no end to the rough dirt road snaking to the Masogo Health Centre.
Auma is eight months pregnant and trudging to an antenatal check-up.
She already has a baby, who at 11 months old is overdue for his measles and yellow fever vaccinations at the same clinic. But Auma, a farmer who asked that we not use her real name, has left him at home.
With the ever heavier little boy strapped on her back, she fears she may not make it to the health centre.
"It's too expensive for me to board a boda boda," says Auma. The boda boda (motorbike) drivers in the rural community in Siaya County charge 200 Kenyan shillings (about R26) for a ride to and from the health centre.
With another baby due, Auma doesn't know when - or if - her son will complete Kenya's routine immunisation schedule for children under one year old. He went for his last shots three months ago, when she could still carry him.
The Kenyan government prescribes vaccinations against diseases such as tuberculosis (TB), polio, whooping cough, tetanus and hepatitis B. At least one vitamin A dose should be given before the first birthday, while vaccinations against yellow fever (in two counties) and measles are given at nine months, according to ministry of health guidelines.
Routine childhood immunisation is cost-effective and successful: it saves millions of lives globally every year, data from the World Health Organisation shows. But the organisation says an estimated 19.4-million infants worldwide are still missing out on basic vaccines and global coverage levels have stalled at about 85%.
Health bodies are grappling with how to reach parents like Auma, the remaining 15% who say they want to immunise their children but have practical difficulties doing so, or those who forget or ignore the importance of vaccinations.
As cellphone access expands globally, scientists say mobile technology can be used to improve vaccine coverage.
In the past few years, several studies from as far afield as Guatemala have shown that SMS messaging can improve disease prevention.
Auma has considered stopping her antenatal check-ups but says she is trudging to her appointment only because she got relentless calls and SMS reminders from the clinic and community health volunteers.
"I would take my child [for vaccinations] too if I had money for transport," she says.
A new study conducted in Kenya has confirmed that many caregivers share Auma's views. The study found that SMS reminders, combined with a cash incentive, significantly improve not only coverage but also timeliness of immunisations, according to the research published in The Lancet medical journal in April.
As part of the study, 2 018 caregivers and their infants from 152 Kenyan villages were divided into four groups and documented for about a year.
One group got only SMS reminders before scheduled immunisation visits and another got SMSes plus an incentive of 75 Kenyan shillings. A third group received SMS reminders plus 200 Kenyan shillings. Participants in the groups who got incentives were given additional money if their child was immunised within two weeks of their vaccination due dates.
A fourth segment, acting as a control group, received no reminders or cash. In this group, the study found that 82% of children were fully immunised by 12 months.
To be considered fully immunised, a child had to receive full courses of vaccines to protect against TB, polio and measles, as well as a five-in-one jab that guards against illnesses such as tetanus, whooping cough and hepatitis B.
In the group in which caregivers got SMS reminders only, 86% of children were fully immunised. The study found that this figure, at 86%, was the same for the group that received 75 Kenyan shillings.
But in the group that got the highest incentive of 200 Kenyan shillings, there was a marked increase in immunisation coverage and 90% of the children in this segment were fully vaccinated.
Timeliness is a crucial element, says Godfrey Bigogo, a researcher at the Centers for Disease Control and Prevention in Kisumu. "The main aim of the study was to find out whether people were immunised at the right time," he says. "In [the case of] delayed vaccination, a potential infection can take advantage of an unvaccinated child."
"SMS reminders, with or without incentives, yielded significant gains in timeliness of measles vaccination," says the study.
The ethics of economic incentives are controversial, but the authors of the study say that the incentive amounts were not coercive and less than one day's working wage.
But incentives should also be cost-effective, they say. "The use of incentives ... might be cost-effective or even cost-neutral if they significantly strengthen the routine immunisation services".