The Zika virus, to most people, is harmless. But to pregnant women it can be devastating because this virus is associated with serious neurologic disorders in newborns.
Professor and Director of York University’s Agent-Based Modelling Laboratory Seyed Moghadas and his then-graduate student, Affan Shoukat (now a postdoctoral fellow at Yale University), led a study that concluded that the vaccination of young women would prevent Zika from infecting these individuals in about 75 per cent of them. The vaccine would also be cost effective at under $16 (US dollars) per vaccination in most countries in the Americas.
“Our findings indicate that targeted vaccination of women of reproductive age is a noteworthy preventive measure for mitigating the effects of Zika virus infection in future outbreaks,” says Moghadas, an expert in mathematical and computational modelling in epidemiology and immunology.
This work was supported by Natural Sciences and Engineering Research Council of Canada. Financial support also came from the Canadian Foundation for Innovation for the establishment of the Areto Computational Cluster at York, used to perform the simulations.
The article, “Cost-effectiveness of Prophylactic Zika Virus Vaccine in the Americas,” was published in the high impact journal Emerging Infectious Diseases (2019).
Economic burden in the billions
The Zika virus is a mosquito-borne virus that was first identified in Uganda in 1947 in monkeys. It was later identified in humans in 1952 in Uganda and the United Republic of Tanzania. Today, 87 countries and territories have reported evidence of mosquito-transmitted Zika infection, according to the World Health Organization (July 2019, WHO).
The economic burden is estimated to be substantial, ranging from $7 to $18 billion in short-term costs and $3.2 to $39 billion in long-term costs.
A vaccine could be a game changer.
Zika virus devastating during pregnancy
To the average person, this virus causes mild disease. Most people never develop symptoms. If they do, these signs include fever, rash, muscle and joint pain or headache, and usually last two to seven days.
People who have contracted Zika face an increased risk of neurologic complications, such as Guillain-Barré syndrome, a rapid-onset of muscle weakness; neuropathy (nerve damage); or myelitis, an inflammation of the spinal cord that can result in paralysis and sensory loss.
Zika virus infection during pregnancy is very dangerous. It can cause infants to be born with congenital malformations – primarily, microcephaly. This is a condition where a baby’s head is much smaller than expected. It means that the baby’s brain has not developed properly during pregnancy or has stopped growing after birth.
Researchers develop simulation model to see how effective vaccine would be
The idea of a vaccine, more specifically how effective this would be, is something that Moghadas has been working on for some time. In this article, he and Shoukat developed a simulation model where the human population was divided into four categories:
- Exposed and incubating;
- Infectious; and
The mosquito population was also divided into groups: susceptible, exposed and incubating, and infectious groups. (Only three groups, not four as the human population. Why? Because once a mosquito is infectious, it cannot recover; it will be infectious for the rest of its lifespan, which is days to weeks.)
The team also looked at country-specific demographics (age and sex distributions and fertility rates), and calibrated it to attack rates, which were based on 2015–2017 outbreaks. The countries ranged from Belize to Brazil, Peru to Panama.
“These attack rates were considered to be the proportion of the population that was infected (representing the level of herd immunity) at the start of simulations for each country in the evaluation of vaccination scenarios,” Shoukat explains. (Herd Immunity is where the majority of individuals in a population have developed immunity to a pathogen. Because so many people within the community are unable to contract the disease, this reduces the likelihood that those who haven’t developed immunity will contract the disease.)
The researchers also factored in the costs associated with the disease and vaccination.
In this simulation, the vaccination coverage was 60 per cent for women of reproductive age. For pregnant women, it was 80 per cent initially and continued at 80 per cent throughout the simulations. Averaging these numbers in their computer simulations, the researchers concluded that the vaccination of young women would prevent Zika from infecting about 75 per cent of these individuals.
They also calculated the reduction of fetal microcephaly during pregnancy (if vaccination had occurred), and found a marked reduction, within the range of 74 to 92 per cent. The median percentage reduction was over 80 per cent in all countries.
The researchers considered both short- and long-term medical costs specific to each country. Short-term costs included physician visits and diagnostic tests for pregnant women. Long-term costs included disability-adjusted life-years (DALYs, the number of years lost due to ill-health, disability or early death) with disability weight (i.e., severe intellectual disability) extracted from a Global Burden of Disease study (published in The Lancet in 2013).
The researchers concluded that “a single-dose vaccination program is cost-effective for all countries studied.” Specifically, the vaccine would be cost effective at under $16 (US) per vaccination.
Figure 1 offers a country-by-country break-down. (Note: “Cost-saving” and “cost-effective” are not the same thing. “Cost-saving” refers to preventive care that decreases costs. If the benefits are sufficiently large compared to the costs, the intervention is “cost-effective” even if it doesn’t save money.)
This work could help to inform health policy
Moghadas emphasizes the policy applicability of this work: “We want to develop knowledge translation methods to bridge existing gaps between theory, policy and practice. Modeling outcomes should be translated to inform health policy development and support decision-making.”
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By Megan Mueller, senior manager, Research Communications, Office of the Vice-President Research & Innovation, York University, email@example.com