Zika Resource Center
Zika Resource Center
April 18, 2018
3 min read

Up to 99% of prenatal Zika infections may be prevented with vaccination

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Approximately 99% of prenatal Zika infections could be avoided through immunization with a vaccine that demonstrates 75% efficacy, according to findings published in the Annals of Internal Medicine.

This strategy would require WHO-suggested vaccination coverage, which includes males and females between the ages of 9 and 49 years.

“Despite the vector-control measures already implemented in many affected countries to combat dengue and yellow fever, Zika continued to spread throughout 2016 and into 2017,” David P. Durham. PhD, fromthe Center for Infectious Disease Modeling and Analysis, Yale School of Public Health, , and colleagues wrote. “Sustained reduction of risk for future outbreaks will likely require a combination of vector control and vaccination, as is necessary for yellow fever, another flavivirus.”

The researchers also noted that to successfully reduce the risk of outbreaks in the future, a multifaceted approach using vector control strategies and vaccination will most likely be required. This approach is currently necessary to prevent yellow fever and outbreaks of other mosquito-borne illnesses.

To measure the frequency of prenatal Zika infections after prioritizing females between the ages of 9 and 49 years for immunization against the virus, and then vaccinating all males between the ages of 9 and 49 years, the researchers used a compartmental model that focused on the spread of Zika from mosquitos to humans. The model was created and adjusted to reflect the mosquito density in 34 countries and territories located within the Americas that have experienced prior outbreaks of the disease.

During analysis, the researchers categorized mosquitos into three cohorts, which labeled the insects as susceptible, exposed or infected. Furthermore, humans were split into five cohorts, including susceptible, exposed, infected, recovered and vaccinated. Durham and colleagues merged information on exposure with age-specific fertility rates, demographics for each country analyzed and rates of infection through sexual encounters. In the model, these people received a vaccine that demonstrated 75% efficacy.

Durham and colleagues estimated that 5,530 Zika infections were contracted prenatally in Puerto Rico between 2016 and 2017 (95% credible interval [CrI], 3,3551-7,503). When a vaccine with an efficacy of 75% was administered to 90% of females between the ages of 9 and 49 years, the number of infections would drop to 358 (CrI, 188-626; 94% reduction; CrI, 92%-95%).

When this vaccination administration strategy was applied to males between the ages of 9 and 49 years alongside females of the same age, only 76 infections would be contracted (CrI, 52-111; 99% reduction; CrI, 98% to 99%).

Durham and colleagues also accounted for an inadequate vaccine supply. If all individuals between 9 and 49 years old — an age range recommended by WHO — cannot be immunized, 5-year age groups were considered for those between the ages of 10 and 49 years. Based on this strategy, a significant number of prenatal infections would be prevented through every vaccination in relation to every country and that country’s fertility rate (P < .001).

The researchers observed that when 20- to 24-year-old women were vaccinated in most countries, including Brazil, Costa Rica and Puerto Rico, vaccination against prenatal Zika infection was most effective. Countries in which females tend to have children later in life, such as Dominica, Guadeloupe and Suriname, vaccination was most effective at decreasing the number of prenatal Zika infections when females were immunized between the ages of 25 and 34 years.

“Significant challenges must be addressed in the development and implementation of a Zika vaccine,” Durham and colleagues wrote. “The decrease in Zika incidence makes traditional methodology for clinical trials infeasible. Even during an epidemic, the speed of Zika outbreaks at the local level necessitates innovative approaches for trial site selection, possibly including simultaneous preapproval and surveillance for many sites.”

“Despite these challenges, our results show the substantial effect that Zika vaccination programs could have in mitigating and preventing future outbreaks,” the researchers continued. “… Given the temporal uncertainty of Zika resurgences, we must continue to develop and test vaccines, ensuring availability as soon as possible.” – by Katherine Bortz

Disclosures: The authors report no relevant financial disclosures.