Oral cholera vaccine provides significantly less protection for children under 5 compared to older children and adults, according to a re-analysis of data from a vaccine trial in India. Findings published in BMC Infectious Diseases suggest re-vaccination is key when children are older.

An international group of researchers used the trial data to compare vaccine efficacy and cholera risk between three different age groups: children younger than 5 years, 5 to under 15 years, and those 15 years and older. The research team included biostatisticians from the University of Washington School of Public Health and Fred Hutchinson Cancer Research Center.

“The natural risk of cholera decreases from one age group to the next,” said first author Youyi Fong, an affiliate professor of biostatistics at the School of Public Health. “However, vaccine efficacy peaked in the middle age group of 5 to 15 year olds.” Fong is an associate member of Fred Hutch’s Vaccine and Infectious Disease Division and Public Health Sciences Division. M. Elizabeth Halloran, a professor of biostatistics and epidemiology at the School of Public Health, and a full member of the Fred Hutch, was a co-author of the study.

Three million cholera cases occur every year, about 100,000 of which are fatal. To understand the oral cholera vaccine’s efficacy among different age groups and how rapidly its protection wanes, the research group re-analyzed results from a 2006 cholera vaccine trial in Kolkata.

In the original trial, more than 66,000 people received two doses of either oral cholera vaccine or a placebo. The vaccine was found to be safe and effective at the two-year interim analysis; it was licensed in India in 2009 and prequalified by the World Health Organization in 2011. The trial’s final analysis found that the vaccine provided protection over five years, at least among older children and adults. However, important questions remained unanswered.

This time, the research group modeled the effect of age categories on both vaccine efficacy and risk of infection in the placebo group. The vaccine was 38 percent effective in those vaccinated before the age of 5 years, 85 percent effective in those 5 to under 15 years, and 69 percent in those 15 years and older.

"The results presented here may be relevant for future routine oral cholera vaccine deployment plans in endemic regions where cholera strikes frequently,” wrote the researchers. “Our analyses show that ­oral cholera vaccine could provide five years of high-quality protection for those over 5 years old with no evidence of waning among adults, suggesting that they might not need to be re-vaccinated frequently to maintain protection, making mass vaccination more cost-effective in cholera-endemic regions.”

The study’s corresponding author is Dennis Chao, senior research scientist at the Institute for Disease Modeling in Bellevue, Washington. Other co-authors include Jin Kyung Park, senior biostatistician at the International Vaccine Institute in Seoul, South Korea, and John Clemens, executive director of icddr,b in Dhaka, Bangladesh.