Nepal is a typhoid-endemic country with one of the world’s highest typhoid burdens. Recognizing the burden of typhoid, alongside the impact on local communities, policy makers in Nepal made the decision to introduce typhoid conjugate vaccines (TCV). In 2022 Nepal administered a nation-wide TCV campaign and reached more than 7.5 million children aged between 15 months and 15 years old. TCV is now available in routine immunization for children at their 15 month visit.
Learning more about TCV immunogenicity
Previously conducted large scale phase three efficacy studies in Bangladesh, Malawi, and Nepal assured that TCV is safe and efficacious against typhoid in both African and Asian settings. Additional studies have continued to provide additional information on TCV, including recent data from Malawi showing at least 4 years of durable protection in children.
Recently, researchers in Nepal conducted a small study with one hundred children vaccinated with TCV at 9 or 12 months. Children were then given a second dose at 15 months. In that study, blood samples were collected at four-time points just before the vaccinations and 1 month after – to measure antibody levels after the first dose and additional protection, if any, from the second dose.
Adding to what we know
The study found that TCV is immunogenic in both 9- and 12-month age groups. However, the group vaccinated at 9 and 15 months had a stronger immune response after the second dose than the group vaccinated at 12 and 15 months.
Although small, this study shows that there could be a beneficial immune response with a second TCV dose after a certain interval. The study predicted that a longer interval between doses might be the reason for a stronger response. Due to interruptions during the COVID-19 pandemic, the study analyzed a small sample and cannot conclusively determine the need for additional doses.
Currently there is no WHO recommendation for a second dose of TCV. WHO recommends a single dose of TCV given between 6 months and younger than 15 years of age in a campaign setting in typhoid-endemic regions and during routine visits thereafter. Policymakers are encouraged to make decisions about TCV introduction in their routine immunization program following the appropriate WHO recommendation.
A single dose TCV remains highly immunogenic in children. As additional children are vaccinated and researchers gather new data, we learn more about the long-term protection of TCV. But what we know now remains unchanged: there is ample evidence to support TCV introduction in typhoid endemic countries and countries with high burden of drug resistant typhoid. TCV protects children against typhoid, helps to reduce typhoid transmission, and can limit further evolution of drug resistant typhoid.
Cover Photo Credit: University of Oxford.
