Study on Bangladeshi children confirms lasting impact of typhoid vaccine

A study published in the international medical journal "The Lancet" yesterday confirmed that a single dose of typhoid conjugate vaccine (TCV) offers strong protection for older children against typhoid fever, though effectiveness declines over time, particularly in younger children.

The TyVOID study, conducted in Bangladesh, tracked the vaccine's effectiveness over five years, reports Oxford Vaccine Group.

According to the study report, within two years of vaccination, the TCV provided high levels of protection between 80% and 96% across all children. However, the vaccine's effectiveness decreased over time, particularly in children vaccinated at a younger age.

Between three- and five-years post-vaccination, the data revealed a significant rise in typhoid incidence among children vaccinated in 2018/2019 and those vaccinated in 2021. Despite this, children vaccinated at two years or older remained well-protected, with effectiveness rates ranging from 59% to 85%.

In contrast, a marked drop in protection was noted in children vaccinated under the age of two, suggesting that a booster dose may be required to maintain immunity for this age group.

"Typhoid is a serious and life-threatening disease, particularly for children in low- and middle-income countries," said Xinxue Liu, associate professor of Medical Statistics and Epidemiology at the Oxford Vaccine Group, and one of the senior authors of the study.

"This study highlights the importance of TCV in reducing typhoid cases and provides valuable information for policymakers on the long-term protection offered by the vaccine," he said.

Dr Firdausi Qadri, senior scientist at icddr,b and first author of the study, explained, "This research extends the original TyVAC trial, monitoring the population for another two and a half years after unmasking in 2021. Our findings suggest a decline in antibody levels in various age groups and indicate that a booster dose around school entry age for those vaccinated under two could sustain protection through the school years."

WHO currently recommends a single dose of TCV for children aged six months and older. However, the study's findings raise questions about the long-term protection of a single dose, particularly for younger children, according to Professor Sir Andrew Pollard, director of the Oxford Vaccine Group.

"Whether a single dose of TCV provides long-term protection remains a priority for research, and it is essential to continue investigating to inform future policy decisions," he said.

Typhoid fever continues to pose a significant health burden, particularly in low-income countries where access to clean water and sanitation is limited. The disease is responsible for an estimated seven million cases and 93,000 deaths annually worldwide, with children under five particularly at risk.

Bangladesh ranked fourth in under-5 mortality caused by typhoid in 2019.

The spread of drug-resistant strains of typhoid has heightened the threat of the disease, reducing treatment options and making preventive measures, such as vaccination, all the more critical. 

The first TCV, developed by Bharat Biotech International, received WHO prequalification in 2017, and in 2018 WHO recommended its use as a key tool for typhoid prevention.

With TCV set to be rolled out in vaccination campaigns across Bangladesh in 2025, the TyVOID study, funded by the Bill and Melinda Gates Foundation, stressed the need for ongoing research to monitor the vaccine's long-term effectiveness and its impact on antibiotic resistance. 

This data will be essential for WHO as it continues to update its guidelines for typhoid prevention and control.