Research against typhoid vaccination confirms durable protection for older children

A single dose of typhoid conjugate vaccine (TCV) provides safe, effective protection against typhoid fever two years after vaccination in all children, and durable protection for older children three to five years after immunization, according to a report by researchers at the Oxford Vaccine Group and the International Center for Diarrhea Disease Research, Bangladesh (icddr,b). But it also shows a decline in protection at later times among children vaccinated at a younger age.

The TyVOID study, published in The Lancetmeasured the effectiveness of a single dose of TCV in Bangladeshi children over a five-year period and confirmed high vaccine protection (80-96%) within two years of vaccination in all children. The article is titled “5-Year Vaccine Protection Following a Single Dose of Vi-Tetanus Toxoid Conjugate Vaccine in Bangladeshi Children (TyVOID): A Cluster Randomized Trial.”

Three to five years after vaccination, the study found a threefold increase in the incidence of typhoid fever in children who received TCV in 2018/2019 and in those who received it in 2021, indicating a decrease in vaccine effectiveness, most pronounced in very young children.

The data showed that children vaccinated at two years of age or older were still well protected three to five years after vaccination (59-85%). However, a more significant decrease in protection was observed in children vaccinated under two years of age, suggesting that a booster dose for these children may be required for lasting protection.

Xinxue Liu, associate professor of medical statistics and epidemiology at the Oxford Vaccine Group, and one of the senior authors of the study, said: “Typhoid is a serious and life-threatening enteric fever that remains a substantial public health problem for children and adolescents. in low- and middle-income countries.

“TCV offers the best opportunity to reduce the burden of typhoid fever, thereby reducing transmission and limiting the further development of resistant strains. This study provides policymakers with additional information on longer-term TCV protection and the importance of continued research and updated guidelines.”

Dr. Firdausi Qadri, senior scientist at the Division of Infectious Diseases at the International Center for Diarrheal Disease Research, Bangladesh (icddr,b), and first author of the study, said: “This TyVOID study followed the original TyVAC cluster randomized controlled trial population. two and a half years after the unmasking in 2021.

“The results indicate a decrease in antibody concentrations in different age groups and suggest that a booster dose around school age for children vaccinated while under 2 years of age could be considered, to maintain protection against TCV throughout the school years when the children are the highest. risk of typhoid.”

Professor Sir Andrew Pollard, director of the Oxford Vaccine Group, said: “The current WHO recommendation is a single dose of TCV for infants and children from the age of six months.

“Epidemiological studies in several countries in Asia and Africa have shown that the incidence of typhoid fever is much higher in children under 16 years of age than in adults, with the peak of cases observed in children between the ages of 5 and 9 years. of TCV provides long-term protection remains a top research priority to advise policy makers.”

Typhoid fever continues to cause a significant disease burden in low- and middle-income countries, which are characterized by inadequate sanitation and limited access to clean water and contaminated food. Globally, there are estimated to be more than 7 million cases and 93,000 deaths annually. In Bangladesh, typhoid fever ranked fourth among the causes of under-5 mortality in 2019.

Furthermore, the escalation of antimicrobial resistance, underscored by the emergence of highly resistant strains in Asia, has reduced available treatment options and increased the public health threat posed by this disease. In a multicenter disease burden study, STRATAA, Bangladesh had the highest rates of cholera.

The first typhoid conjugate vaccine (TCV; Bharat Biotech International) received prequalification from the WHO in 2017, and in 2018 the WHO recommended vaccines as an important tool in the prevention and control of typhoid. By 2025, TCV will be used in childhood vaccination campaigns across Bangladesh.

This report confirms the need for further studies to monitor the typhoid burden following TCV rollout, and to provide additional data on the duration of vaccine protection from high disease burden settings, as well as the effect on antibiotic resistance patterns, which is critical for the WHO to update its data. position.