A single dose of typhoid conjugate vaccine (TCV) offers safe, effective protection against typhoid two years after vaccination in all children, according to a report by researchers at the Oxford Vaccine Group.

The study also found the vaccine offered sustained protection for older children three to five years post-immunisation.

However, it showed a decline in protection over these longer periods among children vaccinated at younger ages.

Typhoid fever continues to place a substantial disease burden on low-income and middle-income countries, marked by inadequate sanitation and limited access to clean water and food.

There are estimated to be more than seven million cases and 93,000 deaths annually worldwide.

In Bangladesh, typhoid fever was ranked fourth among the causes of under-five mortality in 2019.

The TyVOID study assessed the effectiveness of a single dose of TCV in Bangladeshi children over a five-year period.

It confirmed high vaccine protection (80-96 per cent) within two years of vaccination in all children.

The study found a three-fold increase in typhoid incidence three to five years after vaccination in children who received TCV in 2018/2019 and those who received it in 2021, indicating a decline in vaccine effectiveness, most pronounced in the very young.

The data suggested children vaccinated at two years or older were still well protected (59-85 per cent) at three to five years post-vaccination.

However, a more significant decline in protection was seen in children vaccinated at under two years of age, suggesting a booster dose for these children might be needed for sustained 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 issue for children and adolescents in low and middle-income countries.

"TCV offers the best chance to reduce the burden of typhoid, helping to reduce transmission and limiting further evolution of drug-resistant strains.

"This study provides additional information for policymakers on longer-term TCV protection and the importance of continued investigation and updated guidance."

Dr Firdausi Qadri is a senior scientist at the infectious diseases division at the International Centre for Diarrhoeal Disease Research, Bangladesh.

The organisation worked alongside Oxford Vaccine Group on the study which Dr Qadri is first author of.

She said: "The results indicate a decay in antibody concentrations in different age groups and suggest that a booster dose around school entry age for children vaccinated while younger than two years could be considered, to sustain the protection from TCV through the school years when children are at greatest risk of typhoid."