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Introduction to Typhoid Vaccines
Typhoid fever remains a serious public health issue in many developing countries where sanitation and clean water supplies are limited. The bacteria Salmonella Typhi is transmitted through contaminated food or drinking water causing typhoid fever. For years, typhoid fever has been controlled through antibiotics and vaccines. There are two main typhoid vaccines available – the injectable Vi polysaccharide vaccine and live attenuated oral typhoid vaccines.
Vi Polysaccharide Vaccine
The Vi polysaccharide vaccine was the first typhoid vaccine to be licensed for use. It contains purified Vi capsular polysaccharide from Salmonella Typhi bacteria. It provides approximately 70% protection for 3 years after vaccination through injection. However, it is not effective in young children below 2 years of age. Responses to the vaccine begin to wane after 3-5 years requiring booster doses. The injectable nature also makes distribution and administration logistically difficult in mass immunization programs in resource poor settings.
Advancement of Oral Vaccine
In the late 1980s, a live attenuated oral typhoid vaccine was developed using the strain Ty21a. This vaccine provided protection levels similar to the injectable Vi vaccine but offered the significant advantage of oral administration. Clinical studies found it to be safe and over 85% effective. The Ty21a oral vaccine offered longer lasting protection estimated at 7 years along with the ability to immunize from 6 months of age. The ease of oral delivery meant mass immunization programs in endemic areas were feasible. It was prequalified by WHO in 1992 and introduced vaccination programs in several countries.
Tybal Vaccine – A New Oral Formulation
More recently, a new oral typhoid vaccine using the live attenuated bacterial strain CVD 908-htrA was developed by Bharat Biotech. Termed Tybal, it contains roughly a million times fewer bacteria compared to earlier oral typhoid vaccines like Ty21a. This lowers the risk of any adverse effects while still inducing protective immunity. Clinical trials found Tybal to have a protective efficacy of over 94% lasting for at least 3 years after two doses. It can be administered to children over 6 years and adults. With its superior immunogenicity and safety profile, Tybal vaccine has the potential to significantly impact typhoid control.
Mass Immunization Programs Using Tybal
Several Asian and African countries with a high typhoid disease burden have now introduced Tybal vaccine into their Expanded Program on Immunization schedule following WHO prequalification in 2018. In 2020, a nationwide vaccination campaign was conducted in Bangladesh targeting over 30 million people in Dhaka city. This was the world’s largest mass typhoid vaccination campaign to date. Preliminary analysis reported the campaign was successfully rolled out with high coverage rates and no serious adverse events recorded. Early signs indicate a substantial reduction in laboratory-confirmed typhoid cases in the post-vaccination period.
Similar mass immunization programs have also been undertaken in Zambia and parts of Pakistan with vaccination of school going children. Ongoing surveillance data from these regions will provide valuable insights into the public health impact of the vaccine at the population level. The successful implementation of large-scale vaccination campaigns along with the findings from post-introduction studies will help evaluate Tybal’s potential as a game-changing oral vaccine for typhoid control.
Regulatory Approvals and Future Prospects
Based on its safety, efficacy and public health relevance, Tybal vaccine has received marketing authorization from several national regulatory agencies including India, Pakistan and South Africa. WHO has also prequalified Tybal paving the way for global procurement and funding through programs like GAVI. At a significantly lower price point compared to earlier oral vaccines, Tybal is projected to have a major role in achieving the WHO’s goal of typhoid control and elimination in endemic regions. Its integration into routine childhood immunization schedules could break the transmission of the pathogen in communities. Bharat Biotech also plans to evaluate a typhoid conjugate vaccine incorporating Tybal to provide longer lasting protection for adults. Overall, the oral Tybal vaccine holds immense promise as a game-changing public health intervention against the scourge of typhoid fever worldwide.
*Note:
- Source: Coherent Market Insights, Public sources, Desk research
- We have leveraged AI tools to mine information and compile it
