Shigella Vaccines – Scientists Working Towards An Effective Solution

 

Shigella is a bacterial infection that causes shigellosis - a severe diarrheal disease affecting millions worldwide each year especially children in developing countries. With limited treatment options, scientists have been working on developing an effective Shigella vaccine that can help reduce the burden of this infectious disease. In this article, we take a look at some of the key developments in Shigella vaccines research.

The Problem of Shigella

Shigella bacteria are primarily transmitted through contaminated food and water or direct person-to-person contact. The infection causes diarrhea, fever, abdominal cramps, and can sometimes lead to dysentery - a particularly severe form of diarrhea with presence of blood in stool. Symptoms usually start 1-3 days after exposure and most people recover within 5-7 days. However, in certain vulnerable groups like young children and people with compromised immunity, Shigella infection can turn serious and even become deadly in severe cases if not treated properly.

Globally, Shigella causes an estimated 80-165 million cases of diarrhea each year resulting in around 600,000 deaths primarily among young children. While much of the developed world has access to safe drinking water and proper sanitation, Shigella continues to pose a major health challenge in developing nations especially in densely populated areas with poor sanitary conditions. Considering the significant disease burden and limited treatment options, scientists believe an effective vaccine can play a key role in reducing the impact of shigellosis.

Challenges in Shigella Vaccine Development

However, developing an effective Shigella vaccine has posed various scientific challenges over the years. Some of the key difficulties include:

- Identifying serotype coverage: There are four serotypes/species of Shigella - boydii, dysenteriae, flexneri, and sonnei. An ideal vaccine needs to provide protection against multiple prevalent serotypes found across regions.

- Immunogenic response: Shigella primarily causes infection in intestinal mucosa. Achieving a robust mucosal and systemic immune response against the bacteria through oral vaccination has been difficult.

- Antigenic variation: Shigella has a tendency to modify surface antigens through genetic mutation making it difficult for vaccines to provide long term protection.

- Safety issues: Using live-attenuated Shigella strains as vaccines carries a potential risk of reversion to virulence. Killed/inactivated vaccines so far have not shown good immunogenicity.

Progress on Shigella vaccines has thus been slow with none being incorporated into public health programs yet. Scientists have been continuously working on novel approaches and formulations to overcome these obstacles.

Recent Developments

In the past decade, significant advances have been made -

Live-attenuated Vaccines:

Two live-attenuated Shigella vaccines - Shigevac and Sahecvac have shown promise in clinical studies conducted in Bangladesh and Mali respectively. Both vaccines target S. flexneri 2a, a commonly prevalent serotype in these regions. They were found safe and demonstrated good immunogenicity providing around 80% protective efficacy against the targeted serotype. Larger, longer term field efficacy trials are still ongoing.

Conjugate Vaccines:

Conjugating Shigella O-antigens to carrier proteins like tetanus toxoid is a strategy to boost immunogenicity of killed/subunit vaccines. It helps activate T-cell dependent immune response. A bivalent S. sonnei and S. flexneri 2a conjugate vaccine named Shanchol demonstrated 64% efficacy against homologous serotypes in a phase 3 trial in Bangladesh. It got WHO prequalification in 2013 but is not yet part of national programs.

Multivalent Vaccines:

Combining antigens from multiple prevalent serotypes in a single formulation is critical for broader coverage. A tetravalent vaccine covering S. flexneri 2a, 3a and 6 as well as S. sonnei completed phase 1 and 2 testing. Large efficacy trials incorporating more serotypes are still awaited.

Oral Recombinant Vaccines:

Genetic engineering techniques are utilized to develop oral recombinant vaccines expressing Shigella invasion plasmid antigens that can directly activate mucosal immune response. A few candidates targeting S.flexneri have shown promise in early studies and are advancing to further trials.

Future Directions

While continued progress is being made, an ideal globally effective Shigella vaccine is still elusive. Here are some key areas that require further research -

- Expanding serotype coverage: Incorporating additional prevalent serotypes like S. boydii and S. dysenteriae type 1 into multivalent vaccines.

- Improving immunogenicity: Novel adjuvant combinations and delivery platforms to achieve robust mucosal and systemic immune responses.

- Field effectiveness: Large scale real world efficacy studies are needed from disease endemic regions to better understand public health impact.

- Combination therapies: Exploring synergistic effects of vaccines along with other prevention strategies like water/sanitation improvements and oral rehydration.

- Production and delivery: Ensuring vaccines can be manufactured affordably at scale and integrated into routine childhood immunization programs.

With continued global efforts, it seems increasingly promising that we will have a suite of effective Shigella vaccines in the coming years substantially reducing the burden of this deadly diarrheal disease, especially in developing countries.