An Indian cholera vaccine now available produces only 53 per cent protection after two doses.
Using a novel approach, scientists in India have developed a live oral cholera vaccine that is not only more efficacious and hence more protective than the currently available ones but also able to elicit better protection with just one dose. The results of the human clinical trial of the vaccine have been published in July this year in the journal PLoS ONE.
“We were able to achieve 65.9 per cent sero-conversion using only one dose of the vaccine,” said Amit Ghosh who is currently an Emeritus Scientist at the National Institute of Cholera and Enteric Diseases (NICED) in Kolkata. An Indian cholera vaccine now available produces only 53 per cent protection after two doses.
The difference between the existing three vaccines and the candidate vaccine — VA1.4 — being tested goes beyond the level of protection achieved. The most important one from the public health perspective is that the higher protection was achieved using only one dose of the vaccine.
‘Shanchol’, marketed by Hyderabad-based Shantha Biotechnics requires two doses to achieve 53 per cent protection, with the second dose given 14 days after the first. The other two vaccines too need to be given in two doses.
But the biggest public health challenge when a vaccine is given as two doses is to make sure that people come back for the second dose. In reality, there could be a significant number of people not turning up for the second dose; this greatly impacts the achievement of the primary objective of preventive vaccination, especially during cholera outbreaks.
“It is difficult to say” whether it was the use of a live cholera strain (unlike the killed ones used in the other three cholera vaccines) in the vaccine that produced better protection Dr. Ghosh noted. “It’s a speculation that if some antigen that may induceprotective immunity is made by the Vibrio bug only when it is in the intestine, then this protective antigen is absent in the killed bug [used in other vaccines],” he explained.
But the biggest differentiating factor is that unlike the other three vaccines, the strain used in the VA1.4 vaccine does not have the gene that produces the cholera toxin.
“It does happen in nature that due to various reasons one bug may not have the gene responsible for producing cholera toxin,” he said. “NICED [National Institute of Cholera & Enteric Diseases, Kolkatta] screened 1,000s of cholera strains. They identified one and sent it to me at IMTECH [Institute of Microbial Technology, Chandigarh] in mid 1990s to genetically engineer the bug.”
While the general trend at that time was to take a live virulent cholera strain and remove the cholera toxin gene thereby preventing the strain from causing cholera (when the vaccine containing the live bacteria is given), the vaccine still causedsome adverse effects.
“When they remove the toxin gene, other secondary virulent factors present in the cholera bacteria whose adverse effects are normally masked by the presence of the cholera toxin gene emerge,” Dr. Ghosh explained. “These [secondary virulent factors] cause diarrhoea.”
“So we wanted to take a Vibrio bug which is completely devoid of all virulent factors and then manipulate it so the bug has only the immunogenic subunit of the cholera toxin,” he said. The live oral vaccine VA 1.4 was developed by isolating a ‘Vibrio cholerae O1 El Tor’ strain.
The cholera toxin gene is a combination of two different subunits. Subunit A of the toxin gene is the one that causes cholera disease, while subunit B is the immunogenic subunit that is necessary for the virus to produce antigen. The human immune system produces antibodies in response to the antigen produced by a bacteria/virus; antibodies so produced are responsible for killing the bacteria. “We genetically engineered the strain to produce the subunit B,” he said.
“We were probably lucky the approach worked,” he said. “We got a U.S. patent in 10 months of filing it.”
A human clinical trial conducted last year had 44 subjects on whom the vaccine was tested; 43 got a placebo. Two doses were given — the second dose was given 14 days after the first one. The sero-conversion was about 66 per cent on day seven after the first dose was given, and it did not increase further after the second dose was administered. The trial was done in collaboration with the Society for Applied Sciences, Kolkatta.
“The trial was beyond Phase I,” Dr. Ghosh said, “because it looked at sero-conversion [efficacy] and not just safety.” The main objective of Phase I trials is to check for the safety of a candidate drug/vaccine. A larger trial involving more human subjects is being planned.
The vaccine was developed by a collaborative effort of three institutes in India — IMTECH, NICED the Indian Institute of Chemical Biology (IICB), Kolkata. DBT funded the project.