Ring-Vaccination Strategy: Does it work?
Written by Ritika Jhawar
Edited by Charan Karthik
Jan 23rd 2022
Edited by Charan Karthik
Jan 23rd 2022
As defined by the CDC, cholera is “an acute diarrheal illness caused by infection of the intestine with Vibrio cholerae bacteria”. One can get infected by eating/drinking water that is contaminated, and symptoms range from mild to life-threatening. Some of these symptoms include: vomiting, restlessness, low blood pressure, etc. One of the biggest problems after getting infected is the risk of dehydration. If left untreated, this can lead to further complications such as strokes, or even death. Luckily, once patients receive treatment, they recover without any long-term effects. They are still at risk of getting sick if exposed again though, as the infectious bacteria is no longer in their body.
In an attempt to learn more about cholera vaccines and risk of contraction, a study was conducted in Kolkata, India from July to September 2006. The purpose of this research was to understand whether ring vaccination strategies (using oral cholera vaccines - OCVs) were effective in reducing infection rates and protecting those living near cholera cases.
Research shows that OCVs provide a great amount of protection for those who get access to it. In reality, there are always limited doses and resources, resulting in many unprotected communities. Cholera tends to spread through tight-knit groups, in which those who are closest to the cases are at highest-risk. The method proposed to prevent such spread is “ring vaccination.” This involves vaccinating a group of people closest to the cases, in hopes of increasing coverage and reducing the amount of vaccines required within groups.
The format of the trial was randomized, double-blind, and placebo controlled. The participants were from an urban slum in Kolkata, and they were either given vaccines or placebos. They were grouped into dwellings that were randomly assigned, and the cholera surveillance began by gaining information on sanitation, hygiene, water usage, etc. The index controls (first identified case) were those who lived in the slum and had gotten cholera during this surveillance period after being vaccinated. Those who lived around each index case were arranged into “rings” as groups of people who were exposed. These cohorts varied from 10 to 55 meters, and they were also created for “non-exposed” cases.
This variation in distance within a specific time frame helped to track the cholera cases. Vaccine effectiveness (VE) was compared between the cohorts that had most vaccinations versus the ones with fewest. The control groups were the ones with the lowest coverage, and a regression model (shows relationships between different variables) was used to calculate the risk between the vaccination coverages. The overall VE was calculated by comparing transmission rates (regardless of vaccine) between high and low vaccine-covered cohorts, while the indirect VE looked at cases between unvaccinated individuals in vaccine-covered cohorts.
Upon observing the data, the risk was over 11 times higher for those who were exposed to cholera than those who were not. Furthermore, the risk was much higher for those living within 0-25 meters of a case. When looking at the cohorts in this range, only 22% were vaccinated on average. Those who were living within 10 meters of cholera cases and within the time frame of two weeks after onset were at very high risk compared to those who lived the same distance away at the same time. This shows proof of how fast transmission can occur for those who live near each other. There is also high risk for those living as far as 25 meters away, which furthers the argument that any highly populated area requires vaccination when cases arise. Regarding time, the risk decreases after seven days of onset, but intervention is required rapidly in order to prevent further symptoms.
To assess the ring vaccination strategy, the VE of people living within 25 meters, 8-28 days after onset was observed. Those who lived within high vaccination cohorts were very protected compared to those who lived in low vaccinated ones. This includes those who were unvaccinated living in a vaccinated ring. This trend was consistent for five years after the vaccine was received, leading to the theory that revaccination is not required if many have received a dose. It was concluded that the ring vaccination strategy is effective if there is more than 30% coverage.
There are a few drawbacks that need to be considered when understanding this study. While ring vaccination is a strong tool to help protect communities, it is more challenging to apply in the field. It is hard to deliver such a large amount of doses in a short amount of time (first onset), and it would require properly trained teams. Another issue is that this strategy was evaluated a few days after the appearance of the case, so there would have to be other prevention methods within the first few days of diagnosis. Overall, there needs to be a balance found between the production of OCVs and its distribution within highly populated areas. There needs to be further research done on the implementation of vaccines in a safe and effective manner, but ring vaccination seems to be like a strong option if done right.