Typhoid is found throughout much of the world.
Prior posts have noted that vaccines offer what is known as herd immunity [see http://en.wikipedia.org/wiki/Herd_immunity]. An inexpensive vaccine against typhoid fever demonstrates this effect, offers protection across age groups, and is particularly effective in preschool-age children, a large trial in India finds. The same study shows that vaccinating half the people in a neighborhood confers significant protection throughout its population (thus the herd immunity effect).
Despite the availability of two approved vaccines, many countries have lagged in their efforts to confront typhoid, which strikes 21 million people each year and causes 200,000 to 600,000 deaths worldwide. In the new study, researchers took an unusual approach to gauge the public health effect of one of the vaccines. In late 2004, the researchers vaccinated more than 37,000 people in the slums of Calcutta. People in some neighborhoods received an injection of a typhoid vaccine called Vi, while half the people in other neighborhoods, serving as a control sample, received hepatitis A vaccinations. The scientists vaccinated roughly half of the people in each neighborhood and arranged for clinics and hospitals to track any subsequent cases of fever that might be typhoid.
After two years, the researchers found that people who had received the typhoid vaccine had 61 percent fewer cases of the disease than did those who got the hepatitis shot. In children ages 2 to 5, a high-risk group, there were 80 percent fewer cases. Unvaccinated neighbors and relatives of people receiving the typhoid vaccine gained some protection, too. There were 57% fewer cases of typhoid overall in neighborhoods in which half of the people received the typhoid shot compared with neighborhoods where people got the hepatitis shot. That drop was not solely attributable to fewer cases among people who had been vaccinated. By vaccinating some people, pathogens have fewer targets to infect. Since typhoid is typically spread by feces-contaminated food and water, having fewer sick people spreading the microbes lessens overall disease risk.
The study can be found at http://content.nejm.org/cgi/content/full/361/4/335.
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