Intravenous tuberculosis (TB) vaccination is highly protective against the infection in monkeys, compared to the commonly used approach of skin injection, according to a study which may lead to new ways of delivering vaccines against the disease.
According to the researchers, including those from the University of Pittsburgh in the US, even though a vast majority of people across the world are vaccinated against TB, more people die from the disease than any other infectious disease, suggesting that the vaccine may not reliable.
In a study, published in the journal Nature, the scientists showed that simply changing the way the vaccine is administered could dramatically boost its protective power.
"When we compared the lungs of animals given the vaccine intravenously versus the standard route, we saw a 100,000-fold reduction in bacterial burden. Nine out of 10 animals showed no inflammation in their lungs," said study senior author JoAnne Flynn from the University of Pittsburgh.
The researchers said while the Bacille Calmette-Guerin (BCG) TB vaccine, which is made of a live, weakened form of TB bacteria found in cattle, has been around for 100 years, and is among the most widely used vaccines in the world, its efficacy varies widely.
As part of the study, the scientists separated their study subjects, a colony of monkeys, into six groups -- unvaccinated, standard human injection, stronger dose but same injection route, mist, injection plus mist, and finally, the stronger dose of BCG delivered as a single shot directly into the vein.
They then exposed the animals to TB after six months, and monitored them for signs of infection.
According to the study, all of the animals which received the standard human dose had persistent lung inflammation, and the average amount of TB bacteria in their lungs was only slightly less than in the monkeys that received no vaccine at all.
Injected and inhaled vaccines offered similarly modest TB protection, the researchers said.
On the other hand, the intravenous vaccine, the researchers said, offered nearly full protection.
They added that there was virtually no TB bacteria in the lungs of these animals, and only one monkey in the group developed lung inflammation.
"The reason the intravenous route is so effective is that the vaccine travels quickly through the bloodstream to the lungs, the lymph nodes and the spleen, and it primes the T cells before it gets killed," Flynn explained.
The researchers found the vaccine, and the immune system's T cells in the lungs of all the intravenously vaccinated animals.
In the other groups, the scientists noted that BCG was undetectable in the lung tissue, and T cell responses were relatively meagre.
But before this method is translated to humans, the researchers said it must be tested for safety and practicality since an intravenous vaccine requires more skill to administer and carries a higher risk of infection.
"We're a long way from realizing the translational potential of this work. But eventually we do hope to test in humans," Flynn said.