Researchers have a possible answer to one of the world's greatest public health challenges, making vaccines available beyond the reach of refrigeration. They are encasing the vaccine's proteins in silica cages that prevent heat decay. The potential of the approach has been demonstrated for a new tuberculosis (TB) vaccine, which, on its own, could prevent some of the 1.6 million annual TB deaths. If success can be extended more widely this could be among the greatest life-savers in history.
Although the developed world is understandably fixated on the way the antivaccination movement allows diseases to flourish, inadequate refrigeration is a much bigger obstacle to vaccines. The "cold chain", where vaccines and medications are moved from one refrigeration unit to another, requires reliable electricity. Half of vaccine doses are discarded, usually because of exposure to temperatures above 8ºC (46ºF). Even where it is possible to keep vaccines useable, refrigeration costs can be so large there's not enough money to vaccinate everyone.
Dr Asel Sartbaeva of the University of Bath came up with the idea of ensilication, where vaccine proteins are encased in successive layers of silica to keep them stable in the heat. With the silica around them, model proteins can't break down or change form. Sartbaeva refers to the silica as a cage.
The idea is theoretically applicable to many heat-sensitive molecules, but the first medically significant trial, described in Scientific Reports, was conducted on the antigen 85b. Although not able to provide protection against TB on its own, Ag85b is being studied as the potential core of future TB vaccines.
“Our results reveal the potential of ensilication in storing and transporting life-saving vaccines at ambient temperatures globally – in particular to remote areas of developing countries where disease rates are often highest,” Sartbaeva said in a statement.
Given the number of lives at stake, it's unsurprising many other techniques have been tried to deliver immunity without the cold. Some of these have proven promising for particular vaccines, such as MMR (measles, mumps, and rubella). Nevertheless, the advantage of something more wide-ranging is obvious.
Coating microscopic proteins in multiple silica layers might sound like an expensive process, at least if you are imagining a chemical jailer caging each protein molecule by hand. The actual process is much easier, however. Silanol groups are negatively charged and attracted to positively charged amino acid residues in the protein, causing the cages to self-assemble.
Vacuum-filtering and drying the caged Ag85b proteins left Sartbaeva a powder that survived five hours exposure to 100ºC (212ºF), but whose vaccines could be released from suspended animation intact through chemical treatment of the silica. Uncaged Ag85b melts at 74ºC (165ºF) and ceases working long before that.