Understanding Vaccination: The Cold Chain

Delivering vaccines is hard work. One of the hardest—and least-discussed—problems of vaccination is the cold chain, the challenge of keeping a vaccine at cold temperatures all the way from production, through shipping to a given country, through local delivery to a health clinic, and finally delivery into someone's body. As we continue World Immunization Week, let's dig in to this slightly geeky technical problem—one that literally means the difference between life and death.

Why Keep Vaccines Cold?

While some vaccines are stable at a relatively wide range of temperatures (some as hot as 40° C), most vaccines must be kept cold in order to maintain their potency. What's more, some vaccines must be kept in a strictly controlled range of temperatures (typically 2°-8° C) or they go bad. Temperatures too cold or too warm can cause a dose of vaccine to lose its "immunogenicity," or its ability to affect the human immune system.

There's a related problem here: Some vaccines that are able to be exposed to higher temperatures are not labeled as such. This causes workers to refrigerate those vaccines unnecessarily (treating all of them the same), which is a waste of energy.

Why Is This A Challenge?

Almost 50% of health posts providing vaccines have no (or very minimal) access to the power grid. Without grid power, it's hard to have reliable refrigeration (and even if you have grid power, in some parts of the world that means lots of outages—which can be hard on a refrigeration unit).

Transporting cold vaccines to remote areas and then storing them properly in those locations are both serious problems due to the general lack of reliable electricity for cooling. In some parts of the world, we're talking about literally packing a foam cooler with ice and vaccine, then carrying that cooler to a village.

To compound the cold chain's core logistical challenge, we are now able to vaccinate children against more diseases than ever. This is great! But more kinds of vaccines means more vaccine volume to carry around and keep cold. One estimate shows that the volume of vaccine per child has gone from 50cm³ in 1980 to 200cm³ in 2010. That's a fourfold increase in vaccine volume per child, to protect against roughly 2.5 times the number of diseases over the same time period.

Promising Solutions

The best solution to this problem would be the development of vaccines that do not require refrigeration. While work is underway, it may not be possible for certain kinds of vaccines, and even if it becomes possible, that doesn't solve any problems today; the R&D is years out. But, for the record, "thermostable" vaccines are what we hope for down the road—along with better labeling of the ones we have today.

There are also interesting technological solutions in testing. The simplest is a "passive long-term cooler," which you may know better as "a really big, beefy Thermos©-style container." When you make the leap from traditional "tailgate party cooler full of ice" to a precisely engineered vacuum flask, you can extend the cold storage from a matter of hours to an entire month with no power. This is huge, it's cheap, and it's simple. (On the hand, it does require ice...which typically requires refrigeration to create.)

Another approach for health posts without grid power is a Solar Direct Drive. These devices use solar power to drive a compressor, creating ice, then store power in the ice rather than in a battery. This is more resilient to power losses than a traditional refrigerator or battery pack, and a large block of ice can keep the system cool for up to five days even if solar input is low or nonexistent (for instance, on cloudy days).

The final technological solution in the pipeline is Ice-Lined Refrigerators (ILRs). These exist today, but improvements to the basic technology could mean these refrigerators could operate on about 8 hours of grid power per day, and still keep vaccines in the required cool range for several days in the event of a power failure.

Systems Thinking

The cold chain is a logistical problem with many inputs. To improve its performance, we have to think about all sides of the problem: improved vaccines (ideally requiring less refrigeration or less volume); improved refrigeration (requiring less or no power); and improved delivery systems (only deliver the needed amount of a vaccine to a given area "just in time," reducing the need for local storage off the grid). All of these elements are in play.

In a recent pilot program in Nigeria's Lagos State, all these elements were addressed. At the beginning of the trial, half the district health posts had inadequate vaccine stock on hand; at the end, all were properly stocked. After the program, pentavalent vaccination rates had risen by 15% within just one month. (The "penta" vaccine protects against five diseases: diphtheria-tetanus-pertussis (DTP), hepatitis B, and Haemophilius influenzae type B.) Nigeria is just one of three countries (the others being Afghanistian and Pakistan) in which polio is still endemic, so improvements to vaccination there are key to beating polio.

The take-away: by tackling the cold chain problem from multiple angles, we can improve delivery of vaccines, reduce waste, and save both lives and money. That's a goal worth fighting for.