Health workers have to keep vaccines cool at all times - from the moment of their production until they are administered by injection or in other ways.

"This is one of the reasons why we're still unable to vaccinate many children against diseases, which have been wiped out in developed countries - polio, for example," says Sebastian Dietrich, a physician working with Doctors without Borders. "We just can't get the vaccines to very remote areas while keeping them cool."

When Doctors Without Borders vaccinates people in developing countries, they face serious challenges, says Dietrich. Most sensitive vaccines solutions become unusable as soon as their temperature rises above eight degrees Celsius.

Dietrich and his colleagues have to get the vaccines through customs before they go bad, and they have to find storage rooms with good refrigeration, or transport the vaccines by car to remote areas, where there may not even be electricity.

Polio still exists in some countries - part of the problem, is keeping the vaccine cool

And sometimes they fail.

"Vaccines often get warmer than eight degrees - it might simply be because the door of a refrigerator is not closed properly," says Dietrich. "And when that happens, we have to dump the lot."

Once a vaccine becomes ineffective, it fails to trigger an immune response - the vaccination fails.

"If we had vaccines which could leave the cold chain for a week, a day even, that would make a huge difference," Dietrich says. "And we dream of vaccines which do not need refrigeration at all."

Vaccines encapsulated

One approach to developing heat-resistant vaccines uses "nanovaccines." Nanovaccines consist of nano-sized particles of a biodegradable polymer, which encapsulates an antigen - that is, for example, a protein of a pathogen, or the active ingredient.

At this week's National Meeting of the American Chemical Society in Dallas, Balaji Narasimhan of Iowa State University presented his latest results from tests with nanovaccines: his team was able to boost an immune response in mice.

Vaccines have to be refridgerated until they are administered - a challenge in developing countries

"We've shown that it works with rodents, and we're moving forward to show that [it works] in larger animals as well," says Balaji Narasimhan.

His team is focused on vaccines against influenza. But Harasimhan says the particles they used are also suitable for vaccines against tropical diseases. It all depends on the antigen the vaccine producer puts inside the nanoparticles.

One dose, thermally stable and no syringes

One of the biggest potential advantages of nanovaccines is that health workers could store them at room temperature for up to six to 10 months.

"The particles are made of materials that have high-term stability - the only thing that pulls them apart is water," says Narasimhan. They just need to be stored in a dry place.

And people would only need one dose - with no booster shots.

But what makes nanovaccines especially attractive for vaccinating children is that they can be inhaled as a spray - so no syringes either.

Narasimhan is still at the stage of preclinical trials with animals. If his nanovaccines continue to perform well in the lab, it will still be years before they can be used in the field.

Developing new vaccines costs about a billion dollars - and a lot of time

Breaking the chain

So far the only heat-resistant vaccine that exists is MenAfriVac, which is against meningitis. Health authorities have approved it for use even after four day's storage at up to 40 degrees Celsius. In 2012, researchers tested the vaccine successfully in vaccination campaigns in Chad and Benin.

Sebastian Dietrich has worked on the MenAfriVac campaign in Chad.

"It changes everything," he says. It still has to be refrigerated for most of the time, but once the vaccination program starts, you can "just get into the car and go wherever you need to go."

MenAfriVac was developed by the World Health Organization and the Program for Appropriate Technology in Health (PATH). The researchers are now looking into heat-resistant vaccines for other diseases as well.

Dumping vaccines that still work

But it's unclear whether absolutely every vaccine needs refrigeration. Often, this is decided on the basis of what researchers have tested in the lab - and what authorities are willing to approve.

"We know that vaccines against tetanus and hepatitis are relatively stable," Dietrich says. "We could theoretically leave them out of the cold chain for a short time."

But that would go against regulations.

"Officially they have to be cooled. So [if we don't cool them], we have to throw them away."

Vaccine makers tend to run accelerated stability studies, Simona Zipursky, a public health specialist with the WHO recently told "Nature," adding that the standards for vaccine expiration are very conservative.

It's one reason why the MenAfriVac researchers conducted additional stability testing to get their vaccine approved. But running such studies can be costly.

So until non-refrigeration vaccines become widely available, especially for hot, developing countries, health workers will just have to make sure they don't run out of fridges and ice packs.