Thermostability of vaccines

Every year the immunization services in developing countries prevent about

490 000 children from becoming paralysed by poliomyelitis. Over three million deaths

are similarly prevented from measles, neonatal tetanus and pertussis (51). These

achievements are partly attributable to the training of staff in the proper storage and

transport of vaccines and partly to improvements in the cold chain.

However, vaccines are still not being stored and transported properly in many areas.

Questions are often raised as to what should be done with stocks of vaccines that

have been exposed for varying periods to elevated temperatures. There is no simple

and cheap method that can be used in the field to assess whether a vaccine exposed

to ambient temperature has retained at least the minimum required potency, although

the vaccine vial monitors (VVMs) now provided with oral poliomyelitis vaccine (OPV)

can indicate the level of heat exposure of individual vials. Vaccine potency can be

determined only by costly laboratory assays, the results of which are often delayed

for several months. Only a large number of doses can justify sending a vaccine for

retesting (from 2000 doses for poliomyelitis and measles vaccines, to 200 000 doses

for diphtheria-tetanus-pertussis (DTP) vaccine) (45).

A knowledge of a vaccine’s stability, especially of the rate of decline in potency at a

given temperature, can be helpful in determining storage requirements. The present

document updates previously reported information on this subject (54), with particular

regard to the stability of vaccines stored and transported at ambient temperatures or

exposed to freezing temperatures.

In part two of the document, the stability of individual vaccines is analysed, dealing

in the first section with the vaccines most commonly used in national immunization

programmes, ranging from those that are highly stable, such as the toxoids, to the

least stable, such as oral poliomyelitis vaccine. The following sections look at other

viral and bacterial vaccines which are not yet broadly used, or which address diseases

which are of regional rather than global public health importance.