Describe the cold chain and its maintenance.

The Cold Chain

• That last point IS the whole reason the system matters — the failure is invisible, the child appears immunised, and the deficiency is discovered only when an outbreak occurs among the "vaccinated"
• The storage temperatures — +2 to +8°C at the peripheral level (the ILR and the vaccine carrier); and -15 to -25°C for the deep freezer, used for OPV storage and for freezing ice packs
• The critical distinction, which is examined more than any other point in this topic:
• The most heat-sensitive vaccines are OPV, followed by measles-rubella and BCG — these are the live vaccines
• The most freeze-sensitive vaccines are those containing an aluminium adjuvant — DPT, pentavalent, hepatitis B, Td and IPV. Freezing destroys them irreversibly by causing the antigen to dissociate from the adjuvant and the vaccine to flocculate
• The practical consequence follows directly and is frequently got wrong — freeze-sensitive vaccines must never be placed against the walls or at the bottom of an ILR, nor in the deep freezer, nor in direct contact with a freshly-frozen ice pack
• IN practice, accidental freezing IS A commoner and more damaging problem than heat exposure, and this is counter-intuitive enough to be worth stating plainly
• The shake test detects a frozen adsorbed vaccine — a frozen-and-thawed vial shows rapid sedimentation with floccules, and clears within about 30 minutes, whereas an unfrozen vial remains smooth and uniformly turbid when compared side by side with a deliberately frozen control
• The equipment of the chain — walk-IN coolers and freezers at regional stores; deep freezers and ILRs (ice-lined refrigerators) at district and PHC level; cold boxes for transport and for storage during power failure; vaccine carriers with four conditioned ice packs for the session site; and day carriers for short trips
• The ice-lined refrigerator is designed for the Indian setting — its lining of water-filled tubes holds the temperature for many hours during a power cut, and it is top-opening so that cold air does not spill out when it is opened
• Conditioning of ice packs is essential and is routinely omitted — a frozen ice pack must be left at room temperature until water begins to move inside when shaken, before it is placed in a carrier
• An unconditioned ice pack freezes the adjuvanted vaccines beside it, which is the commonest single cause of cold chain failure at the session site
• Monitoring devices:
• The vaccine vial monitor (VVM) is a heat-sensitive square within a circle printed on the label, which darkens cumulatively with heat exposure
• Its interpretation — the vaccine is usable while the inner square is lighter than the outer circle, and must be discarded once the square is the same shade as, or darker than, the circle
• The VVM monitors heat only and gives NO information about freezing, which is precisely why the shake test remains necessary
• Other devices — the freeze indicator (freeze-tag); dial thermometers with twice-daily temperature recording on a chart; and eVIN, which provides real-time electronic monitoring of stock and temperature and has transformed the visibility of failures
• The common failures to identify in practice — unconditioned ice packs; storing adjuvanted vaccine against the ILR wall; keeping food or drugs in the vaccine refrigerator; opening the refrigerator too frequently; failure to record temperature twice daily; power failure without a contingency plan; and overstocking beyond capacity
• Freezing does more damage than heat, which is the opposite of what most people assume about vaccine storage
• Adjuvanted vaccines are destroyed by freezing irreversibly, as the antigen dissociates and the vaccine flocculates
• Never place DPT or pentavalent against the ILR wall, nor in the deep freezer, nor beside an unconditioned ice pack
• Condition ice packs until water moves inside, which is the step most often skipped at session sites
• The vaccine vial monitor records heat only, which is precisely why the shake test still exists
• Discard once the square matches or darkens past the circle, and use the vial while the square remains lighter
• OPV is the most heat-sensitive vaccine, followed by measles-rubella and BCG, all of them live
• The shake test compares against a frozen control, and a damaged vial sediments with floccules within half an hour
• Cold chain failure is invisible, so the child appears immunised and the deficiency shows only in an outbreak
• The ILR holds temperature through a power cut, and opens from the top so cold air does not spill out
• Keep no food or drugs in the vaccine refrigerator, since every extra opening costs temperature stability
• Record temperature twice daily on the chart, which is the simplest discipline and the most often neglected
• Take four conditioned ice packs to a session, and never a freshly frozen one straight from the freezer