Thermostability of Vaccines
Thermostability of Vaccines
Mrudula K. Lala
Kamlesh R. Lala
Cold chain is the system of transporting, storing and distributing vaccine in a potent state at recommended temperature till it is administered to an individual. It is the vital link between the child and immunity in immunisation against vaccine preventable diseases (VPD). However potent a vaccine may be, if cold chain is not maintained from the manufacturer to the place of vaccination, the vaccine efficacy greatly suffers(1). Most vaccines lose their potency by heat and sunlight and hence need protection from both(2).
In spite of better development of safe transport and storage facilities, there are still weak points in the cold chain(3). The avoidable errors are reported even from developed countries and include high temperature during storage and transport, exposure of adsorbed vaccine to freezing temperature, refrigerator without thermo-meters and lack of regular recordings of temperature, refrigerator not being used exclusively for vaccine, failure to discard unused vaccine and use of reconstituted vaccines after exposure at ambient tempera-ture(4). Each exposure of the vaccine to an ambient temperature has a cumulative effect on reducing its potency. This is of concern in view of the fairly frequent reports of VPD occurrence in populations thought to have been well immunized(5).
There is no simple and cheap method that can be used in the field to assess whether an exposed vaccine has retained at least the minimum required potency. The available methods like accelerated degradation test, viral titers and biological assays are costly and time consuming taking several months. But the vaccine vial monitor (VVM) now provided with OPV can indicate the level of heat exposure of that particular vial. Know-ledge of vaccine?s stability, especially of the rate of decline in potency at a given temperature can be helpful in determining its storage requirements.
Vaccine Vial Monitors (VVM)
VVMs, which measure exposure to heat, are time and temperature sensitive labels attached to vials of vaccine at the time of manufacture. Through a gradual colour change they warn about the falling potency. They are designed to meet the vaccine?s heat stability curve as per WHO or manufacturer requirements(6). VVMs were first introduced on OPV vials supplied to UNICEF and WHO in 1996.
The information delivered by a VVM is simple. If the inner square is of lighter colour than the outer reference ring, the vaccine can be used. If the inner square is of the same colour or darker than outer ring, the vaccine should not be used (Fig. 1). The VVMs can be seen as a catalyst for much needed changes in strategies of vaccine distribution via the cold chain. It will definitely reduce distribution costs and increase flexibility in handling of vaccines in the field, thus helping to make operations more effective.
Operationalisation of Cold Chain
The cold chain is not simply a series of warehouses and refrigerated containers, isothermic boxes and portable ice-boxes, it also involves intermediate phases in which transporters, programme administrators, warehouse workers and vaccinators have a part to play. The cold chain involves two complementary aspects: (i) the set chain represented by the refrigerator, (ii) the mobile chain represented by isothermic boxes and iceboxes, and (iii) the third and an important aspect is the personnel in charge of cold chain(5,7).
(a) Walk in cold rooms (WIC): They are located at regional levels and are meant to store vaccines up to 3 months and they serve 4-5 districts.
(b) Deep freezers and domestic/ice lined refrigerators (ILR) (300 L): They are supplied to all districts and WIC locations to store vaccines. They are used for long term storage of vaccine below ?20? C and also for making ice packs. OPV and measles vaccines can also be stored in deep freezers. The temperature monitoring is done twice a day with an alcohol thermometer for deep freezer and a dial thermometer in case of ILR.
(c) Small deep freezers and ILRs (140 L) are provided to PHCs and clinics.
(d) Cold boxes or isothermic boxes are well insulated, solid and hermetically (air tight) sealed boxes. They are supplied to all peripheral centers and used for trans-portation of large amount of vaccines and to carry them for several days. They are also useful during electricity failure. Cold accumulators are placed between the boxes of vaccine and the sides of a box. A paper or polythene is kept between vaccine and accumulators to prevent freezing of vaccines because of direct contact.
(e) Vaccine carrier or ice boxes are used to carry small quantity of vaccines for distribution or to carry the vaccines to the outreach place of immunization session. It is surrounded by cold accumulators from inside.
(f ) Day carriers are of the size of a small lunch box and are used to carry still smaller quantity of vaccines, but can be used only for a few hours. Here two fully frozen ice packs are used.
(g) Ice packs should be made of plain water. Never add salt to the water. Water should be filled in the icepack up to the level marked. Dry carbon dioxide can also be used instead of water.
(h) Distribution of vaccines: Only small quantity of vaccine is distributed to the periphery as break in cold chain is common at the periphery because of either lack of knowledge or electricity failure. Vacuum flask is never used for an outreach place.
Domestic refrigerator is used for short-term storage of vaccines. The usual temperature of an ordinary chamber of the refrigerator is between 4?C to 10?C and that of ice chamber or freezer compartment is between 0?C to ?4?C. Tips for proper functioning of the refrigerator are provided below(1,2,5,7).
Keep the refrigerator away from sunlight and at sufficient (10 cm) distance from the wall. Place cold accumulators in freezer compartment. They can be of use during electricity failure or in iceboxes. The shelves should be filled with water filled plastic bottles, which help to maintain temperature during electricity breakdown. These bottles are not to be used for drinking purpose. The temperature of the refrigerator is recorded at least twice a day. The length of electricity breakdown is to be noted and during this period vaccines are to be protected by putting them in isothermic boxes. When a layer of ice measuring 5 mm or more forms on the sides of the freezer compartment, the refrigerator should be defrosted. During defrosting, the vaccines should be temporarily placed in an isothermic box.
Tips for proper storage
No foods or drinks or other drugs are to be kept in a refrigerator. It is to be used exclusively for storage of vaccines. To maintain the temperature, it is opened only when necessary. No vaccine should be placed in the door panel or in baffle tray (baffle tray may contain water and this may freeze to ice). Direct contact of vaccine with ice is to be avoided. An empty space must be left between packages for free air circulation. Minimum stock of vaccine is to be kept, say for a month only. Avoid freezing of diluents as the vial may burst when frozen. Correct storage of vaccines in clinical set up is shown in Fig. 2.
Thermostability of Vaccines
Knowledge of a vaccine?s stability, especially the rate of decline in potency at a given temperature, can be helpful in determining the storage requirements(3). Adsorbed diphtheria and tetanus toxoids are the most stable of the vaccines commonly used in UIP and OPV is the most sensitive to heat. Opened vials which have not been fully utilized after reconstitution should be discarded within one hour, if no preservative is present (most live virus vaccine) or within 3 hours, or at the end of the session when the vaccines containing preservative are used(4). Reconstituting the vaccine with warm diluent may be harmful and vaccine loses its potency within hours.
Diphtheria and tetanus toxoids
Adsorbed diphtheria and tetanus toxoids in monovalent form or as components of combined vaccines are the most stable of the commonly used vaccines. They are stable at elevated temperatures, even for long periods of storage, but they may change their appearance and potency when frozen. The freezing of adsorbed vaccines (DPT, DT, TT, HB) constitutes an absolute contraindication for their use as they are associated with a reduced immune response or an increased incidence of local reaction. This is not because of the characteristics of the toxoids them-selves, but because of an aluminium-based adjuvant, which changes its structure. The freezing point for adsorbed DTP vaccine is between ?5?C and ?10?C. The freezing time depends upon the number of doses in the vial and the temperature. It takes about 110 to 130 minutes at ?10?C. When vaccine is frozen, the aluminium oxide loses its colloidal structure and is broken down into crystalline parts, which may cause aseptic abscesses at the injection site and make the vaccine ineffective. Frozen adsorbed vaccines contain granular or flaky particles when thawed. When shaken, they sediment within 30 minutes, leaving a deposit below a column of transparent fluid. This indicates that the vaccine has been frozen (shake test)(7,8). The thermostability is as shown in Table I. Several studies showed an insignificant decrease in potency when the vaccine was stored for 1.5 years at 18? C, for 6-12 months at 24? C and for 2-6 months at 37? C(9,10).
Thermostability of this vaccine is as described in the Table I. The effect of freezing the vaccine is same as with DPT. There are no data available for acellular pertussis vaccine and so the stability profile similar to that of other protein vaccines is to be expected i.e., relatively good thermostability, poor resis-tance to freezing and shelf life of 2-3 years at 2?C to 8?C(11).
Hepatitis B Vaccine
HB vaccine is a liquid suspension consisting of puri