5.2.5 Vaccine wastage rates and wastage factors
Using calculations like the one in Table 5.3 enables you to determine the number of vaccine doses you need. However, some vaccine doses may be wasted during the year for various reasons (Box 5.1 on the next page). The wastage rate is the percentage of vaccine doses that are wasted. The general guideline on the amount of vaccine wastage that is considered acceptable for different types of vaccines is also shown in Box 5.1.
Box 5.1 Vaccine wastage
Some reasons for vaccine wastage:
- Some unused doses may have to be thrown away, e.g. because they have passed their expiry date or lost their labels.
- Some doses may be spoilt for one reason or another (e.g. vaccines damaged by storage at the wrong temperature).
- Some vials or ampoules may be broken during transport and handling.
Acceptable vaccine wastage rates:
- For liquid vaccines supplied in single or two-dose vials (e.g. pentavalent vaccine and PCV10), a wastage rate of 5% is acceptable.
- For OPV, a wastage rate of 10% is considered acceptable.
- For liquid vaccines supplied in multi-dose vials of 10 or more doses, a wastage rate of 15% is acceptable.
- For reconstituted vaccines, wastage rates of 50% for BCG and 25% for measles vaccine are considered acceptable.
Calculating the wastage factor
The wastage factor is the factor (number) that you multiply your estimated vaccine needs by, in order to allow for some doses being wasted. We use the letters ‘wf’ to represent the wastage factor in the following equation:
- wastage factor (wf) = 100 ÷ (100 minus the % wastage rate)
- where the wastage rate is the number of doses wasted, expressed as a percentage.
Note the ‘brackets first’ rule. In equations that include brackets around some of the numbers, you always calculate the answer to whatever is inside the brackets first, before you do anything else.
If the wastage rate is 30%, what is the wastage factor?
Wastage factor = 100 ÷ (100 – 30) = 100 ÷ 70 = 1.43 Therefore, if 30% of the doses are wasted, the wastage factor will be 1.43.
Now you have learned how to calculate the basic values you need to estimate your vaccine needs for the target population. These values are the target population (pt), the number of doses in the schedule (dn), the immunization coverage target (ic) and the wastage factor (wf).
5.2.4 Immunization coverage targets