## Calculating annual vaccine needs from previous consumption

For this calculation, we use an equation based on the stock of vaccine at the beginning and end of a particular period, the vaccines received during that period, and the vaccines lost, destroyed or thrown away during that period. The equation is given below:

- Annual vaccine needs (in doses) = (i+r) – (f+l)

Remember the ‘brackets first’ rule; calculate the sums inside the brackets *first*, before you do anything else.

The equation includes the following numbers of doses:

- initial vaccine stock at the beginning of the period (i)
- vaccines received during the period (r)
- stock remaining at the end of the period (f)
- lost, destroyed or expired doses (l).

Use the above equation to calculate the annual vaccine needs (in doses) for pentavalent vaccine in a particular coverage area*, *in which the initial stock at the beginning of the year was 250 doses, and the quantity received during the year was 1,250 doses. At the end of the year there were 500 doses remaining in stock. The quantity lost during the year was 125 doses.

The answer is calculated as follows:

Annual vaccine needs | = (i+r) – (f+1) |

= (250 + 1,250) – (500 + 125) doses | |

= 1,500 – 625 doses | |

= 750 doses |

In this example, the estimated annual pentavalent vaccine need based on previous consumption is 750 doses.

In the example given earlier, the estimated annual pentavalent vaccine need, based on the *size of the target population*, was 718 doses. However, the estimate based on the *previous year’s consumption *(see above) was 750 doses. If both these imaginary examples had come from the same health facility, what explanations could you suggest for the difference between the two estimates?

Clearly the health facility staff used *more* vaccine doses (750) in the previous year than was predicted from the size of the target population (718). There are several possible reasons for this, including:

- There were more surviving infants in the coverage area than the 200 estimated from calculating 4% of the total population size of 5,000.
- The health facility staff achieved a higher immunization coverage rate in the previous year than the 80% target they were asked to deliver.
- They wasted more doses of vaccine than expected.