Title: Vaccine supply and stock management
Course: Level 3 - Immunization > Section: Cold chain, vaccine supply and stock management

Estimating Vaccine Needs

Vaccine management involves estimating the number of vaccine doses, diluents and injection equipment (e.g. syringes, needles) needed for a particular population over a stated supply period to ensure for an efficient and effective immunisation session you need to have an adequate supply of vaccines of acceptable quality.

Efficient management of the immunisation program is determined by calculating vaccine needs accurately. If the vaccine needs for your Health Post are not estimated correctly, and the wrong amounts are supplied by the Health Centre, this may result in a shortage of vaccines or excess stock.

There are three methods used to calculate vaccine needed for your health post. These are:

- The size of the target population for immunisation.
- Previous vaccine consumption data.
- The size of the scheduled immunisation sessions.

Calculating vaccine needs base on target population

The target population is the number of people who are eligible for vaccination with a particular vaccine. We use the letters "pt" to represent the target population in calculations.

- For BCG vaccine, the target population is all live births (i.e. complete expulsion from the mother, regardless of the duration of pregnancy, showing any evidence of life).

In Ethiopia, it is estimated that all live births and surviving infants account for 4% of total population, while 23% of the population is a figure for women of child bearing age.

Immunisation coverage rate is the percentage of the eligible population that has been agreed as your objective for immunisation with each of the EPI vaccines this year.

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 shown below.

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.

The wastage factor is the factor (number) that you multiply your estimated vaccine needs by, 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.

Therefore, vaccine needed for your immunisation program based on the size of target population is calculated by using the equation:

Annual vaccine needs = pt x dn x ic x wf

The equation uses the following values:

- Target population (pt).
- Number of doses in the schedule (dn).
- Target immunization coverage (ic) expressed as a decimal number (not a percentage).
- Wastage factor (wf).

Calculating vaccine needs based on previous consumption:

Even though some adjustments are acceptable because of possible increase in population size since the last year, you can calculate vaccine need based on the previous year use using the equation:

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

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).

Calculating vaccine needs to be based on the size of immunisation sessions:

If you can not determine the rates of vaccine wastage or vaccine stock management is not important, you can use immunisation sessions to calculate your vaccine need. The equation is:

Annual vaccine needs = posts x weeks x sessions x vials x doses

Where,

- Posts = number of immunisation sites.
- Weeks = number of weeks the service is delivered during the year.
- Sessions = number of immunisation sessions per week.
- Vials = number of vials opened per immunisation session
- Doses = number of doses per vial.

Ordering vaccines:

Whenever you order a vaccine, you should take into account the following considerations

- Avoid stock shortages, especially when mass immunisation campaigns are planned.
- Avoid stock excesses, by not ordering excess stock, or exceeding the recommended storage periods.
- Avoid situations where vaccines expire during their storage period.
- Ensure that the other necessary stocks (e.g. diluents, syringes, needles; wick and paraffin or kerosene for refrigerators, etc.) are ordered at the same time as the vaccines.
- Organize stock using the principle of "bundling" for all supplies required delivering an immunisation session.
- Ensure that there is adequate cold chain storage facilities (both in capacity and temperature.

Calculating quantities of vaccine for a particular supply period:

The vaccine needs for a specific storage or supply period (in this case, 12 months or 48 working weeks) can be calculated using the following equation:

q period = (q year ÷ 12) x p supply when the supply period is given in months, or

q period = (q year ÷ 48) x p supply when the supply period is given in working weeks.

where:

- q =vaccine needs for the period
- q =annual vaccine needs
- p supply = supply period (in months or weeks).

Calculating the minimum stock level:

The Minimum stock level (also known as the re-order level) implies the least amount that you should have in stock or the level which, when reached, initiates a re-order; usually expressed as the number of weeks/months of supply. It is the amount of stock you will use in the time between placing and receiving an order plus the buffer stock. The minimum stock level is the level below which stocks should never drop without having placed an order.Usually, the minimum stock is taken as 25% of the total vaccine needs for the supply period.

The minimum stock level can be calculated using the following equation:

smini = qperiodx 25% (or 0.25)

Where, smini is the minimum stock level and qperiod is the vaccine needs for the period.

Calculating the maximum stock level:

The Maximum stock level implies the largest amount of stock that you should have, usually expressed as the number of weeks/months of supply. It is the minimum stock plus the amount of stock used between orders. The maximum stock level is set to guard against excess stock which results in losing vaccines to expiration before use.The maximum stock level is the maximum number of vaccine doses that should be present in the refrigerator immediately after a new supply has been collected from the Health Centre.

The maximum stock level can be calculated using the following equation:

smaxi = qperiod + smini

Where, smaxi is the maximum stock level, qperiod is the vaccine needs for the period, and smini is the minimum stock level.

Calculating the critical stock level:

The critical stock level (or "time to order" level) is the number of vaccine doses in stock at the time when it is necessary to place a new order. The delivery time is the time interval between the day the vaccines are ordered and the day that you collect them from the Health Centre.

The critical stock level can be calculated using the following equation:

scritical = qdelivery + smini

- scritical is the critical stock level.
- qdelivery is the number of doses needed during the delivery time, and
- smini is the minimum stock level.

The number of doses required during the delivery time (qdelivery) can be calculated using the following equation:

qdelivery = (qperiodxtdelivery) ÷ n

- tdelivery is the number of days between placing the order and collecting new vaccines.
- n is the number of days in the supply period (i.e. the period that the health facility maintains its vaccine stocks at, or above, the minimum level).

Finally, some vaccines are very sensitive to strong light and their exposure to ultraviolet light causes loss of potency. Consequently, they must always be protected from sunlight or fluorescent (neon) light. BCG and Measles vaccines are equally sensitive to light (as well as to heat). Normally, these vaccines are supplied in vials made from dark brown glass, which gives them some protection against light damage, but care must still be taken to keep them covered and protected from strong light at all times.