Immunization Module: Vaccine Supply and Stock Management

Study Session 5  Vaccine Supply and Stock Management

Introduction

This study session deals with how to determine vaccine needs and how to keep your stock of vaccines in a reliable way, so that your immunization programme will not be interrupted as a result of shortage of supply. We also explain how to avoid wastage of vaccines by reducing the risk that the stock will pass its expiry date. The study session involves a lot of calculations using basic mathematics (addition, subtraction, multiplication and division) and using letters to represent values that you will need to measure or estimate. You are advised to apply your own data after each of the examples given here, to help you understand how the calculations apply in your setting.

Learning Outcomes for Study Session 5

When you have studied this session, you should be able to:

• 5.1  Define and use correctly all of the key words printed in bold. (SAQs 5.1, 5.3 and 5.4)
• 5.2  Calculate the size of the target population in your community and forecast their vaccine needs, using three different methods. (SAQs 5.1 and 5.2)
• 5.3  Calculate vaccine wastage rates and wastage factors and apply them to orders of vaccine stocks to minimise wastage. (SAQs 5.1 and 5.3)
• 5.4  Calculate the minimum and maximum quantities of vaccines required for a stated supply period and manage the critical stock level effectively. (SAQ 5.4)

5.1  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. In order to run an efficient and effective immunization session you need to have an adequate supply of vaccines of acceptable quality. This is essentially dependent on reliable planning and monitoring. You might have heard from mothers in the community that the health facility ran out of vaccines and their child could not be immunized. On the other hand, health facilities may have an excess stock of vaccine that has passed its expiry date and has to be thrown away. You should try to ensure that these situations do not arise in your Health Post (Figure 5.1).

Figure 5.1  Fura kebele Health Post, in the Southern Nations, Nationalities and Peoples Region (SNNPR) of Ethiopia, has been commended for meeting high performance targets. (Photo: Janet Haresnape)

Before we move on, remind yourself of the meaning of some key terms from previous study sessions.

Determining your vaccine needs accurately is important because it allows you to manage your immunization programme efficiently. Good vaccine management:

• enables you to use vaccines efficiently, and reduce wastage
• avoids shortages or excess accumulation of vaccines.

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 shortage of vaccines or excess stock.

We now consider how you can determine the vaccine needs for your Health Post. There are three methods based on:

• size of the target population for immunization
• previous vaccine consumption data
• size of the scheduled immunization sessions.

You will learn about each of these methods in the sections that follow. But you should note that vaccine requirements are likely to vary a lot from one kebele to another; the stocks required for your immunization programme may be very different from the examples used in this study session.

5.2  Estimating vaccine needs based on the size of the target population

5.2.1  What is the 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.

• [You learnt about tetanus toxoid (TT) vaccine and all the other the vaccines available in the EPI in Ethiopia in Study Sessions 2 and 3]
• For BCG vaccine, the target population is all live births (i.e. complete expulsion from the mother, regardless of duration of pregnancy, showing any evidence of life).
• For all other vaccines in the Expanded Programme on Immunization (EPI) in Ethiopia, the target population is all surviving infants (i.e. survive to their first birthday).

To calculate vaccine needs based on the target population, you need to know the size of the target population, the number of doses required according to the EPI schedule, and the percentage immunization coverage rate you have been given as the target in your annual activity plan (Figure 5.2).

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Figure 5.2  Annual activity plan performance for immunizations in Fura kebele with a total population of 6,348 people in 2003 E.C. (Ethiopian calendar, or 2010 in the European calendar). (Photo: Janet Haresnape)

In Figure 5.2, the column headed ‘eligible’ gives the number in the target population for each vaccine. You can see that the health workers in this Health Post planned to immunize 100% of the eligible children and 100% of the pregnant women (TT+2 PW, pregnant women receiving more than 2 doses of TT vaccine). Columns headed P refer to ‘planned’ numbers for immunization, and A are the ‘actual’ numbers immunized — in the first quarter of the year, they exceeded their targets for all categories except TT+2 NPW (non-pregnant women of childbearing age receiving more than 2 doses of TT).

Next, we will show you how to use data from an area similar to that of your kebele to calculate the target population for vaccination, so you can construct an annual activity plan for immunization like the one shown in Figure 5.2.

5.2.2  Calculating numbers in the target population

Table 5.1 (on the next page) shows an estimate of the target population for immunization in a kebele with a total population of 5,000. A figure of 4% of the Ethiopian population is usually taken to estimate the coverage required for all live births and surviving infants. You can assume that (on average) about 23% of the Ethiopian population will be women of childbearing age, of whom about 4% will be pregnant in a typical year.

You estimate the target population by multiplying the total population by the percentage in a particular category; for example in Table 5.1:

Expressing a percentage as a decimal number is easy, e.g. 90% is the same as 0.9; 23% is the same as 0.23, and 4% is the same as 0.04.

• The estimated number of women eligible for TT vaccination (Figure 5.3) is 5,000 x 23% (or 0.23) = 1,150
• The estimated number of children eligible for all other EPI vaccines is 5,000 x 4% (or 0.04) = 200, if you take the simple average of 4%. If you use the Ministry of Health indicators of live births (3.6%) and surviving infants (3.4%), the numbers will be 170 and 180 respectively.

Figure 5.3  About 23% of the Ethiopian population are women of childbearing age (pregnant and non-pregnant combined). (Photo: Ali Wyllie)

5.2.3  Calculating required doses based on the immunization schedule

In addition to calculating the number of people in the target population, you also need to know the number of doses required in the EPI schedule in order to immunize everyone (total coverage). You learned about the EPI schedule in Study Sessions 2 and 3, but we repeat the key points in Table 5.2. We use the letters ‘dn’ to represent the number of doses in calculations.

5.2.4  Immunization coverage targets

You now know how to calculate the annual vaccine needs for immunization of everyone in the target population. The EPI policy is to immunize 100% of the target population, but you may not be able to reach everyone in the target population in one year (for example, if your kebele includes very remote areas). Your annual action plan will include a percentage target from the woreda health office for the immunization coverage rate — the percentage of the eligible population that has been agreed as your objective for immunization with each of the EPI vaccines this year. The performance of your Health Post will be measured against this target.

If the annual target is less than 100% for a particular vaccine, you can estimate the number of vaccine doses you will actually need as follows. Start with the number of doses required to immunize everyone in the target population (i.e. total coverage), and multiply that number by the agreed (lower) percentage target for immunization coverage in your setting. We use the letters ‘ic’ to represent the percentage target for immunization coverage in calculations. Now look at the example in Table 5.3. (Your targets will be different.)

Note that you can use the same method of calculation to estimate the number of ampoules of specific diluents required to reconstitute the freeze-dried vaccines (BCG and measles), and your requirement for injection equipment.

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.

5.2.6  Calculating annual vaccine needs from the size of the target population

To calculate the annual vaccine needs for your immunization programme from the size of the target population, you need to multiply together the four values explained in Sections 5.2.1 to 5.2.4 of this study session. The calculation can be summarised using the equation shown below:

• 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).

Now we turn to the second method of calculating vaccine needs.

5.3  Estimating vaccine needs on the basis of previous consumption

This method is based on the consumption of vaccines during the previous reporting period (usually the previous year). Some adjustment may be necessary if you believe that there has been any increase in the population size since the previous vaccine consumption was recorded. This method is useful for a Health Post where the stock management is good, but there is insufficient information on immunization objectives and targets for the next action plan. It is also useful when placing short-term orders.

In a kebele where stock management is efficient, there is likely to be good information on previous vaccine consumption. For example, you can probably see from the red bars in Figure 5.4 that 97 babies were immunized at birth with BCG in this kebele in 2001 (Ethiopian calendar). So this number could be used to predict the BCG vaccine requirement in the following year.

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Figure 5.4  EPI coverage for infants aged under one year during a five year period at Fura kebele, SNNPR, Ethiopia; years are in the Ethiopian calendar (E.C.). (Photo: Janet Haresnape)

5.3.1  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).

5.4  Estimating vaccine needs based on the size of immunization sessions

Now we show you how to calculate vaccine needs based on the size of immunization sessions. This method is appropriate if you cannot determine the rates of vaccine wastage, or vaccine stock management is not good. The equation you should use to estimate the annual vaccine needs by this method is given below:

Annual vaccine needs = posts x weeks x sessions x vials x doses, where:

• posts = number of immunization sites
• weeks = number of weeks the service is delivered during the year
• sessions = number of immunization sessions per week
• vials = number of vials opened per immunization session
• doses = number of doses per vial.

Table 5.4 shows how to calculate vaccine needs based on the size of immunization sessions in an imaginary example, using a vaccine supplied in multi-dose vials containing 10 doses per vial.

So the vaccine needs for this particular multi-dose vaccine, based on the size and number of vaccination sessions = 960 doses per year.

5.5  Ordering vaccines

In the previous section you learnt how to estimate vaccine needs. Vaccines are very expensive, so it is important to ensure that the ordered vaccines will be used and not spoiled. The World Health Organization (WHO) recommends that every order for vaccines should take into account the considerations in Box 5.2. There are some new terms in this box which are explained below it.

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Figure 5.5  This stock record should be kept up to date at all times to ensure that stock shortages and stock excesses are avoided.

Bundling refers to the practise of organising related stock all together in ‘bundles’ consisting of the correct numbers of all the items you need during an immunization session:

• good quality vaccines and diluents
• auto-disable (AD) syringes (or single-use disposable syringes and needles, if these are still used)
• safety boxes, etc.

Auto-disable (AD) syringes were mentioned in Study Session 4, and are specially modified disposable syringes with a fixed needle; an AD syringe is automatically disabled after it has been used once, because the plunger cannot be pulled back a second time. The WHO recommends that immunization programmes use AD syringes for all vaccinations, to prevent re-use of contaminated injection equipment.

The cold chain is fully described in Study Session 6, and refers to the network of refrigerators, cold stores, freezers, cold boxes and vaccine carriers, and their efficient organisation and maintenance, so that vaccines are kept at the right temperature at all times. This ensures that vaccines keep their potency during their transportation and storage at all stages, from leaving the factory to the point of administration to the target population.

Storage of vaccines is a major challenge. Vaccines are easily damaged during storage if the cold chain is not maintained. Therefore it is important to divide the vaccine supply into manageable periods for the purpose of ordering stocks. A Health Post will usually have a shorter supply period (four weeks) than a health centre. The next section explains how to calculate the quantities of vaccine needed for a specific supply period.

5.5.1  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_period = vaccine needs for the period
• q_year = annual vaccine needs
• p_supply = supply period (in months or weeks).

You can see an example of vaccine needs calculated in weeks in the following question.

5.5.2  Calculating the minimum stock level

The time between the date of ordering vaccines and the date when you collect them may not be as soon as you would wish it to be. There may be an increase in vaccine demand that you want to respond to quickly, but you may find there are unexpected delays in restocking. Therefore, you should always aim to keep a minimum amount of vaccine in stock. The minimum stock level is the minimum number of vaccine doses that should be in the refrigerator when the next supply of vaccines is collected. 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:

• s_mini = q_period x 25% (or 0.25)

where s_mini is the minimum stock level and q_period is the vaccine needs for the period.

5.5.3  Calculating the maximum stock level

You have learned that vaccines are easily damaged and that they are very expensive. It is not good practice to keep an unnecessary amount of vaccine in stock. 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:

• s_maxi = q_period + s_mini

where s_maxi is the maximum stock level, q_period is the vaccine needs for the period, and s_mini is the minimum stock level.

If your supplies of vaccine exceed your maximum stock level, it may be wise to consider returning some vials to the higher level.

5.5.4 Calculating the critical stock level (or ‘time to order’)

It is important to be aware of your vaccine stock, and to place your next order at the right time. The critical stock level (or ‘time to order’ level) is the number of vaccine doses in stock at the time when it is absolutely necessary to place a new order. ‘Time to order’ calculations take into account the level of vaccine consumption while waiting for the new supply. This precaution is necessary to prevent the vaccine stock from dropping below the minimum stock level before the new order can be collected. 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:

• s_critical = q_delivery + s_mini , where:

• s_critical is the critical stock level
• q_delivery is the number of doses needed during the delivery time, and
• s_mini is the minimum stock level.

• q_delivery = (q_period x t_delivery) ÷ n , where:

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

• t_delivery 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).

Now attempt Activity 5.1 to calculate the critical stock level for an imaginary Health Post. You will need paper and a pen or pencil to help you make the calculations.

In the next study session you will learn about keeping vaccines and diluents safe and effective by maintaining the cold chain. This is vitally important in reducing wastage, as well as maintaining the potency of the vaccines.

Summary of Study Session 5

In Study Session 5, you have learned that:

1. Vaccine management is important because vaccines are easily damaged and they are very expensive.
2. Accurate estimation of all stock requirements avoids shortages and prevents wastage from excessive orders; good vaccine stock management is essential to the smooth running of immunization sessions.
3. There are three methods of estimating annual vaccine needs, based on:
   • size of the target population
   • previous consumption levels
   • size of immunization sessions.
4. These methods can also be applied to estimates of other immunization supplies, such as diluents and injection equipment.
5. It is essential to keep the EPI Vaccine and Injection Materials Stock Record up to date; this will help you to avoid stock shortages and stock wastage.
6. It is good practise to organise stocks of vaccines, diluents, syringes, needles and safety boxes together in bundles.
7. It is important to ensure that vaccine stocks do not fall below the recommended minimum level, or rise above the recommended maximum level, and that you order new supplies when the stock falls to the critical level.

Self-Assessment Questions (SAQs) for Study Session 5