Dilution and Dosing

Part of Vaccines

Preparing vaccines at correct concentrations to maximize immune response while minimizing adverse effects.

Why This Matters

Too little vaccine and the immune system is not adequately stimulated — the person may not develop protective immunity. Too much vaccine and adverse reactions become more severe, potentially causing harm. Between these extremes lies the therapeutic window: the dose range that reliably stimulates protection without unacceptable risk.

Determining and reliably reproducing the correct dose is one of the most practically important steps in vaccine preparation. Without precise analytical equipment, achieving exact dosing is difficult — but achieving reasonable dosing with careful technique and simple tools is possible. Historical vaccine programs operated with far cruder measurement than modern ones and still achieved effective immunization.

Understanding the principles of dilution allows practitioners to scale concentrations, prepare working stocks from concentrated preparations, and estimate doses when definitive assays are unavailable.

Units of Measurement in Vaccines

Vaccine doses are expressed in various units depending on what is being measured:

Colony-forming units (CFU) or plaque-forming units (PFU): Living organisms quantified by their ability to form colonies (bacteria) or plaques (viruses) in culture. These units directly reflect the number of viable organisms.

Optical density (OD) or turbidity: For bacterial vaccines, the cloudiness of a suspension correlates with organism concentration. Comparing to a known standard (McFarland standards — tubes of barium sulfate suspension with defined turbidities) allows approximate CFU estimation without culture.

Mass: Some inactivated vaccines are dosed by mass of antigen (micrograms). This requires accurate weighing equipment.

Empirical units: Historical vaccines often used empirical units — doses defined by their biological effects rather than physical measurements. A “skin test dose” or “protective dose” was calibrated against a biological endpoint.

In a rebuilding context, turbidity standards and empirical biological calibration are most accessible.

Making McFarland Turbidity Standards

McFarland standards allow approximate bacterial concentration estimation by visual turbidity comparison.

Standard preparation: McFarland 0.5 (approximately 1.5 × 10⁸ CFU/mL for E. coli):

1. Prepare 1% barium chloride solution (barium chloride in water).
2. Prepare 1% sulfuric acid solution.
3. Mix 0.5 mL barium chloride solution with 99.5 mL sulfuric acid solution.
4. The resulting fine precipitate suspension has a defined turbidity equivalent to approximately 1.5 × 10⁸ bacteria/mL.
5. Seal in a glass tube with wax. Store in the dark at room temperature. Valid for 6 months.

Using the standard: Prepare bacterial suspension in saline. Compare turbidity visually by holding tubes side-by-side against a white background with black lines behind them. Adjust suspension by adding more bacteria (more turbid desired) or more saline (less turbid desired) until it matches the standard.

Different McFarland standards (1, 2, 3, etc.) are prepared by adjusting the barium chloride/sulfuric acid ratio and represent higher concentrations.

Serial Dilution Technique

Serial dilution is the fundamental technique for reducing concentrations by known factors.

1:10 serial dilution:

1. Label 8 tubes with dilutions: 10⁻¹ through 10⁻⁸.
2. Place 9 mL sterile saline or broth in each tube.
3. Add 1 mL of original stock to tube 1; mix thoroughly by rolling between palms or using a sterile rod. This is a 1:10 dilution.
4. Transfer 1 mL from tube 1 to tube 2; mix. This is 1:100 (10⁻²).
5. Continue transferring 1 mL to each successive tube.
6. Result: each tube is 10× more dilute than the previous.

Key rules:

• Change tools between tubes to avoid carryover errors
• Mix each tube completely before transferring
• Work quickly to minimize evaporation effects
• Label before beginning, not during

Calculating working dilutions: If a vaccine stock has an estimated concentration of 10⁹ CFU/mL and the target dose is 10⁶ CFU per 0.5 mL injection, you need:

• Target concentration in injection: 10⁶ ÷ 0.5 mL = 2 × 10⁶ CFU/mL
• Required dilution: 10⁹ ÷ 2 × 10⁶ = 500×, or roughly 1:500
• Prepare 1:500 dilution: e.g., 1 part stock + 499 parts saline

Dose Determination Without Assays

Without laboratory assays, dose determination relies on:

Historical reference: Known effective doses from historical vaccine programs provide starting points. These are generally expressed in ways that could be reproduced without laboratory equipment — skin test doses, volumes at defined turbidity, lesion size criteria.

Biological titration: Inoculate groups of animals with different concentrations of vaccine. The lowest concentration that produces measurable immunity (assessed by subsequent challenge or seroconversion) defines the minimum effective dose. The concentration that produces unacceptable adverse events defines the maximum tolerable dose. The working dose falls between these.

This requires animals and time but can be accomplished without laboratory equipment.

Dose-response observation: In early human use of a new vaccine, start low (1/10 of estimated effective dose) and increase in subsequent subjects while monitoring for response and adverse effects. This graduated approach, while ethically complex, was used by Jenner and Pasteur.

Reconstitution of Lyophilized Vaccines

Freeze-dried (lyophilized) vaccines are supplied as dry powder that must be mixed with liquid before use. Reconstitution errors are a major source of dosing mistakes.

Principles:

1. Use only the specific diluent supplied with the vaccine, or sterile water if no diluent provided. Never use saline for vaccines that specify water, and vice versa — ionic content affects stability.
2. Add diluent slowly to powder, not powder to diluent.
3. Swirl gently; do not shake vigorously (can denature proteins).
4. Check for complete dissolution — no visible particles (except known adjuvant turbidity).
5. Use within the time specified. Most reconstituted vaccines expire in 4-6 hours.

Volume accuracy: Reconstitution volume determines final concentration. If the vial specifies 0.5 mL diluent for 10 doses, adding 1 mL gives half the intended concentration per dose — inadequate protection. Use accurate volumetric measures (calibrated syringes or pipettes).

Multi-Dose Vials

A single vial intended for multiple doses requires careful management:

Dose extraction:

• Use a new sterile needle and syringe for each dose
• Never re-enter a vial with a needle that has touched a patient
• Keep the vial stored under appropriate conditions between doses (usually cold)

Discarding:

• Vials without preservative: discard any remaining dose after the session (typically 6 hours or end of day)
• Vials with preservative (phenol, thimerosal): may be used for up to 28 days if kept cold and not visibly contaminated
• Any vial with changed appearance (turbidity, color, particles): discard

Recording: Note the opening date and time on every vial. Track doses extracted to identify when vials should be discarded.

Volume and Route Considerations

The route of administration affects the appropriate volume:

Route	Typical Volume	Site
Intradermal (ID)	0.05-0.1 mL	Forearm skin
Subcutaneous (SC)	0.5 mL	Upper arm fat
Intramuscular (IM)	0.5-1.0 mL	Deltoid muscle
Oral	0.5-1.5 mL	Swallowed
Scarification	Minimal (applied to skin abrasion)	Upper arm

Injecting more than the intended volume by the wrong route can cause local tissue reactions. For intramuscular injection, injection too deep (hitting bone) or too shallow (into fat) changes pharmacokinetics.

Always confirm the route before preparing dose volume. A dose prepared for intramuscular delivery that is given intradermally will cause a localized inflammatory reaction and may be partially wasted.

Practical Dosing in a Rebuilding Context

With crude measurement tools, aim for approximate correctness:

• Prepare graduated measurement vessels from known container volumes
• Use consistent collection techniques (same syringe, same technique each time)
• Err on the side of slightly higher doses for inactivated vaccines (lower risk of adverse effects from excess dose)
• Be more cautious with live vaccines (dose excess can cause more disease-like reactions)
• Document every dose preparation method so it can be reproduced consistently