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 January 16, 2007

VETERINARY SERVICES MEMORANDUM NO. 800.203 

Subject: General Licensing Considerations: Compatibility of Components 

To: Veterinary Biologics Licensees, Permittees, and Applicants 

 Directors, Center for Veterinary Biologics 

I. PURPOSE 

This memorandum provides guidance for evaluating the compatibility of components in 

combination biological products formed by assembling previously licensed products. 

II. CANCELLATION 

This memorandum cancels Veterinary Services Memorandum No. 800.203, dated 

May 28, 2002.

III. BACKGROUND 

The products covered by this memorandum are vaccines and similar prophylactic 

immunobiologicals, such as bacterins or toxoids, which contain antigens intended to 

actively stimulate an immune response in the recipient. Antigenic fractions with 

previously established efficacy in licensed products may be combined to form a new 

polyvalent product. It must be verified that the efficacy of each fraction has not been 

compromised in the new product when compared to a product with known efficacy. 

IV. DEFINITIONS 

 A. Component Interference - Component interference is an adverse alteration of the 

 expected immune response to one antigen by the presence of another antigen or 

 component in the same product. 

 B. Excessive Interference - Interference is excessive when there is reason to believe 

 the product’s efficacy against disease has been decreased by the alteration of the 

 immune response due to interference. 

 C. Fraction - A fraction of a prophylactic immunobiological product refers to an 

 antigen (organism) and the form in which it appears (e.g., modified live, 

United States 

Department of 

Agriculture

Animal and Plant 

Health Inspection 

Service 

Washington, DC 

20250 
Veterinary Services Memorandum No. 800.203 2

 inactivated, subunit, toxoid, vectored, etc.). 

D. Reference Product - A reference product is a product for which efficacy in the 

 target species has been directly demonstrated. 

 E. Test Product - A test product is a new polyvalent product formed by combining 

 fractions from previously licensed product(s) and/or adding new antigens to licensed 

 combinations. 

V. GUIDELINES 

Material submitted to support the efficacy of a test product must include information verifying 

the absence of excessive component interference. Support for the absence of excessive 

interference may be provided by one of the following. 

 A. Efficacy Study - A satisfactory efficacy study conducted with the test product may be 

 used to verify the absence of excessive interference on the efficacy of the vaccine 

 antigen that was challenged. Efficacy studies may be conducted when licensing any test 

 product and must be conducted for the following: 

1. Avian and Fish Products – Conduct an efficacy study for each fraction of new 

 polyvalent products intended for use in poultry or fish. 

2. Mammalian Products – Conduct an efficacy study for each fraction of new polyvalent 

 products intended for use in mammals if the new product differs significantly from 

 the licensed products in composition, production methods, or recommended 

 vaccination regimen. 

B. Existing Information - Submit convincing objective data documenting the absence of 

 excessive interference. Such information may include previous studies or documented 

 experience with the fractions comprising the test product. 

C. Potency Test - Validated potency tests which accurately reflect a fraction’s efficacy may 

 be sufficient support for the absence of excessive interference. Acceptable potency tests 

 have been limited to the in vivo tests found in the Standard Requirements for Leptospira 

 species, Clostridium species, and the equine viral encephalitides (Code of Federal 

 Regulations, Title 9, Part 113). 

D. Comparative Serology - In comparative serology studies, serum titer is used as an 

 indicator of the overall immune response. If the humoral responses to vaccination with 

 the test and reference products are equivalent, then the critical assumption is made that 

 cell-mediated processes and their relative contributions to the overall immune response 

 are unlikely to be materially affected as well. A change in that balance may be signaled 

 by an antibody response that is either increased or reduced. To support the lack of 

 excessive interference by comparative serology, conduct a study in the target species 

 comparing the serological responses between a group vaccinated with the test product 
Veterinary Services Memorandum No. 800.203 3

 and a group vaccinated with a reference product. In many instances, the serologic 

 response of interest will be a serum titer, and the group geometric mean serum titers 

 (GMT) would be compared. The absence of excessive interference would then be 

 supported if the GMTs were equivalent. For the typical comparative serology studies 

 described in this section, lack of excessive interference would be supported by the 

 noninferiority of the test product GMT relative to the reference product GMT, and full 

 equivalence need not be demonstrated. If, however, the GMT of the new product is 

 dramatically higher than the reference product GMT, further study of the potential impact 

 on the overall protective immune response may be warranted. 

 1. Methods 

 a. Equivalence - Two values are equivalent if they differ by less than an amount 

 which is considered meaningful in a clinical or practical sense. The range within 

 which two values are considered equivalent is termed the equivalence margin. 

 b. Formulation - Formulate the test and reference products from the same bulk lot of 

 each antigen common to both products. The potency of each common antigen 

 should be the same in both products. 

 c. Serum titer - Use a validated assay that measures an antibody response that has 

 been shown to be related to efficacy. 

 2. Design - Design the study for statistical analysis and inference by accepted 

 equivalence methodology. 

 a. Route of administration - Equivalence should be evaluated for each route of 

 administration separately. 

 b. Serum collection time -Collect serum near the time of peak response to the 

 reference product. If serum is collected on more than one occasion, adjust for 

 multiplicity by estimating simultaneous confidence intervals for all occasions. 

 Important differences at any time point may be an indication of excessive 

 interference. If there is reason to believe the product’s antibody response profiles 

 over time are different, design the study so that the profiles may be directly 

 evaluated. 

 c. Extraneous variance - The objective of a serological interference study is to draw 

 conclusions about the immunogenicity of the test product based on the average 

 serological response of vaccinated animals. Such studies may benefit from 

 reducing the impact of assay variance on interval estimates. Studies intended to 

 do so should be designed to include multiple titrations on each serum specimen. 

3. Criteria 

 a. Noninferiority - The serological noninferiority of the test product must be 

 demonstrated. Serological noninferiority means that the expected GMT of the 

 group vaccinated with the test product is not likely to be lower than the expected 

 GMT of the group vaccinated with the reference product by more than the 

 equivalence margin. 

 b. Lower margin - Protocols proposing serological equivalence studies must 
Veterinary Services Memorandum No. 800.203 4

 explicitly state the noninferiority margin. For studies of component 

 interference, use a 63% margin, which aims to show that the test product GMT is 

 at least 63% of the reference product GMT, unless another criterion is justified. A 

 63% titer ratio corresponds to a difference of about two thirds of a twofold 

 dilution in a serial dilution assay. (The 63% margin does not necessarily apply to 

 applications other than component interference studies.) 

 c. Confidence - For studies of component interference, use a 0.05 level of 

 significance and a confidence interval based on one minus twice the significance 

 level, i.e. a 90% confidence interval. For noninferiority, compare only the lower 

 end of the confidence interval to the lower end of the equivalence margin. For full 

 equivalence, no part of the confidence interval should lie outside the equivalence 

 margin. 

 d. Lack of seroconversion - If more than a few animals in a group do not 

 seroconvert, it is not appropriate to estimate the group GMT. In that case, the 

 presence of many nonresponders indicates the need to evaluate whether or not 

 serology is suitable to evaluate component interference. If a serological 

 comparison is considered valid because the distribution of serum titers is typical 

 of the product, use statistical methods that are appropriate for the nature and 

 distribution of the data. (For example, a mixture model or a nested model which 

 simultaneously evaluates the fraction of animals that seroconvert and the GMT of 

 those animals that have seroconverted.) 

 4. Field Studies - The variability of the humoral response in some cases may require 

 that a serological equivalence study include more subjects than available for an 

 experimental study. Serum derived from appropriately designed field studies may be 

 used to study serological equivalence. For example, subjects in a field safety trial may 

 be randomized to new and existing products. While such subjects would not 

 necessarily be seronegative, they would emulate the target population as well as the 

 rest of the field safety study sample. 

/s/ Andrea M. Morgan (for) 

John R. Clifford 

Deputy Administrator 

Veterinary Services