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EFFICACY STUDIES

(including Interference Studies)

Overview

Each antigenic fraction of a licensed/permitted product must be supported by an efficacy 

study acceptable to APHIS. If label claims for cross protection are desired, separate 

efficacy studies must be performed to support each claim. Likewise, separate efficacy 

studies must be performed to support multi-syndrome claims for diseases with more than 

one distinct disease syndrome. 

Once efficacy has been proven for an antigen in a given product formulation, that antigen 

often may be combined with other antigens in related products with reduced requirements 

for efficacy. If each antigen in a proposed new product previously has been proven 

efficacious individually (or in other combinations), it may be necessary only to 

demonstrate that the antigens do not excessively interfere with each other in the new 

combination. 

After licensure, efficacy-type studies may be performed to qualify reference serials for 

potency tests, or to confirm appropriate revaccination intervals. Although reference 

qualification studies may sometimes utilize slightly smaller treatment groups than pivotal 

efficacy studies, all other guidelines for review and interpretation of results apply. 

Flow of Information

1. Efficacy reports are routed directly to the reviewer upon receipt. 

2. Electronic data submitted with efficacy reports are posted by the program 

assistant to the electronic mail log. The existence of electronic data is noted in the 

mail log record by an associated Statistical Data File document record. 

3. Most efficacy studies warrant evaluation by the statistics unit. A preliminary 

review of the report should be done as soon as possible to confirm if statistical 

input is necessary. If statistical input is desired, preliminary information should 

be filled out in the efficacy licensing study summary (see chapters 4.4.2 Efficacy 

study licensing summary and 4.4.2.1 Efficacy study licensing template for 

additional information). Adding the preliminary information to the efficacy 

licensing template facilitates statistical review. The report may then be forwarded 

to Statistics. . Statistics will return the submission to the reviewer with written 

comments. 

4. When the reviewer has reviewed the submission and prepared a response, it is 

handled like all other correspondence (see Office Procedures chapter). 

5. The reviewer then completes the final portion of the Efficacy Study Licensing 

Summary to document the basis for the regulatory decision. 

6. If the study is accepted to support licensure, the reviewer evaluates the cosubmitted Draft Individual Summary (submitted as part of the single-tier 

initiative) for major deficiencies. 

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additional 0.5log10 overage has historically been added, so that in general, modified live 

products have been released at a level of 1.2 logs greater than the minimum immunizing 

dose observed in the efficacy study. For most live bacterial vaccines, the bacterial count 

required throughout dating has been expected to be twice that of the product used in the 

efficacy study. To explore the possibility that release titers may be lowered based on 

scientifically sound data, Draft Document 440 was posted for public comment. If a firm 

chooses to determine serial release (for products that are tested using an in vitro potency 

test) using the guidance in Draft Document 440, the following points should be 

considered: 

• Overage for assay variability should be based on data evaluated during assay 

validation. When serial release is based on a well validated assay, antigen overage 

to account for assay variability should not be as high, as for an assay in which a 

great deal of variability is observed. Therefore, historical overages for assay 

variability may be decreased based on the appropriate data.

• Codified antigen overages, (for example as described in 9 CFR 113.330, 113.331, 

113.332 etc) must still be met, regardless of guidance in Draft Document 440. 

• Final release specifications and throughout dating specifications are based on 

confirmation of dating study data. Initial specifications may be adjusted based on 

confirmation of dating study data. 

3. The efficacy study should be performed according to the study protocol. Firms are 

strongly encouraged to submit study protocols for review prior to initiating pivotal 

studies (see chapters on Protocols and Statistics). Ideally, the protocol also is 

appended to the final report for reference. 

4. The characteristics of animals used in the efficacy (and safety) study will be used 

to define permissible labeling statements. These include the minimum age at 

vaccination and any special recommendations (e.g., use in pregnant, lactating 

animals).

5. Each animal in the study should be uniquely identified. An exception is often 

made in large poultry studies or fish studies, where animals are identified by 

house/tank instead of individually. 

6. Animals enrolled in efficacy studies should be immunologically naïve for the 

antigen under study. The report should include pre-vaccination data that demonstrate 

the animal’s eligibility for inclusion in the study. For some ubiquitous antigens and 

depending on the sensitivity of the screening assay, it may be necessary to use 

animals with low pre-existing titers. In such cases, it may be prudent to block the 

animals by titer when allocating them to treatment groups.

7. The study should include the proper types of treatment groups. A separate 

treatment group should be included for each proposed route of administration. (Note: 

Needle-free administration is treated as a distinct route of administration 



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on the basis of serology compared to the generation before it. Over time, such practices 

can result in substantial drops in required efficacy. 

If serological equivalence can be demonstrated, no further data are usually required. If, 

however, serological equivalence cannot be demonstrated, then a host animal efficacy 

study is usually necessary to demonstrate that an antigen in a new combination remains 

adequately efficacious. If adding a new fraction to a previously licensed multivalent 

product, the firm should formulate the new fraction at the MID and the other fractions 

at/above release values to demonstrate lack of interference. If a firm is proposing initial 

licensure of a multivalent product, it may be acceptable to formulate the efficacy serial 

with all fractions at the MID, which would require only a single serial to be formulated as 

the efficacy serial, if there are no known issues regarding interference with the fractions 

involved. An exception to this would be a multivalent product that contains Newcastle 

Disease Virus and Avian Bronchitis Virus. Since there are known issues regarding 

interference with these viruses, the antigen being tested should be formulated at MID and 

other fractions at/above release values. 

If a firm proposes doing a serological interference study to demonstrate lack of 

interference on one fraction, and a vaccination/challenge study to demonstrate lack of 

interference on another fraction, it may be acceptable to use the same vaccine formulation 

and thus the same animals in both studies 

Serology is often acceptable to demonstrate lack of excessive interference when it would 

not be acceptable to demonstrate efficacy directly. The rationale for this is that with an 

interference study, we are attempting to gain confidence that the immune response 

elicited by the new product combination is qualitatively and quantitatively similar to 

another matched product for which efficacy has been demonstrated. We are seeking to 

demonstrate consistency of response, and serology is usually considered to be an 

acceptable indicator for this purpose. If the serological response is not considered to be 

equivalent, then the conclusion simply should be that the immune response elicited by the 

product may have changed. No attempt should be made to use serology to determine the 

impact that the altered immune response has on the overall efficacy of the product. 

Serological equivalence should not be confused with serological noninferiority. In the 

past, it was often considered acceptable to generate serological titers with the proposed 

product that are substantially higher than that obtained with the product of proven 

efficacy, just as long as the response was not inferior. The scientific wisdom of this is 

questionable, given our current knowledge of type I vs type II immune responses. If the 

immune response in the proposed product is shifted to a more predominantly type II 

response, it may be characterized by a higher antibody titer but a weaker cellular 

response and may be less protective overall. More is not necessarily better. 

For certain antigens with a codified in-vivo potency test that is adequately linked to 

efficacy (e.g., certain clostridial products), demonstrating that the new combination 

passes the potency test may be acceptable. For poultry products, it is common to perform 

a host animal efficacy study to demonstrate lack of excessive interference.
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Reviewing Interference Studies

1. Determine whether demonstrating lack of excessive interference is an acceptable 

alternative to demonstrating efficacy directly:

1.1. The new product combination must contain the same adjuvant system (in the 

same concentration) and have the same schedule and route of administration as does 

the licensed product that will be used as the basis for comparison. It must use the 

same Master Seeds at the same minimum concentrations.

1.2. Efficacy must have been demonstrated directly for the product used as the basis 

for comparison In other words, a product that was licensed on the basis of 

interference studies cannot, in turn, serve as the basis for licensure of another product. 

2. Ensure that the serials being compared are matched except for the antigen of interest. 

They should be made from the same bulk antigen lots, with equal amounts of antigen. 

3. Ensure that data are generated for each antigen in the licensed product. For example, 

if a fourth virus is being added to a licensed 3-way viral product, serological titers against 

each of the 3 original viruses must be compared to ensure that the addition of the fourth 

antigen did not interfere with any of the other viral antigens. 

4. For serological studies: 

4.1. Compare the geometric mean titers from each treatment group. Ensure that 

serological equivalence is demonstrated to an acceptable degree of confidence. 

Currently, a 63% equivalence margin with a 0.05 level of significance is acceptable 

for serological noninferiority. The estimated confidence interval must not have a 

lower limit less than 63%. 

Although VS Memorandum 800.203 states that we are looking for serological 

noninferiority, any product testing outside of these parameters (higher OR lower) 

should be suspect. Use your professional discretion when evaluating interference 

data in which the new product generates substantially higher titers. 

4.2. Ensure that the serological assay used to demonstrate equivalence is adequately 

validated (especially with respect to repeatability) and properly controlled. 

5. If serological equivalence cannot be demonstrated initially, the firm may elect to:

5.1. Increase the group sizes for serological comparison. Data may be generated from 

appropriately designed field studies randomized to the proposed and existing product. 

5.2. Challenge vaccinated animals to demonstrate protection with the proposed 

product. If this is done, the review considerations for pivotal efficacy studies apply.