An important element of WHO's mission with respect to immunization devices is to update and revise product standards to ensure that requisite product performance is maintained.

Revision of product standards may be required:

• to preserve vaccine potency if this is at risk
• when data and results from operational studies reveal that product improvement is needed
• following repeated performance issues
• when national immunization programmes indicate (additional) programmatic needs.

Product specifications and requirements therefore evolve too, impacting products for which prequalified status is sought, as well as products that have already been prequalified.

Prequalification Holders may be affected in two ways:

• retroactive compliance
• grandfathering.

Retroactive compliance

The need for demonstration of retroactive compliance for an existing prequalified product is decided on a case-by-case basis. The cost, feasibility and timeline for implementation and compliance are discussed with manufacturers during a consultation period.

Thereafter, Prequalification Holders of concerned products must ensure that their prequalified products meet the requirements of the revised and/or new WHO standards.

Grandfathering

Grandfathering is the process whereby prequalified products are exempted from having to demonstrate compliance with relevant, new product specifications.

As of July 2018 WHO has applied a grace period whereby a prequalified product must comply with relevant, new specifications within two years of those specifications being issued. If a prequalified product does not meet that deadline it will be deemed non-compliant and delisted.