Patent of addition: to apply or not to apply?

often, after filing a patent application, improvements, additional uses and refinements surface. To ensure patent protection, the applicant can file for either a new application or a patent of addition. While filing a new application is a straightforward option, the applicant cannot appropriate the priority date of the previously filed application (or 'parent application').

In contrast, a patent of addition permits a patent applicant to appropriate the priority date. A patent of addition facilitates the addition of new subject matter to an existing disclosure of the parent application, while retaining the priority date for claims based on the original disclosure. A patent of addition is designed to let the applicant add subject matter, unlike a divisional application. For example, the applicant can:

modify the existing disclosure based on improvements; 

expand existing descriptions (eg, to ensure that claims for an important feature are fully supported); or 

add paragraphs disclosing new subject matter. 

The claims of the patent of addition can be directed to a combination of new and old subject matter, or exclusively to the new subject matter.

Claims that are limited to subject matter shared by the patent of addition and the parent application inherit the priority date of the parent application. As a result, claims in the patent of addition can have a priority date that is more than one year before the filing of the patent of addition. In contrast, claims that include subject matter that is first disclosed in the patent of addition are entitled only to the filing date of the patent of addition.

More specifically, contrary to the perception that the patent of addition inherits the priority date of a parent application, certain claims may have their own priority dates. Only a claim with all of its limitations fully supported by the disclosure of the parent application is entitled to the parent’s earlier priority date. All other claims are given the priority date corresponding to the patent of addition’s filing date.

For example, consider a scenario where a patent family includes a parent application, P, and a later patent of addition, PA, claiming priority to P. Application P is published or patented, disclosing a process with steps 1 to 10. Later, application PA is filed, disclosing additional step 11 and claiming both the 10-step process and a process with steps 1 to 11. PA’s claim of the 10-step process is entitled to P’s priority date, but PA’s claim of the 11-step process is not. As a result, references that are filed or published after P’s priority date but before PA’s filing date can be cited against PA. Moreover, P itself can be cited as prior art against PA’s claim for the 11-step process, including steps 1 to 10. However, such citation of P as a prior art remains restricted to testing novelty against the 11-step process.

A patent of addition effectively enables an applicant to keep a patent family alive, make incremental improvements and add embodiments that might not justify their own patent applications without needing to file parallel applications. More specifically, the applicant need not pay a renewal fee for a patent of addition, thereby saving costs. A patent of addition application can serve as a vehicle for future divisional application, and even further patent of addition applications.

While a patent of addition remains a convenient tool for applicants to graft additional features onto previously filed patent applications, it may not always be the right tool for the job. A standard patent term is 20 years after the application’s earliest priority date. The patent term of a patent of addition is thus limited to its parent application’s term. For example, a patent of addition filed in 2015 that claims priority to a parent application filed in 2010 will expire in 2030, as opposed to 2035 if the new disclosure had been filed as a standalone patent application. The shortened patent term applies to all claims of the patent of addition, even those that are not entitled to the parent’s earlier priority date. This reduced patent term can potentially mean a loss of significant revenue from products or methods that are encompassed by patent claims which are supported by the new disclosure in a patent of addition.

Further, when a patent of addition is filed based on newly introduced subject matter, it may be also appropriated by opponents to question whether disclosure in the parent application enables a skilled person to arrive at the invention. More specifically, it may be asserted that the new matter, as introduced in the patent of addition, is actually necessary to make and use the invention, and is thus critical for enablement in the parent application. Therefore, the introduction of new subject matter in the patent of addition may leave the door open for a patent challenger to assert the invalidity of parent application claims due to non-enablement.

Comment
A patent of addition application, while an attractive option, has its disadvantages, such as a shortened patent term and the absence of an earlier priority benefit with respect to claims with new subject matter.

A patent of addition is not the only way to obtain patent protection for new subject matter related to a previously filed, pending or granted application. In situations where support for the new subject matter of the patent of addition is not clearly set out in the parent application, it may be advantageous to file a separate, unrelated application to utilise the distinctiveness of the new claims, and to obtain the benefit of a full patent term.

Accordingly, a patent applicant must make an informed decision by weighing up an earlier priority date for some claims against a shortened patent term for all claims in a patent of addition.