IP Cases & Articles

Secret prior art: a trap for the unwary?

In almost all countries around the world, a patent application is only published after 18 months from the filing date, or where priority has been claimed from the priority date. This raises the interesting question of how the system should handle a second application which is filed after a first patent application has been filed but before the first application has been published. This situation is known as “secret” prior art since, in general, the applicant of the second application has no way of knowing that the first (unpublished) application exists.

Under the usual rules of prior art, only public disclosures are available to be cited against an application. As the first application has not yet been published, it ordinarily would not be citable against the second application. However, if the first application cannot be cited against the second application this could lead to the undesirable outcome where both applications could be granted to the same subject-matter such that the applicant of either granted patent could prevent the other from working the invention and could prevent the invention from ever coming to market.

So, given the desire of governments to avoid potentially extremely valuable patents being blocked from coming to the market, what can be done?

The basic approach taken is to allow the first application to be cited against the second application but to apply various limitations on how the first application can be used, with a view to balancing the rights of both applicants while avoiding overlapping rights that could prevent the invention from coming to market.

However, even within this basic approach there are a wide range of options that states can adopt. Unfortunately, global harmonisation has not yet been achieved. While there are many aspects of variance between states, two key aspects of significant variation are:

  1. what patentability criteria secret prior art can be used for; and
  2. whether self-collision is possible.

The different approaches can be seen in the summary table shown below for a selection of patent offices.

State UK Europe US Japan Korea China
Citable purposes Novelty only Novelty only Novelty and obviousness Novelty only Novelty only Novelty only
Self-collision Yes Yes No No No Yes

As can be seen, most states strike the balance between the rights of the two applicants by stating that the earlier application can only be cited for novelty purposes. The thinking is that as long as there is no overlap between the two applications (that is, the second application is novel) then the mutual blocking situation described above can be avoided. However, the US takes a very different approach and allows secret prior art to be cited both for novelty and obviousness purposes.

From one perspective, this US approach is a hold-over from before the 2012 America Invents Act: the US previously used a “first to invent” standard, as opposed to the current “first inventor to file” standard. Under the “first to invent” standard, what mattered was who first made the invention, with it being possible to “swear behind” an earlier filed application if an inventor could prove that they had invented the invention earlier. Accordingly, the usage of the earlier invention as secret prior art for both novelty and obviousness naturally followed.

The second key aspect, as shown in the table above, is whether self-collision, that is, the applicant’s own prior applications, can be cited. As can be seen there is a far more even split on this issue with the UK, Europe and China taking the view that there is no special status to the applicant’s own applications and accordingly these should be fully citable, just as if they were an application by a third party. In contrast, the US, Japan and Korea take the opposite view that they should not citable.


As we have seen, this is an area where the variety of approaches taken across the world can lead to applications which are granted in one state being refused in another. This often comes as a surprise to applicants who have become used to most aspects of patent law having a good degree of harmonisation across the world.

While there have been moves to harmonise the rules on secret prior art across the world as part of the Substantive Patent Law Treaty, the negotiations have stalled since 2006. As such, it appears that there are no immediate prospects for the law in this area to be harmonised. In the meantime, if you have any questions or concerns regarding this area your D Young & Co representative is here to help.

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