Malware writers are not above analyzing their competitors malicious code. The Xpaj file infector, complete with bootkit functionality, can run under Windows x86 and Windows x64. It can be conjectured that this variant of the virus is merely a test version. The next modification of the malware could be fully-functional and perhaps its authors will implement a mechanism for infecting 64-bit executables, including kernel-mode drivers (which will of course involve disabling signature checking)”]
Source: https://securelist.com/xpaj-reversing-a-windows-x64-bootkit/36563/