For the best understanding of this product refer to the following link: http://www.steripen.com/stuff/contentmgr/files/1/a6350a3b2838d63edab45ad4623448d7/misc/mini_english.pdf
UV does kill giardia or at least damage giardia so it can't reproduce.
Does UV inactivate Cryptosporidium (Beaver fever) and Giardia?
Cryptosporidium and Giardia are what is called a protozoan cyst. Protozoa can be described as microscopic, single celled microorganisms which live in water and are quite a bit larger in comparison to other microbes. The majority of protozoan cysts are parasitic. Both Cryptosporidium and Giardia are parasitic. These organisms are in a dormant phase when in water but when they enter a host (being any kind of mammal) they release colonies and begin to breed, ultimately causing severe diarrhea and dehydration over a prolonged period of time.
Cryptosporidium and Giardia were not microorganisms of concern until approximately 10 years ago when Milwaukee was hit with a waterborne disease epidemic. Milwaukee drinking water is surface water, which had become contaminated with high concentrations of Cryptosporidium at the time. Over 100,000 people came down with Cryptosporidiosis and over 400 people died. The USEPA recognized that they needed to consider these organisms and include them within their drinking water guidelines. Testing was done and it was found that chlorine was NOT effective against either of the protozoa. Testing was then conducted using UV technology with initial failure due to improper test procedures. Ultimately it was proven that UV is in fact very effective against Cryptosporidium and Giardia. The dose levels required to inactivate these cysts are actually quite low; less than 10 mJ/cm² for 99.9% reduction of both Cryptosporidium parvum and Giardia lamblia.