Since 1 October 2008, the CDC has tested 1,146 seasonal influenza A (H1N1) viruses for resistance against oseltamivir and zanamivir. It was found that 99.6% of the samples were resistant to oseltamivir while none were resistant to zanamivir. However, when 2009 Influenza A (H1N1) virus samples were tested, only 4% (of 853 samples) showed resistance to oseltamivir (again, no samples showed resistance to zanamivir). A study conducted in Japan during the 2009 H1N1 pandemic concluded that infants exposed to either oseltamivir or zanamivir had no short term adverse effects. Both amantadine and rimantadine have been found to be teratogenic and embryotoxic (malformations and toxic effects on the embryo) when given at high doses in animal studies.