TABLE 3.

Dose of H5N1 virus required to overcome protective immunity provided by H9N2 infection^a

Dose (CLD50) of H5N1 challenge virus	Protection of chickens (no. sick/no. dead)	H5N1 virus in surviving chickensb
		Trachea	Cloaca
10	3/2	+ (8/8c)	2.75d (4/8)
25	7/5	+ (5/5)	2.68d (5/5)
50	10/10	+++	+++
100	10/10	+++	+++
150	10/10	+++	+++
200	10/10	+++	+++

^aTen chickens per group were infected with 10³ CID₅₀ of A/Chicken/HK/G9/97 (H9N2) 30 days prior to challenge with various doses of A/Chicken/HK/728/97 (H5N1). Challenged chickens were daily monitored for death and tracheae and cloacae were swabbed for detection of H5N1 influenza viruses. The presence of H5N1 influenza viruses was determined by inoculating samples into four eggs. Results were evaluated by embryo death and HA test. 

^bH5N1 viruses were detectable only in the undiluted samples (less than 10 ELD₅₀). H5N1 influenza viruses were detectable 2 to 5 days postchallenge. +++, H5N1 titers over 10⁶ ELD₅₀; chicken did not survive. 

^cNumber of chickens positive for H5N1 influenza viruses/number that survived. 

^dAverage log ELD₅₀. H5N1 influenza viruses were detectable from 3 to 10 days postchallenge.