TABLE 2.

Protection against challenge with lethal doses of PR8 in mice immunized with M2KO virus^a

Weeks postimmunization	Immunogen	PFU of virus inoculated per mouse	No. of survivors/no. of mice tested	Virus titer (mean ± SD log10 [PFU/g]) from indicated sourcec
				Lungs	Nasal turbinates
4	M2KO	3 × 106	8/8	ND	ND
	M2KO	3 × 105	8/8	ND	ND
	M2KO	3 × 104	8/8	6.3 ± 1.5	5.1 ± 0.3
	M2KO	3 × 103	7/8	6.9, 6.6	6.2, 6.0
	M2KO	3 × 102	0/8	8.0 ± 0.1	6.3 ± 0.3
	Inactivated PR8b		0/8	7.6 ± 0.1	5.9 ± 0.2
	PBS		0/8	7.5 ± 0.1	5.9 ± 0.1
12	M2KO	3 × 106	8/8	ND	ND
	M2KO	3 × 105	8/8	ND	ND
	M2KO	3 × 104	8/8	7.2 ± 0.6	6.0 ± 0.3
	M2KO	3 × 103	7/8	7.4 ± 0.6	6.3 ± 0.2
	M2KO	3 × 102	0/8	8.5 ± 0.2	6.9 ± 0.2
	Inactivated PR8		0/8	7.3 ± 0.2	6.7 ± 0.3
	PBS		0/8	7.8 ± 0.3	6.2 ± 0.6

^aTwenty-two BALB/c mice per group were intranasally immunized with the indicated amounts of M2KO virus, inactivated PR8, or PBS (50 μl per mouse). Half of the mice were challenged with a 50% lethal dose of wild-type PR8 of 100, 4 weeks postimmunization, and the remaining mice were challenged 12 weeks postimmunization. Eight mice per group were monitored for survival for 14 days after challenge. Three mice per each group were sacrificed on day 3 postchallenge to measure virus titration. When virus was not recovered from all three mice, individual titers were recorded.

^bVirus particles equivalent to 3 × 10⁶ PFU (32 hemagglutination units) were used.

^cND, not detected (detection limit, 10 PFU/ml/lung).