Table 2.

Clinical illness during the first wave of the 1918 pandemic in the spring and summer and protective effect against illness during the second wave in fall 1918, British military and civilian communities.

Population, illness status	Illness in fall	No illness in fall	RR (95% CI)	Protective effect (95% CI)	P
British Grand Fleeta (N = 90,000)
Illness in spring	186	10,127	0.28 (0.24–0.32)	72%(68%–76%)	<.001
No illness in spring	5195	74,492
Twelve civilian British communitiesb (N = 24,706)
Illness in summer	240	2623	0.65 (0.57–0.73)	35%(27%–43%)	<.001
No illness in summer	2831	19,012

^aData on influenza illness in May and September 1918 [10].

^bData from a British pandemic report [11].