Prior immunity helps to explain wave-like behaviour of pandemic influenza in 1918-9

BMC Infectious Diseases

Table 2 Observed odds ratios (OR) and 95% confidence intervals to test for evidence of immune protection from wave to wave.

POPULATION	OR for SA	OR for SW	OR for AW
South Shields	1.199 (0.045 - 11.364)	0.937 (0.036 - 8.735)	1.508 (0.292 - 6.549)
Leicester	0.292 (0.158 - 0.533)	0.871 (0.521 - 1.446)	0.655 (0.437 - 0.979)
Wigan	0.266 (0.011 - 2.186)	0.174 (0.007 - 1.419)	0.284 (0.061 - 1.090)
Newcastle	0.258 (0.071 - 0.816)	1.945 (1.284 - 2.936)	0.959 (0.536 - 1.692)
Manchester	0.972 (0.709 - 1.329)	1.976 (1.182 - 3.286)	1.267 (0.635 - 2.481)
Blackburn	1.033 (0.406 - 2.522)	0.782 (0.290 - 1.996)	1.729 (0.763 - 3.821)
Widnes	0.340 (0.175 - 0.648)	0.397 (0.230 - 0.677)	0.219 (0.091 - 0.505)
London police	0.449 (0.117 - 1.515)	0.000 (0.029 - 2.933)	1.125 (0.324 - 3.610)
Cambridge Uni	0.248 (0.165 - 0.371)	0.439 (0.261 - 0.734)	0.915 (0.604 - 1.380)
Clifton College	0.302 (0.164 - 0.551)	1.055 (0.657 - 1.687)	1.756 (1.044 - 2.946)
Haileybury	1.059 (0.634 - 1.766)	0.337 (0.199 - 0.567)	0.945 (0.550 - 1.620)
Finchley School	0.282 (0.152 - 0.518)	1.241 (0.368 - 3.841)	0.230 (0.060 - 0.774)
*ALL*	0.621 (0.529-0.729)	1.105 (0.946-1.291)	0.972 (0.831-1.138)

Odds ratios (OR) and confidence intervals [33] were calculated from the data of Table 1. The numbers affected in the summer (S) wave, for example, are obtained by summing totals for S- -, SA -, S-W and SAW. An odds-ratio for SA of less than 1 (eg 0.292 for Leicester) shows a tendency for those affected in the summer wave to be less affected in the autumn wave, relative to those not affected in the summer wave. For six populations over the SA comparison, the OR (in bold type), are reduced significantly below 1, providing ostensible evidence of immune memory and protection following the summer wave. In contrast, the results of model fitting (Table 4) are consistent with effects of wave to wave immunity in all populations.

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